UNIVERSITY OF BIRMINGHAM Chemical Engineering Design Project
Rod Mill design for potash processing
Lincoln Smith 935495
The design of a rod mill for a potash processing plant at Boulby, Boulby, UK is described, including operating principals, sizings and other equipment design considerations. The design process is gone through in a stepwise manner in order to make the design process clear. A single mill with internal diameter of 3.2 metres is i s chosen for the process.
Rod Mill Design For Potash Processing b y Lincoln Smith is licensed under a Creative Commons Attribution 3.0 Unported License. License.
Contents 1.
Size Reduction .......................................................................................................................................................... 4
2.
Description ................................................................................................................................................................ 5 2.1
3
4
2.1.1
Rod Mills vs Ball Mills ........................................................................................................................ 5
2.1.2
Operating principle ............................................................................................................................. 5
2.1.3
Design Considerations ....................................................................................................................... 6
2.2
Rod Mill Components .................................................................................................................................. 7
2.3
Discharge Arrangement .............................................................................................................................. 9
Particle Sizes .......................................................................................................................................................... 10 3.1
Size Distributions ....................................................................................................................................... 10
3.2
Mean Diameters .......................................................................................................................................... 11
Mill Design .............................................................................................................................................................. 12 4.1
6
7
Mill Sizing ...................................................................................................................................................... 12
4.1.1
Mill Sizing by Power Requirement ............................................................................................ 12
4.1.2
Mill Power Correction Factors .................................................................................................... 13
4.1.3
Mill Sizing by Residence Time ..................................................................................................... 13
4.2
Number of Lifters ....................................................................................................................................... 13
4.3
Rod Mill Charge ........................................................................................................................................... 13
4.3.1
Rod Volume ......................................................................................................................................... 13
4.3.2
Rod Size ................................................................................................................................................ 14
4.3.3
Number of Rods ................................................................................................................................. 15
4.4
5
Rod Mills ........................................................................................................................................................... 5
Speed ............................................................................................................................................................... 15
4.4.1
Critical Speed ...................................................................................................................................... 15
4.4.2
Operating Mill Speed ....................................................................................................................... 16
Auxiliary Equipment ........................................................................................................................................... 16 5.1
Motor Size ...................................................................................................................................................... 16
5.2
Piping .............................................................................................................................................................. 17
5.3
Hydrocyclones ............................................................................................................................................. 17
Control Strategy .................................................................................................................................................... 18 6.1
P & ID Explanation ..................................................................................................................................... 18
6.2
Startup / Shutdown ................................................................................................................................... 19
Control Implementation .................................................................................................................................... 19 7.1
Ball Mill Level Control .............................................................................................................................. 20
7.2
Rotation Speed Control ............................................................................................................................ 20
7.3
Residence Time ........................................................................................................................................... 21
7.4
Operator Interaction ................................................................................................................................. 21
8
Hazards .................................................................................................................................................................... 21 8.1
Noise ................................................................................................................................................................ 21
8.2
Dust Exposure .............................................................................................................................................. 21
8.3
Static Electricity .......................................................................................................................................... 21
9
Materials of Construction ................................................................................................................................. 22 9.1
Linings............................................................................................................................................................. 22
9.2
Pipes ................................................................................................................................................................ 23
10
Costs ..................................................................................................................................................................... 24
10.1
Fixed Costs .................................................................................................................................................... 24
10.2
Operating Costs ........................................................................................................................................... 25
10.2.1
Utility Costs ......................................................................................................................................... 25
10.2.2
Maintenance Costs ........................................................................................................................... 25
11
Conclusion .......................................................................................................................................................... 27
12
References .......................................................................................................................................................... 28
13
Appendicies ....................................................................................................................................................... 31
Appendix A - Mill Process Block Flow Diagram ........................................................................................... 31 Appendix B – Size Reduction Process P & ID ................................................................................................ 32 Appendix C - Particle Size Distributions ......................................................................................................... 33 Appendix D - Visualising Particle Size Distributions ................................................................................. 34 Appendix E – Calculating the crusher crusher power requirement........................................... ..................... ........................................... .......................... ..... 34 Appendix F - Energy Requirement Requirement / tonne......................................... ................... ............................................ ............................................ ..................................... ............... 35 Appendix G – Correction Factors ....................................................................................................................... 35 Appendix H – Residence Time and mill Volume .......................................................................................... 36 Appendix I - Using Solver to calculate the mill dimensions .......................................... .................... ............................................ .......................... .... 37 Appendix J - Number of Mill Mill Lifters......................................... ................... ............................................ ............................................ ........................................... .............................. ......... 37 Appendix K – Rod Diameters ............................................................................................................................... 38 Appendix L - Calculating Calculating the number of rods required required ............................... ........................................... ...................... ......................... 39 Appendix M – Motor Sizing ................................................................................................................................... 39 Appendix N - Hydrocyclone Pipe Diameter ................................................................................................... 40 Appendix O - Hydrocyclone Sizing .................................................................................................................... 41 Appendix P – Process Specification Sheet ...................................................................................................... 42 Appendix Q – Plant Wide P & ID ......................................................................................................................... 44 Appendix R – Mechanical Drawings .................................................................................................................. 44
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1.
Size Reduction
Size reduction is an essential part of the potash production processes. After removing sylvite ore from the ground a series of crushing operations reduce the size of the rock. The milling stage (or fines crushing stage) forms the final stage of the size si ze reduction process. The breakup of the rock liberates the entrained minerals as well as impurities (gangue). This allows separation in later stages of the process ( Gupta, et al, 2006 ).
Figure 1 – Initial Size reduction process (from initial report)
The majority of the potash process uses saturated brine as a transport fluid. Particles are transported through through the process as a suspension or slurry. Hydrocyclones classify the mill output by particle size into a fines and coarse stream. These are treated separately in the rest of the process, forming different products, used for different applications. The mining operation causes the formation of potash fines, which are more difficult to process than particles with larger diameters (GoodQuarry, 2011). In our process, coarse KCl particles are separated by flotation, which are then dewatered in solid bowl centrifuges and dried in fluidised bed dryers. Some potash is upgraded in size by a compaction process for use in particular fertiliser products. products. The small particle sizes in the fines stream make them unsuitable for direct use as fertilisers. After flotation using a suitable flotation agent, the fines stream requires heating. A subsequent selective crystallisation process crystallises potash out of the resulting solution. The stream is then dewatered and dried by centrifuges and fluidised bed dryers. Suspended particles from waste streams are separated using centrifuges, and the liquid stream saturated with potash and halite is recycled back into the process, maximising recovery.
The flotation, centrifugation and particle fluidisation drying stages mentioned, are all dependant on the particle size of the ore. Particle size is a crucial parameter of the effectiveness effectiveness of downstream processes, and size reduction is crucial in ensuring a high quality product.
2.
Description
2.1 Rod Mills 2.1.1 Rod Mills vs Ball Mills Tumbling type mills are used for the grinding of potash rock ( Couper, et al, 2010a). Rod mills are used at the existing site at Boulby ( Holyfield, et al, 1998). The balls in a ball mill have a greater surface area to weight than rod mills, and therefore are more suited to fine grinding (Couper, et al, 2010a). Extreme fine particles are not easily separated by froth flotation; therefore a rod mill is more suited to the process.
Figure 2 – Cut away diagram of a typical rod mill (Metso, 2010)
2.1.2 Operating principle A rod mill is used for the grinding of rock from particle sizes as large as 25mm to between 2 – 0.1 mm mean particle size (Practical Action, 2010). Tumbling mills use the action of falling masses to grind particles to appropriate sizes. In a rod mill, rods are used, typically filled to 45% of the mill volume ( Couper, et al, 2010b). These are lifted by the rotating action of the mill before cascading downwards and causing particle breakup. There are three principal mechanisms for particle breakup in a ball mill:
Impact breakup, due to the fall of the particles onto the rods
Attrition breakup Page 5 of 44
Abrasion breakup (Practical Action, 2010)
2.1.3 Design Considerations To withstand the abrasive forces of the rock and severe impacts, as it is grinds rock to smaller sizes, the lining of the mill must be carefully selected. selected. For this reason, they are commonly manufactured manufactured from manganese or chrome-molybdenum steels ( Metso, 2010), however other linings are available. The speed of rotation is a function of the diameter. They commonly spin between 20 and 30 rpm. The motor used to drive the mill commonly spins at 150 – 250 rpm (ie. low speed motors are used to drive the mill) ( Kanda, 2007 ). ). To avoid the problems associated with dust formation, saturated brine solution is added. Practical Action identifies dust formation and subsequent inhalation as the “most serious long term threat from minerals processing.” ( Practical Action, 2010) The sylvite ore at the Boulby mine contains 38 potash and 51% common rock salt and 11% of other insoluble materials which must be removed in the purification process (Rowson , 2010), or 38% KCl, 50% NaCl, 12% insoluble’s (Holyfield, 1995 ). ). These can be liberated once the particle sizes are smaller than 1mm (Holyfield, 1995 ). ). The feed of sylvite ore slurry is controlled as the mill must be contain the correct level of slurry for effective & efficient grinding to occur. Classifiers are used to select over, and undersized particles so that they can either be recycled back into the mill or passed into different parts of the process. Hydrocyclones Hydrocyclones are most suitable for classification (over screens and sieves) due their history of use in size reduction circuits. Their ability to accept particle sizes between 40 and 400 microns makes them particularly suited to classifying the product from a rod mill, which produces particle sizes between 10 and 200 microns. The mill needs to be carefully chosen, as it is factors such as its size and operation conditions which ultimately controls the particle sizes of the potash ore.
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2.2 Rod Mill Components Table 1 and Figure 3 show and describe the main components components of a rod mill.
2
3
9
11 1
7
8
10 6
5
4 Figure 3
No.
Part
Description
1
Bearing
Supports the load of the rotating section and allows it to turn freely.
2
Mill shell
Section where grinding takes place. It takes the form or a cylindrical steel shell with a replaceable i nner lining. Man-holes on the body are beneficial for cleaning the steel balls, replacing the liners and repairing the machine. The thickness is approximately approximately 1/100th of the length.
3
Drive
Transfers energy from the motor and gearbox, into the rotational energy of the ball mill. Two principal ways exist to do this:
Rubber rollers - rotate around the outside of the entire length of the mill shell Gear and pinion – large gear on one end of the mill is driven by a pinion, connected to the gearbox.
To maximize lifetime, this should be reversible so both flanks of the gear teeth can be used. ( FLSmidth Minerals, 2008) 4
Motor
Transfers electrical energy into kinetic energy to power Page 7 of 44
the mill. The large power requirements of the mills require high power, reliable, reliable, efficient, efficient, low speed, speed, motors. motors. AC synchronous motors fit this requirement. Where the power requirements are high, two motors and drive systems may need to be used. ( GE Motors, 2008) 5
Clutch & Gearbox
Allows the motor and gearbox to be engaged and disengaged with the drive system. Air clutch systems are commonly used. (GE Motors, 2008) Ball mills require a high starting torque to accelerate their contents. (Agrawal, 2001 ) The clutch is required to deliver the power at the correct rate such that it does not damage the motor.
6
Lubrication System
To protect the bearing and drive mechanism, allow it to move freely, minimising wear and damage. Lubrication can be supplied in three ways
Oil mist system Grease Circulating oil
If lubrication to the bearings stops working, the mill must shut down to prevent it from damage. 7
Inlet
To control the feed into the mill shell.
8
Discharge Outlet
To control the outlet from the mill, and retain any material requiring further grinding.
9
Lining
Helps with the abrasion of the potash rock, and protects the mill shell from wear. Linings need to be replaced regularly due to erosion of the material. (see section 3.5)
10
Charge
Rod charge, cause particle break-up by cascading down the sides of the mill. Rods have a practical maximum length of 6 metres. Longer rods bend causing undesirable tangling of the charge ( Gupta, et al, 2006 )
11
Lifters
Part of the lining, used to lift the mill charge and prevent slip on the walls of the mill. There are a variety of lifter designs available. Table 1
(Zoneding, 2009)
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2.3 Discharge Arrangement There are several different arrangements for discharging the milled ore slurry from the mill shell. The discharge arrangement must allow the grinding media to be retained within the mill, while discharging ground potash ore particles. Figure 4 shows the possible arrangements: trunnion overflow discharge, diaphragm or grate discharge, end peripheral discharge, center peripheral discharge. Trunnion overflow discharge and diaphragm or grate discharge are the most popular (Metso, 2010). In the mill in our potash production process, the simplicity of the trunnion overflow discharge mill makes it the most suitable. In grate or diaphragm discharge, discharge, a slotted full diameter grate with a lifters, convey milled from the bottom of the mill, beyond the diaphragm to the discharge opening. In overflow discharge a gradient forms between the feed inlet and discharge openings. A reverse rotating screw retains larger particles and the rod charge inside the mill. (Metso Minerals - Ball mill, 2010)
Feed
Discharge
Trunnion Tru nnion Overflow Di scharge Feed
Discharge
Diaphram / Grate Discharge Feed
Discharge
Feed
Center Peripheral Discharge
Feed
End Peripheral Discharge Discharge
Figure 4 Adapted from: (Metso Minerals, 2010, 2010, Rod Mills) & (Gupta et al, 2006)
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3 Particle Sizes It is important to investigate the particle sizes because this helps define the reduction ratios required for the design. The reduction ratio is the ratio of the initial particle size to final particle size ( Zhang, 1998), and needs to be considered when designing the mill. The run of mine ore is not delivered to the plant as a mono-modal distribution. Instead the size of the ore takes the form of a size distribution. distribution. The particle sizes of the feed ore and output products products are given in the design brief.
3.1 Size Distributions Histograms shown in figures 5 and 6 represent the particle size distributions distributions shown in appendix C. 0.20%
0.050% 0.040%
0.15%
n o r c i 0.10% M / n o i t 0.05% c a r F
m0.030% μ
/ n0.020% o i t c a r f 0.010%
0.000%
0.00%
0
10000
20000
30000
40000
50000
-0.010%
Size / microns
0 -0.05%
Figure 5 – Run of mine ore particle size histogram
500
1000
1500
Particle Size / Microns Figure 6 – Mill output size histogram
The crushing & milling operations need to take the input distribution (figure 5) and transform it into the output distribution (figure 6). The output feed consists of fine and course streams. These data has been averaged, assuming 15% fines and 85% course, to obtain a single mill output size distribution for use in design calculations. Distribution data is given as mesh passing sizes (the proportion of the feed passes through different sizes of classification screen.) A maximum particle size is assumed. Run of mine ore
50mm (50000 μm)
Mill output
2mm (2000 μm) Table 2 – Assumed Maximum Particle Sizes
Tumbling mills accept feeds with maximum sizes not greater than 25mm. ( Gupta, 2006) The purpose of the primary gyratory crusher is to reduce the particle size to below 2500 μm. There are two methods which could be used to determine the distribution of the mill input.
2000
Assume a constant reduction ratio and reduce all particle sizes by this amount. Page 10 of 44
Assume screening takes place, and only particles greater that 25mm are crushed in the gyratory crusher.
A screening process before the gyratory crusher is more suitable. This method ensures energy is not wasted by milling particles too small as the rod mill input distribution would have a smaller standard deviation than the run of mine ore. 0.14% 0.12% n 0.10% o r c i m0.08% / n o 0.06% i t c a r F 0.04%
0.02% 0.00% 0
500
1000
1500
2000
2500
Size / microns
Figure 7 – Rod Mill Input Size Distribution Histogram
The feed is screened at 2350 μm. Particle sizes larger than this are reduced. Gyratory crushers have a reduction ratio of between between 3:1 and 10:1. The geometric geometric mean of these numbers, 5.5:1, is used. This gives the size distribution histogram given in figure 7 as the feed input to the mill. Alternative ways of showing these particle size distributions are given in appendix D.
3.2 Mean Diameters There are several important average sizes which are important for the mill design:
Sauter mean diameter d80 passing diameter d50 passing diameter
Particle Diameters (given in microns)
Run of mine ore
Mill Input
Mill Output
d32
7473
324
222
d80
35500
1380
620 Table 3 – Mean Particle Sizes
The d80 is found from reading from the particle cumulative distribution. The sauter mean diameter is calculated as follows:
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Where f is is the fraction of particles of a particular particle diameter, d. This diameter was used to calculate the reduction ratio needed in the mill. From these calculations, a reduction ratio of 1.46 : 1 is required to mill the ore to the desired size.
4 Mill Design 4.1 Mill Sizing 4.1.1 Mill Sizing by Power Requirement The size of the crusher is essentially determined by the power required for the crushing process. Bond’s Law determines the energy ener gy required for grinding.
Equation 1
2002) (Kanda, (Holdich, 2002) (Kanda, 2007 )
Where : W i = Material Work Index d o = d 80 80 diameter of material entering the mill d i = d 80 80 required diameter of material leaving the mill.
Appendix E shows how this equation is used to calculate the design power requirement of the crusher of 535kW. Once a safety factor of 10% has been applied, the design power is 589 kW. This is not consistent with Cohen, 2005 , who suggests that 10 – 20 kWh per tonne is required for grinding. A design power of 589 kW represents an energy requirement of 1.15 kWh / kg (See Appendix F). The soft nature of the potash rock makes these low energy requirements a reflection of the reality of the situation. Ideally, the largest and most efficient crushers should be used to keep costs low. Large diameter crushers have very high starting torque and hence the motors require large starting currents to start them moving. (Agrawal, 2001) For this reason, multiple trains of crushers should be considered. A single crusher, with a power rating of 630 kW could be used, or two separate rod mill trains, each with a power rating of 380 kW ( MegaIndustry ,2011). The capacities of the mills are determined by the residence time of the particles in the mill. Two mills might allow room for expansion in capacity, however for simplicity of design, one mill will be chosen. This power of mill has an internal diameter of 3.2m and a length of 3.6 metres. For the purposes of my design, this mill will be chosen.
A sensitivity analysis of the power requirement to work index identifies that the power requirement is a linear function of the potash ore work index. Different sources quote different values for the work index, and the work index of the mine ore is likely to vary as different Page 12 of 44
deposits are found in the mine. The linear relationship means that as long as suitable power adjustment factors factors are applied, as above, the rod mill should be able to handle changes in hardness of rock as required.
4.1.2 Mill Power Correction Factors The mill power calculated in section s ection 4.1.1, calculates power requirements from the original mill used to derive this empirical equation. Correction Correction factors can be applied to the mill power, using the mill dimensions selected. Appendix G shows that the total correction factor which needs to be applied is 2.2. A simple method of adjusting the mill size for the correction factor calculated calculated is to double the number of mills.
4.1.3 Mill Sizing by Residence R esidence Time An alternative method of sizing the mill, involves discovering the residence time, the average time a particle spends in the mill, and using the known volumetric flow rate to calculate the size. The residence time in a rod mill can be measured using a radioactive tracer. A study on a ball mill in Chili by Yianatos with dimentions of 3.05m diameter by 4.24m, observed the mean residence time to be 108 seconds. This mill has a volume of 30.8 m 3 (Yianatos, 2005 ). ). The volume of the mill selected above has a total volume of 28.95m 3; therefore a residence time of 108 seconds is likely to be a good order of magnitude estimate for the residence time in the selected mill. Appendix H shows how a residence time of 71 seconds has been calculated for the selected mill. Conversely, the mill size can be calculated assuming a residence time of 108 seconds. This gives a mill volume of 45m 3. A rule of thumb for the ratio of length to diameter in a rod mill is that the length is 1.5 times the diameter (Couper et al, 2010b). Using Excel’s Solver, as shown in a ppendix I shows that the mill would have a diameter of 3.4 m and a length of 5.1 m.
4.2 Number of Lifters The number of lifters used to raise the balls in the ball mi ll is given by Gupta, et al , 2010.
Equation 2
Where D is the mill diameter. The calculation in appendix J, shows that 21 lifters are required in the ball mill and these will be 48 cm apart. The length of these lifters must be greater than half the radius of the balls to allow them to be lifted above the horizontal.
4.3 Rod Mill Charge 4.3.1 Rod Volume The rods in a rod mill usually occupy 45% of the internal volume of the mill ( Gupta, et al, 2006). The mill is filled with rock and the action of the mill charge rods cause particle break up. Over
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filling the ball mill can cause a cushioning effect which which absorbs the impact of the rods. Under filling causes excessive rod-to-rod contact, slowing the breakage rate. Ideally, the rods should sit in parallel alignment. In practice, accumulation of particles near the feed causes the rod charge to become mal-aligned, as shown in figure 6. This is actually an advantage because this spacing at the feed end preferentially grinds larger particles, resulting in a narrow size range. (Couper, et al, 2010a) The density of the slurry (solid concentration) affects the rod charge and must be carefully controlled ( Gupta, et al, 2006).
Rotary action of mill
Feed
Discharge
Slurry
Greater wear of rods here
Rods
Figure 8 – Alignment of rods in the mill
4.3.2 Rod Size The rods in the mill are 152 mm shorter than the length of the mill ( Gupta, et al, 2006). For the mill length of 3.6 meters, the rods should be 3.448 m long. This allows room for the rods to fall in the mill, whilst remaining parallel to other rods. The initial rod diameter is related to the diameter of the mill by equation 2.
Equation 3
Where: F 80 80 = d 80 80 = feed 80% passing diameter D = inside diameter of the mill (2.2m) W i = work index (~8 kWh / tonne for potash ore)
= solids density = fraction of the critical speed
(Gupta, et al, 2006)
Mill diameters vary between 1.6 m and 6.6m ( FLSmidth, 2011). Rod mills mills are often bought ‘off the shelf’ and as such come in fixed sizes. The rod charge is also available in fixed diameters, bought off-the-shelf. Typical rod diameters vary between 25 mm and 150 mm ( Couper, et al, 2010). A range of rod diameters is often chosen, to allow smaller rods to fill the voids between larger rods as shown in figure 11, increasing the mill efficiency. Page 14 of 44
Analysis of this empirical equation found in academic literature has found that it does not give mill or rod diameters which are used in practice or consistent with sources in the literature which suggest rod diameters sizes should be between 25 and 150 mm, as shown in appendix K. A more suitable method is to leave the rod sizing to the manufacturer, who will have more experience with rod selection.
Rotary action of rod mill
25 mm rods (filling voids)
75 mm rods
Figure 9 – Varying rod diameters in a rod mill
The RAEng statement of ethical principles state that engineers should “perform services only in areas of current competence.” ( ” (RAEng, 2009) Accurate rod sizing is outside my level of current competence and should be left to another engineer. This being said, the rods could be assumed to lie in the size range given above, and rod sizes of 25mm and 75mm could be used due to the relative soft nature of the potash ore.
4.3.3 Number of Rods The volume of the mill is 28.9 m 3. A 45% rod mill charge volume is assumed; therefore the rods will occupy 13 m 3. The packing of the rods is not 100% efficient, and voids will form in between the rods. For straight, cylindrical rods, packing can be assumed to be 75 – 90% efficient. Based on these assumptions the volume occupied by the rods will be between 9.8 and 11.7 m 3. This calculation is shown in Appendix L Based on the mill volume, a suitable number of rods for the mill is 700. The mass of this rod charge will be 82 tonnes. This is approximately the weight of the rest of the mill ( WeirMinerals, 2007 ). ).
4.4 Speed 4.4.1 Critical Speed The critical speed is the speed at which the rotational centrifugal force overcomes overcomes the gravity force acting on the balls and mill charge, causing the rods to stick to the outside of the mill wall rather than cascading. Grinding action is reduced or stopped ( Gupta, et al, 2006). The critical speed of a ball mill is given by an empirical equation (equation 3).
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Figure 10
For a mill diameter of 3.2 metres, the critical speed is 24 rpm. As the mill diameter decreases, the critical speed increases and vice versa.
Variation of Critical Speed with Mill Diameter 45 40 m35 p r / 30 d e 25 e p 20 S l a 15 c i t i r 10 C
5 0 1.2
1.8
2.4
3
3.6
Mill Diameter / m
Figure 11
4.4.2 Operating Mill Speed A rule of thumb suggests that rod mills should operate at 50 – 65% of the critical speed. This is between 12 and 16 rpm.
5 Auxiliary Equipment 5.1 Motor Size An AC synchronous motor will be used to deliver power to the mill. During normal operation, the motor is required overcome the frictional forces in the bearings. The motor should be sized si zed for the maximum power draw required rather than the mill power requirement of 630 kW. Maximum power will be drawn when the motor is starting up. To size a motor, the torque and rotational speed of the motor need to be known ( Oriental Motor U.S.A. Corp, 2000). The mill torque is related to the inertia of the mill (I) by equation 4.
Where:
Equation 4
Torque time (seconds) Angular momentum vector Page 16 of 44
The angular momentum vector is a function of the inertial forces on the mill and the rotational speed (angular velocity) as in equation 5
Equation 5
The inertia can be calculated by equation 6. The inertia is a function of mass of material at the edge of the mill (M) and the distance from the centre.
Equation 6
The mill motor should be sized to operate up to the critical speed of the mill. This is 24 rpm. At this speed, maximum power is required. At the critical speed, the rods will be pressed against the exterior of the mill. The radius from the centre to the rods is required to find the inertia of the rotating mill. Appendix M shows the calculation calculation steps showing how a motor motor size of 50 000 kW is required.
5.2 Piping Slurry must be pumped to hydrocyclones at a velocity of 2 m s -1 to 3 m s -1, to prevent particles from settling. Higher than this and excessive wear occurs ( Arterburn, 2010). This is the velocity which the pump must achieve. The internal pipe diameter of the pipe to the hydrocyclone hydrocyclone can be specified using this information. A pipe with internal diameter of between 6 and 7 cm will be suitable for providing the required flow rate. This has been calculated by knowing the required flow rate, as well as an empirical pipe sizing equation given in chapter 5.5 of Coulson & Richardson ( Sinnott, 2009c) (see appendix N). As long as the internal diameter lies within this range, the specific pipe can be chosen from pipe manufacturer data sheets.
5.3 Hydrocyclones Hydrocyclones Hydrocyclones classify the stream into fines and coarse according to the specific weight of the particles .They require a solids concentration concentration of 30% by mass for efficient operation without without increasing operation pressures ( Abulnaga, 2002). The most important slurry property for hydrocyclone hydrocyclone separation is the volumetric slurry density. The mass balance for the process process section is shown in table 4. Only the total flow rates are shown as this section of the process only involves particle sizes and not compositions. Flow-Rate
Tonnes / day
Tonnes / hour
Ore Input
12217
509
Saturated Brine added
12217
509
Total Mill Throughput
24434
1018
Coarse
22789
865
3665
153
Fines
Table 4
The solids concentration by mass is 50%, and is 35% by volume. Page 17 of 44
The hydrocyclone hydrocyclone can be sized according to the method described by Arterburn. The calculations are shown in appendix O and the results shown in table 5.
Hydrocyclone Hydrocyclone Internal Diameter
0.35 m
Number of Hydrocyclone Units
5
Area of Inlet
Length of Vortex Finder
0.12 m
Length of cylindrical Section
0.35 m
Minimum Orifice Size at apex
0.125 m
Cone Angle
13o
Height of cone
0.76 m
350
350
120
760
Table 5
Image from (Sinnott, 2009)
6 Control Strategy 6.1 P & ID Explanation The P & ID for the crushing and milling process is shown in appendix B. This is a working document and will need to be adapted as the plant design progresses. It has been adapted from the block flow diagram in appendix A, which shows major equipment and processes needed to transport materials. This was used for completing hazard study 2. A conveyor is used to transport the ore from the mine to the primary gyratory crusher. This passes a screen to ensure large material does not enter the ball mill. A tank is used to mix the particulate solids and liquid streams. To ensure the correct amount of saturated brine is added, the flow of solids must be measured from the ore and recycle streams. The resulting slurry from the mixing tank is fed into the ball mill. Two methods are available to do this:
A centrifugal pump Feed under gravity
Minimising the number of components keeps the design simple, less expensive, and makes the design intrinsically safer. These principles underpin my mill design. Therefore Therefore the slurry should be fed under gravity. The level of the slurry inside the ball mill is controlled by varying the input and output flow rates. The speed of the motor is maintained constant to ensure optimum operation. operation. After Page 18 of 44
scrubbing, three hydrocyclones remove separate fines from coarse streams. The low reduction ratio of the ball mill and presence of a recycle streams from downstream downstream process, makes the mill suitable to operate in an open loop system; that is one without recycle of coarse particles. The most suitable actuated valve for all pipelines is a Globe valve, because they allow accurate control of flow rate of liquid streams ( Sinnott et al, 2009a). The attrition scrubbing operation removes insoluble insoluble slimes, allowing them to settle on the bottom of the tank for removal.
6.2 Startup / Shutdown The large starting torques required to start the the motor, mean it will be easier to start the mill when empty of slurry. Operating the mill when the slurry is not present could cause excessive wear damage to mill components, components, therefore the mill will be rotated at a speed at which the rods do not cascade, but simply rotate in the mill. This is likely to be between 10 – 30% of the critical speed, or 3 – 7 rpm. Once at this speed, slurry will be discharged from the storage vessel and into the mill. As this happens the mill speed will be increased until it reaches its operating speed of between 12 and 16 rpm. Table 6 illustrates these steps. Start-up Procedure 1
Close all valves
2
Start primary crushing circuit
3
Allow brine storage tank to fill Start mill motor - allow to reach 10 - 30% of critical speed
4
Open mill inlet valve
5
Wait until the mill reaches 45% capacity
6
Open outlet valve and commence automatic control system Start automatic motor speed adjustment control Table 6
In the event of emergency process shutdown, the flow to the mill needs to be stopped and the rotation of the mill needs to be stopped. This is achieved by closing the inlet valve, and cutting the power to the motor. The inertial forces of the motor is likely to keep it spinning for a considerable amount of time, therefore it is recommended to use the AC synchronous motor as a generator, allowing it to act as a brake on the mill. For normal shutdown operations, the same procedure applies. Process downstream of the mill, such as scrubbing and hycrocyclone separation can be shut down after the flow to the mill is shut down. Downstream flows are dependent on the overflow from the mill therefore shutting down upstream also shuts down downstream classification operations.
7 Control Implementation Page 19 of 44
An open loop system, without recycle is proposed for the mill. This is for a number of reasons:
Upstream crushing processes are screened. Downstream flotation separation processes lack an absolute dependence on particle size, instead simply liberated particles with correct surface chemistry The bottom product from the coarse flotation circuit is put through an additional mill to reduce the particle size to ensure all material is liberated.
To design the control strategy for the unit operation, the following need to be considered:
What needs to be controlled? c ontrolled? How and where is it measured?
What is the measured value being compared to?
The mill is responsible for reducing the size of the particles, enabling liberation of the minerals by downstream processes. Ultimately it is the particle size of the output which needs to be controlled. In practice this particle size takes the form of a distribution as shown in figure 5. Recycle streams make off-line control unnecessary for this section of the potash process and a fully on-line control system is proposed. A number of variables affects the particle size output of the ball mill: Controlled Variables
Mill motor speed (as a fraction of the critical speed)
Input flow rate
Uncontrolled Variables
Actual diameter of the milling rods (will be less than initial diameter due to wear) Number of rods in the mill Residence time inside the mill
7.1 Ball Mill Level Control It is difficult to measure the level inside the ball mill as the contents are always rotating. The correct level in the mill due will be maintained due to the overflow outlet. An appropriate control solution is to use a negative feedback control loop to control the input to the mill, and measure the tank output to ensure flow rate into equals the flow rate out of the ball mill. The flow rate of the stream can be measured using a pressure differential based flow meter such as an orifice plate or a venturi flow meter. An orifice plate flow meter will be more suitable because of their reduced cost and size compared to venturi flow meters (Yoder, 1998). This flow sensor is connected to valve V-3 on the P & ID in appendix B.
7.2 Rotation Speed Control The inertial forces of the ball mill make changing rotational rotational speed highly energy intensive. A better control strategy for ball mill speed is to monitor the speed of rotation. The rotation speed may vary with constant motor power output due to changes of the mill charge, flow rates and
Page 20 of 44
slurry densities. Therefore the rotational speed of the mill should be measured and power delivered to the motor to varied, in order to keep the rotation speed constant.
7.3 Residence Time The residence time of the mill is a property of the mill itself, the main parameters being the distance from inlet to outlet (mill length) as well as the slurry flow rate (higher flow rates will decrease the residence time of a particle in the mill.)
7.4 Operator Interaction A distributed control system system is used to control the plant. This is inherently safe as it minimises operator control and responsibility. responsibility. Manual valves add extra complexity complexity to the process, and add extra plant items with the potential to go wrong. The globe valves chosen have the ability to be opened or closed manually if required. Operators Operators will need to evaluate sensor readings respond by making any adjustments to set points. They have responsibility to adjust set points to ensure the process is operating efficiently and economically economically as well as responding appropriately to any fluctuations fluctuations in the potash price. They have the responsibility to evaluate the mill wear, and schedule periods of plant maintenance.
8 Hazards Hazard study 2 identified several important design considerations. The most significant hazardous events are noise from the mill, electric fires caused by static charge build up, and chronic exposure to sylvite dust.
8.1 Noise An exclusion zone will be implemented around the rod mill. Rod mills can produce noise as loud as 100 decibels (WeirMinerals, 2007 ). ). For this reason a 5 metre exclusion zone around the equipment will be used when in operation. The exclusion zone will also prevent people from accessing the rotating parts when in operation. Mill linings will be chosen which provide a noise damping action.
8.2 Dust Exposure The Control of Substances Hazardous to Health (COSHH) regulations apply if personal -3
exposures of sylvite dust exceed 10 mg m eight-hour TWA (total inhalable dust) ( HSE, 1998) The eight-hour TWA limit is a time weighted average (TWA) whereby exposures exposures in a 24 hour period are treated as a single uniform exposure for 8 hours. ( HSE, 2004). Exposure can cause possible reduced lung function. All dry processes will be completely enclosed. This includes the conveyors which transport material into the mixing tank, E-5 on the P & ID in Appendix B.
8.3 Static Electricity
Page 21 of 44
Static electricity is generated by the moving parts of the rod mill, as well as by flowing liquids. Employees working around around the ball mill can also be sources of static electricity. A copper wire connecting the part of the machine where static charge builds up to a water pipe will prevent the build-up of static charge. However, the mill shell rotates and is i nsulated by the lubrication fluid around the bearings making grounding more difficult. A partial solution to this problem is to humidify the air surrounding surrounding the rod mill, allowing charges to leak off the mill. This would help reduce the risk, as well as providing providing a better working environment for operators. ( Paul O. Abbe, 2008) A humidification system, as well as grounding a non-moving component of the mill such as the motor should prevent the build-up of charge on the mill. The recommendations recommendations for controlling undesirable static electricity laid out in BS 5958-2 should be followed. It states that “charge
separation occurs between the liquid and the internal surface of the pipe, producing electrostatic electrostatic charges on both the liquid and the pipe.” It r ecommends avoiding flammable atmospheres, achieved achieved by humidifying the air, as well as earthing pipelines and choosing high conductivity materials of construction. ( British Standards Institution, 1991)
9 Materials of Construction 9.1 Linings An important consideration in mill design is the design of the linings. The internal components of the mill must be able to cope with the abrasive forces of the potash rock and impact forces from the rods. Additionally, materials must be resistant to corrosion to withstand the environment where high concentrations concentrations of chloride ions are present. Liner lifetime must be maximized to keep maintenance costs low, and to prevent catastrophic failure. Two types of material are commonly used for mill liners: steels and rubbers. The advantages and disadvantages of steels commonly used used for mill linings is given in table 7.
The relative soft nature of the potash ore, means that a high chrome iron is most likely to be the most suitable lining material if a steel lining is used. Rubbers suitable for mill linings need to have high tensile strengths (<20MPa), be hard, and able to be stretched 5 to 6 times its length without damage ( Powell, et al, 2006) The rubbers used are a mixture of natural and synthetic rubbers. The principal advantages of rubber linings are their noise damping properties as well as corrosion resistance, weight and cost. Different mill manufacturers have different rubber compositions, such as Metso’s Skega rubber lining. ( Metso, 2010) Composite linings are often used for mill linings to bring together the wear resistance and abrasive properties of steel with the noise damping and corrosion resistance properties of rubbers. The two separate materials are fastened together using a chemical bond and a mechanical attachment to give a secure fastening for the life of the lining. Metal lifter bars can be used in combination with rubber linings, such as with Metso’s Skega Poly-met Poly -met lining. (Metso, 2010) (Moller, 2003) Page 22 of 44
The high concentration of chlorine irons, makes pitting corrosion likely where steels are present in the mill, therefore the most appropriate mill lining material a rubber one. The linings in the mill are usually between 65 and 75 mm thick ( Gupta, et al, 2006 ).
Austenitic manganese steels (AMS)
Advantages
Disadvantages
Work hardens under stress
Deforms with impact, making solid liners difficult to remove
Is, tough and can withstand repeated impacts without fracture
Low Carbon Chrome Molybdenum Steels
Good wear characteristics
High Carbon Chrome Molybdenum Steels
Good wear resistance
Nihard Iron
High wear resistance
Comparatively Comparatively low impact resistance
Good impact resistance Brittle
Good abrasive properties High Chrome Irons
Very high wear resistance Good abrasive properties
Chrome Molybdenum White Irons
Excellent wear resistance Excellent abrasion properties
Higher cost compared to high chrome ions. Table 7 Adapted from (Powell, et al, 2006)
In the chosen mill, the lining will be 70 mm thick rubber with high chrome iron lifter bars. This combines the advantages of rubber linings (deforming under impact) with the hardness advantages of steel.
9.2 Pipes Mild steel pipelines which is commonly used for a pipeline construction material, cannot be used to transport the slurry due to corrosion c orrosion problems. Resistant materials must be used. From the corrosion chart in Appendix B of Coulson & Richardson volume 6 ( Sinnott, et al, 2009b), the materials listed in table 8 are resistant to sea water: Aluminium
High Nickel Iron
Aluminium Bronze
Platinum
Brass
Silver
Copper
Austenitic Ferricr Stainless Steel
Gunmetal
Tantalum
High Silicon Iron
Tin
Page 23 of 44
Nickel-Copper Nickel-Cop per Alloys
Zirconium Table 8
A number of other factors play a part in material selection for pipelines, including formability, formability, tensile strength and costs. Based on prior knowledge of materials, either copper or austenitic ferricr stainless steel should be used for pipeline construction; however a more thorough analysis of the benefits of each would be required and checked over with pipeline manufacturers (Sinnott, et al, 2009).
10 Costs 10.1 Fixed Costs Equipment costs are determined by the manufacturer. The equipment cost is essentially a function of the mill size and mill length. Where data is available, cost estimating equations such as equation 14 given in chapter 6 of Coulson and Richardson’s chemical engineering design can be used.
Figure 12 ( Sinnott, et al, 2009).
Ce = Equipment Cost a & b= cost constants (given in a data table) S = Size parameter (for a rod mill this would be the diameter or volume, depending on the data table used N = exponent, dependant on equipment type Matche.com, uses similar formulas to calculate costs of commonly used equ ipment. A rod mill diameter of 3.2 metres would have a total cost of $1 350 000 in 2007 (Match.com, 2007 ). ). Updating to 2011 prices using an inflation rate of 4% RPI (BBC, 2010 ) gives a total cost of $1.58 million , or £972 thousand (XE.com, 2011 ). ). This is likely to be a definitive estimate, and accurate to
Page 24 of 44
Variation of Purchased Rod Mill cost with Mill Diameter ) s i s a B t s a o C f l u G S U 7 0 0 2 ( $ / t s o C
2500
s d n a s 2000 u o h T 1500
1000 500 0 1.2
1.8
2.4
3
3.6
Mill Diameter / m
Figure 13
Figure 15 shows how rod mill purchased costs vary with mill diameter. In practise not all mill diameters are available, and accurate equipment costing must be obtained direct from manufacturers (IChemE, 1988 ). ). More accurate estimates may be obtained by using commercial mill cost estimating software, such as the CostMine Equipment cost Calculator ( InfoMine, 2011).
10.2 Operating Costs The operating costs for this section can be broken down into two main sections:
Utility Costs
Maintenance Costs
10.2.1
Utility Costs
Utility costs are determined by the power draw of the motor required. The 50 000kW motor, can be assumed to be operating at low power for most of its operation, say 40%, requiring a power of 20 000kW. Electricity for processing plants can cost 5 p/kWh (Rowson, 2010). Assuming operation for 324 days per year, and 24 hours per day, this gives a running cost of £24 000 per day, or £7.7million per year.
10.2.2
Maintenance Costs
The most significant maintenance cost will be the cost of replacing the mill lining when it wears down. The conditions of mill linings need to be monitored by evaluating the performance of the mill as well as by visual inspection at regular intervals. intervals. It is recommended recommended that they are replaced every year. The rubber linings used on this mill are easier and cheaper to replace than steel linings. Accurate cost information of liners is available direct from manufacturers, manufacturers, however
Page 25 of 44
could be assumed to be a fixed percentage of the total mill cost, say 10 – 40%. On this basis the replacement liners would cost between approximately £100 thousand and £390 thousand. This does not include the labour costs associated with replacing the mill, or the loss of output, however this downtime can be included in the 41 days in a year the entire plant will be offline for maintenance. On this basis, the maintenance costs can be assumed to be the geometric mean, of approximately £280 thousand.
The total operating costs are approximately £8 million per year,
Page 26 of 44
11 Conclusion The grinding stage of the potash production process features a single 3.6 m by 3.2 metre diameter rod mill, capable of processing 510 tonnes of slurry per day. The mill grinds the potash sylvite ore from an 80% passing size diameter of 1380 microns to an 80% passing size diameter of 620 microns, allowing the entrained potash mineral to be liberated by downstream flotation operations. The composite composite mill lining features 50 mm high chrome chrome iron lifter bars with a 70 mm thick rubber lining, minimising noise in the immediate area surrounding surrounding the mill creating c reating a better working environment for employees. The 50 000 kW AC synchronous synchronous motor provides effective control of mill speeds, and reliable, smooth startup. A humidification system and grounding of the motor reduces the risk of static charge build-up on the mill shell. The large electrical power requirement of 20MW, to drive the motor for the rod mill makes the estimated utility costs £7.7 million per year. Additional costs are associated with mill maintenance. The use of ratio control accurately controls controls the level of slurry in the mill. The grinding system has the benefits of a simple open loop grinding system due to screening of upstream process streams to recycle particles larger than 2350 microns (2.3 mm). Classification and separation into fine and coarse streams is achieved via 5 hydrocyclones, hydrocyclones, each with a diameter of 35cm.
Page 27 of 44
12 References Abulnaga, B., 2002. Slurry Systems Handbook, Ch 7-7-3 (Hydrocyclones) McGraw-Hill, ISBN: 007-137508-2, Available via Knovel Agrawal, K., 2001. Industrial Power Engineering and Applications Handbook, P. 140, Chapter 10.5, General Problems in electric motors and their remedy, Butterworth-Heinemann, Butterworth-Heinemann, ISBN: 978-0-7506-7351-8, Available through ScienceDirect Arterburn, R., A., 2010, The Sizing and Selection of Hydrocyclones, Accessed: 20/3/2011, 20/3/2011, Available at: rockservices.net/83_sizing_sel rockservices.net/83_sizing_select_cyclones.pd ect_cyclones.pdf f BBC Business News, 2011. UK Inflation rate rises to 4% in January, Accessed 20/3/2011, Available at: bbc.co.uk/news/business-1246 bbc.co.uk/news/business-12462901 2901 British Standards Institution, 1991. BS5958-2, Control of Undesirable static Electricity Cohen, H. E. 2000. Communition, Section Section 2.1, Solid - Solid Separation, Introduction, Introduction, Ullmann's Encyclopedia Encyclopedia of Industrial Chemistry, Chemistry, Available online via Wiley Online Library Couper, J. R., Penney, R. W., Fair, J. R., Walas, S. M., 2010a. Chemical Process Equipment Selection and Design - 3rd Edition, Chapter 12, Disintegration, Agglomeration, Agglomeration, and Size Separation of Particle Solids, P.365 - 374, Elsevier, Online version available at: knovel.com/web/portal/browse/d knovel.com/web/portal/browse/display?_EXT_K isplay?_EXT_KNOVEL_DISPL NOVEL_DISPLAY_bookid=2 AY_bookid=2781&Vertical 781&VerticalID=0 ID=0 Couper, J. R., Penney, R. W., Fair, J. R., Walas, S. M., 2010b. Chapter 0 - Rules of Thumb: Summary Chemical Process Equipment - Selection and Design - 3rd Edition, Elsevier, Online version available at: knovel.com/web/portal/browse/d knovel.com/web/portal/browse/display?_EXT_K isplay?_EXT_KNOVEL_DISPLA NOVEL_DISPLAY_bookid=2 Y_bookid=2781&Verti 781&VerticalID=0 calID=0 FLSmidth, 2011. Rod Mills – FLSmidth Rod mills are able to grind coarser material than ball mills, Updated: 14/1/2011, Accessed: 20/3/2011, 20/3/2011, Available at: flsmidth.com/enUS/Products/Fertilizer+ US/Products/Fertilizer+Minerals/Potas Minerals/Potash/Grinding h/Grinding/RodMills/RodMills /RodMills/RodMills GE Energy, 2010. Motors Product Line, Accessed: 20/3/2011, Available at: www.geenergy.com/prod_serv/products/motors/en/downloads/GEA17491A_MotorLineCard.pdf GE Motors, 2008. Quadramatic – Large Synchronous SAG and Ball Mill Drive Systems, Accessed: 20/3/2011, Available at: geenergy.com/prod_serv/products/motors/en/downloads/deam1043.pdf GoodQuarry, 2011. Why minimise quarry waste and quarry fines?, Accessed: 20/3/2011, Available at: goodquarry.com/art goodquarry.com/article.aspx?id=50 icle.aspx?id=50 Gupta, A., Yan, D.S., 2006. Tubular Rod Mills, Chapter 8, Mineral Processing Design and Operation, Elsevier, ISBN: 978-0-444-51636-7, Accessed: 26/2/2011, Available online through ScienceDirect.com ScienceDirect.com at: sciencedirect.com/science/book/978 sciencedirect.com/science/book/97804445163 0444516367 67 Holdich, R., 2002. Fundamentals Fundamentals of Particle Technology, Technology, Chapter 11, Crushing and Classification, Loughbrough, Midland Information Technolgy and Publishing, ISBN: 0-9543881-0-0, Available free online at: particles.org.uk particles.org.uk
Page 28 of 44
Holyfield, G. W., Brown, D. W., 1998. Application of a filter press in the recovery of potash at Boulby mine, Cleveland Potash Ltd & The University of Nottingham, Available online through ScienceDirect HSE, 1998. Health hazards from dusty cargoes during the loading and unloading of ships, HSE Information Sheet, Accessed: 13/3/2011, Available at: hse.gov.uk/pubns/dis2.pdf hse.gov.uk/pubns/dis2.pdf HSE, 2004. Workplace Exposure Limits - Calculation Methods, Health and Safety Executive, Accessed: 13/3/11, Available at: hse.gov.uk/coshh/calcmeth hse.gov.uk/coshh/calcmethods.pdf ods.pdf IChemE, 1988. A Guide to Capital Cost Estimating, 3rd Edition, IChemE, ISBN: 0-85295-220-1 InfoMine, 2011. About the mill and mine equipment cost calculator, Accessed: 20/3/11, 20/3/11, Available at: calc2007.costs.infomine.co calc2007.costs.infomine.com/about.asp m/about.aspx x Kanda, Y., Kotake N., 2007. Comminution Energy Energy and Evaluation in Fine Grinding, Chapter 12, Handbook of Powder Technology, Yamagata University, Japan, Available online through ScienceDirect Matche.com, 2007, Size Reduction Cost, Accessed: 20/3/2011, 20/3/2011, Available at: matche.com/EquipCost/SizeReduction.htm MegaIndustry, 2011. Zhengzhou Mega Industry Co. Ltd., Rod Mill, Detailed product description table, Accessed: 20/3/2011, Available at: megaindustry.en.alibaba.com/pr megaindustry.en.alibaba.com/product/293 oduct/293214379214379209812633/rod_mill.html Metso, 2010, Rod Mill, Metso Minerals, Accessed: 20/3/2011, Available at: tinyurl.com/6zj574y Metso, 2010, Mill Linings, Mill Trommels and Trunnion Linings, Metso Minerals, Accessed: 20/3/2011, Available at: tinyurl.com/64tc4zj tinyurl.com/64tc4zj Moller, J., 2003. The best of two worlds - A new concept in primary grinding wear protection, Minerals Engineering, Volume 3, Issues 1-2, Pgs. 221 – 226, Available online through ScienceDirect. Oriental Motor U.S.A. Corp, 2000. Motor Sizing – With such a wide variety, how do I pick the right one?, Accessed: 20/3/2011, Available at: charysoft c harysoftware.com/OM/WP ware.com/OM/WP-Motor-Sizing -Motor-Sizing.pdf .pdf Paul O Abbé, 2008. Ball mill Handbook, Accessed: 13/3/11, Available at: pauloabbe.com/productLines/ pauloabbe.com/productLines/millingEquip millingEquipment/ballMillH ment/ballMillHandbook.ht andbook.html ml Powell, M., Smit, I, Radziszewski, P., Cleary, P., Rattray, B., Eriksson, K-G., Schaeffer, L., 2006. The Selection and Design of Mill Liners, P. 331, Advances in Comminution Practical Action, 2010. Mineral Processing: Milling, Available through Practical Answers, Accessed: 20/3/2011, Available at: practicalaction.org/practicalan practicalaction.org/practicalanswers/product_ swers/product_info.php?pro info.php?products_id=1 ducts_id=145 45 RAEng, 2009, Statement of Ethical Principles, Accessed: 6/3/2011, Available at: raeng.org.uk/societygov/ raeng.org.uk/societygov/engineeringeth engineeringethics/pdf/Statem ics/pdf/Statement_of_Eth ent_of_Ethical_Principles.pd ical_Principles.pdf f Rowson, 2010. Potash Design Brief, Available online through University of Birmingham VLE (WebCT) Sinnott. R., Towler, G., 2009a. Chapeter 5, Valve Seclection, Pgs. 235 - 236, Coulson & Richardson’s Chemical Engineering Design, 5 th Edition, Elsevier, ISBN: 978-0-7506-8551-1
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Sinnott. R., Towler, G., 2009b. Appendix Appendix B – Corrosion Chart, Coulson & Richardson’s Chemical Engineering Design, 5th Edition, Elsevier, ISBN: 978-0-7506-8551-1 Sinnott. R., Towler, G., 2009c. Mechanical Mechanical Design of Piping Systems, Chapter 5.5, Pg. 221, Coulson & Richardson’s Chemical Engineering Design, 5 th Edition, Elsevier, ISBN: 978-0-7506978-0-75068551-1 WeirMinerals, 2007. Vulco – Wear Resistant Linings, Accessed: 20/3/11, Available at: weirminerals.com/pdf/MLS%20NA0707u%20Mill%20Linings%20bro.pdf XE.com, 2011. XE Quick Cross Rates (USD – GBP), Accessed: 20/3/2011, Available at: www.xe.com Yianatos, J., Bergh, N., Bucarey, R., Rodrı´guez J., Dı´az, F., 2005, The effect of fines recycling on industrial grinding performance, Minearls Engineering, Ch. 18, Pgs. 1110 – 1115, Available via ScienceDirect Zhang, J., 1998. Particle Technology Study Notes, Chapter 3 - Size Reduction, University of Newcastle Upon Tyne, Accessed: 26/2/2010, Available at: http://lorien.ncl.ac.uk/ming/pa http://lorien.ncl.ac.uk/ming/particle/cpe124p3 rticle/cpe124p3.html .html Zoneding, 2009. Products-Beneficiation Machine, Zhongding Heavy Duty Machine Manufacturing Manufacturing Co. Ltd., Accessed: 26/2/2010, 26/2/2010, Available at: zd-ballmill.com/proinfo/9.html zd-ballmill.com/proinfo/9.html Cover Photos:
AgroSpace, 2009. Potash Fertiliser Prices continue to go up due to increases in crop yield, Accessed: 20/3/2011, 20/3/2011, Available at: agrospace.blogspot.com/20 agrospace.blogspot.com/2009/01/pota 09/01/potash-fertilizer-pr sh-fertilizer-pricesicescontinue-to-go.html BusyTrade.com, BusyTrade.com, 2011. Grinding Steel Rod, Accessed: 20/3/2011, Available at: a452646120.en.busytrade.com/products/info/1895916/Grinding-Steel-Rod-grinding-SteelBar-rod-Mill-bar-Mill-grin.html
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13 Appendicies Appendix A - Mill Process Block Flow Diagram Recycled Brine Solution
Flow from intermediate crusher (crushed particulate solids)
Transport to tank (Conveyor)
Slurry fed under gravity
Feed Controller
Ball Mill Grinding Stage
Discharge through grate / overflow
Pumping to Flotation Cells (Coarse & Fines)
Hydrocyclones
Pumping to Hydrocyclones
Scrubbing
Mixing Tank
Recycle from Coarse Hydrocyclone
Adapted from (Zhengyuan (Zhengyuan Powder Engineering Engineering Equipment Co. Ltd, Ltd, 2010)
Page 31 of 44 of 44
Appendix B – Size Reduction Process P & ID Run of Mine Ore
E-10
E-08
E-07
FIC
FT
Liquid Discharge
Recycled Brine
E-09 V-03
Fines Flotation SC
E-04 M
FC
Classification
ST FC
E-03 FT
FT
E-06 V-02
E-05
Scrubbing V-01
E-02
E-01
Coarse Flotation
KCl Rich Stream
WasteSeperation & Disposal Process
Page 32 of 44 of 44
Appendix B – Size Reduction Process P & ID Run of Mine Ore
E-10
E-08
E-07
FIC
FT
Liquid Discharge
Recycled Brine
E-09 V-03
Fines Flotation SC
E-04 M
FC
Classification
ST FC
E-03 FT
FT
E-06 V-02
E-05
Scrubbing V-01
E-02
E-01
Coarse Flotation
KCl Rich Stream
WasteSeperation & Disposal Process
Page 32 of 44 of 44
Appendix C - Particle Size Distributions Mill Output Size / μm
Run of Mine Ore Cumulative
Size / μm
Perent ent 2000
100%
1180
94%
850
89%
600
Cumulative Percent
50000
00%
32000
75%
79%
25400
62.7%
20
60%
9400
3%
210
26%
4750
43%
100
5%
2350
39%
1180
22%
850
17%
600
12
Appendix C - Particle Size Distributions Mill Output Size / μm
Run of Mine Ore Cumulative
Size / μm
Perent ent 2000
100%
1180
94%
850
89%
600
Cumulative Percent
50000
00%
32000
75%
79%
25400
62.7%
20
60%
9400
3%
210
26%
4750
43%
100
5%
2350
39%
1180
22%
850
17%
600
12
500
10%
425
9%
300
7%
2
%
106
3%
75
%
50
2%
30
1%
10
0%
Page 33 of 44
Appendix D - Visualising Particle Size Distributions 100% e g a t n e c r e P e v i t a l u m u C
100% e g a t n e c r e P e v i t a l u m u C
80% 60% 40% 20% 0%
80% 60% 40% 20% 0%
0
10000
20000
30000
40000
50000
0
Particle Size / microns
500
1000
1500
Particle Size / microns
Figure 14 - Size Reduction Process Input Cumulative Size Distribution
Figure 15 - Mill Output Cumulative Size Distribution
100% e g a t n e c r e P e v i t a l u m u C
80% 60% 40% 20% 0% 0
500
1000
1500
2000
2500
Particle Size / microns
Figure 16 – Rod mill input Cumulative Size Distribution
Appendix E – Calculating the crusher power requirement For potash, the work index is 8.88 kWh / tonne. ( Couper, et al, 2010a) Kanda, et al gives the work index of potash as 8.05 kWh / tonne.
The feed rate is 12217 kg / day, or 510 kg / hr
The power requirement for crushing is approximately This is 535 kW.
2000
Page 34 of 44
To ensure the power is sufficient, a safety factor of 10% is applied. The design power requirement is 589 kW
Appendix F - Energy Requirement / tonne The energy required for grinding per tonne can be calculated by dividing the power requirement (or the amount of energy used in kWh in 1 hour) by the flow rate of solids. Flow rate of solids = 510 tonnes / hour Energy requirement (from appendix …) = 589 kW Energy Required per tonne =
Appendix G – Correction Factors Fact Factor or Descr Descript iptio ion n
Appl Ap plied ied?? – Yes / No
F1
Correction for dry grinding
No - Wet Grinding
F2
Correction for wet open circuit grinding in ball mills
No – Rod Mill
F3
Correction for mill diameter
Yes
Calculation
for D > 3.81 m
Where D is the internal diameter
– No Applied when:
F4
Correction for oversize feed
F5
Correction for fineness of grind
No – Only applied when 80% of product < 75 µm. (Determined from figure 18)
F6
Correction for low reduction ratio
Yes
(section 4.3.2)
D = 3.2 m
F6 is applied when outside the following range: Page 35 of 44
F7
Correction for low R in ball milling
No – Rod Mill
F8
Correction for feed preparation
Yes – Closed circuit crushing is used to prepare the feed
FT
Appendix H – Residence Time and mill Volume Total Flowrate (including saturated brine) = 24434 tonnes / day = 283 kg / s
Assume a slurry density of 1500 kg m-3
Assume a 45% charge volume in the mill. mi ll.
Total mil volumetric flow rate =
Volume required for liquid in mill
Mean residence time =
For a residence time of 108 seconds:
For a charge volume of 45%, Total internal mill volume =
Page 36 of 44
Appendix I - Using Solver to calculate the mill dimensions Mill Diameter
3.4
m
Mill Length
5.1
m
Mill X-sectional Area
8.9
m
45.0
m
Mill Volume Ratio Mill Length / Diameter
2
3
1.5
The following formulas were used in each respective cell:
Solver was used to set the ratio of mill length to diameter to 1.5, whilst changing the mill diameter.
Appendix J - Number of Mill Lifters
To find the distance these lifters are apart, the circumference is divided by the number of lifters.
Page 37 of 44
Appendix K – Rod Diameters Table … shows calculated mill diameters based on rod diameters being between 1” and 4.5” using equation ... and the data below. Only at impractical rod diameters, does the mill diameter fall within the acceptable range. Table … shows the reverse of this calculation, with mill diameters varying between 1 and 4 metres to give the rod diameter. As is shown, the calculated values do not fall within the ranges of mill and rod diameters which are used in practice. Rod Diameter mm
Mill Diameter / m
Rod Diameter / mm
inches
Mill Diameter / m
1
13.4
2.5
0.1
832
1.25
12.7
6.25
0.25
21
1.5
12.1
12.5
0.5
1.3
1.75
11.7
25
1
0.08
2
11.3
37.5
1.5
0.02
2.25
11.0
50
2
0.005
2.5
10.7
62.5
2.5
0.002
2.75
10.4
75
3
0.001
3
10.2
87.5
3.5
0.0006
3.25
10.0
100
4
0.0003
3.5
9.8
112.5
4.5
0.0002
3.75
9.6
4
9.5
Table 9
Table 10
Work Index: 8.88 kWh / tonne Criticial Speed: 0.7 d80: 1380 μm Potash density: 1.993 tonnes / m 3
Page 38 of 44
Appendix L - Calculating the number of rods required For the purposes of this calculation, only 75mm diameter rods will be used.
The number of rods is calculated by dividing the volume occupied by the individual rod volume:
Appendix M – Motor Sizing The mass of material on the outside of the mill must first be found. The internal mill diameter is known, which allows the distance from the centre axis to the mill charge to be found.
Similar calculations can be done to calculate the volume and masses of the shell and the rubber lining, shown in table ... Thickness
Volume
Shell
20mm
0.76 m3
Lining
70mm
2.5 m3
Rods
300mm
10.5 m3
Total
Dentsity
Average = 6655 kg m -3
Mass / Tonnes
6 4 82 92 tonnes
Table 11
Page 39 of 44
Assuming a time of 4 minutes from no rotation till full rotational speed:
The mill is powered through a gearing system. The low speed motor will spin between 150 and 250 rpm.
These motor sizes appear appropriate because the large AC synchronous synchronous motors available from General Electric have power ratings between 750 kW and 75 000 kW. A motor with a maximum power output of 50 000 kW is suggested for the mill ( GE Motors, 2010).
Appendix N - Hydrocyclone Pipe Diameter Method 1
This approximately 8 litres per second. For a flow-rate of 2-3 m s-1,
Upper Bound:
Lower Bound:
The pipe internal diameter must lie between 6 and 7 cm. Method 2
From Coulson & Richardson’s Richardson’s Chemical Engineering Engineering Design, Volume 6 for a stainless steel pipe with turbulent flow:
Equation 7
(Sinnott, 2009c) Page 40 of 44
Where: doptimum is the internal pipe diameter in mm
– –
G is the flowrate in kg / s 11.7 kg s-1
is the slurry density assumed to be 1500 kg m-3.
This lies within the range given by method 1 above.
Appendix O - Hydrocyclone Sizing Method adapted from Arterburn, 2010. Step 1- Materials Balance on hydrocyclone hydrocyclone
Stream
Flow-rate
Input Slurry
0.28 kg / s
0.182 m 3 s-1
Overflow (fines) – 15%
0.042 kg / s
0.027 m 3 s-1
Underflow (coarse) – 85%
0.238 kg / s
0.155 m 3 s-1
Step 2 – Calculate D80
From figure 18, in Appendix D, fines can be classified at 425 microns, as 15% of particles lie below this size. This is the specified micron size. For an efficient separation, 98.8% of particles should be smaller than the specified micron size, hence the multiplier is 0.54.
Step 3 – Calculating Correction Factors
Where Cv = Volumetric solids concentration = 35%
Where
Page 41 of 44
Step 4 – Calculating D50 (base)
– –
Hyy ’’
Calculating Hydrocyclone Diameter
From D50 (base),
Calculate Number of Units required
A 0.35 m diameter hydrocyclone has a flow-rate of 700 US gallons per minute (gpm) or 0.0441 0.0 441 m3 s-1.
Therefore 5 hydrocyclones are required to achieve the separation.
–
Calculating apex size
Total underflow per unit =
Therefore the apex size is 12.5 cm
The angle at the apex is 13 o, therefore the height of the cone can be calculated.
Appendix P – Process Specification Sheet PREP. BY CHKD. BY PROCESS SPECIFICATION
APPROVED
SOLIDS DRYING/HEATING/COOLING/MILLI DRYING/HEATING/C OOLING/MILLING NG
DATE
CLIENT
N. Rowson
ISSUE
1
PROJECT NO.
1
2
3
LOCATION
Boulby, North York Moors, UK
ITEM NO.
E - 01
NO. OFF
PLANT
PotentialAsh Potash Plant
PDF NO.
1
ELD NO.
SERVICE
Milling of Potash Rock
ISS
1018 tonnes / hr
DUTY
MATERIAL HANDLED
SELECTION / AREA
4
Grinding Circuit
SHEET 1
OF
1
Saturated Brine Sylvite Ore – See Appendix C. for composition
SPECIFIC GRAVITY
ANGLE OF REPOSE
Not required
MAXIMUM LUMP SIZE
2350 microns / 2.3cm
BULK DENSITY
Page 42 of 44
DESIGN BASIS
AVERAGE PARTICLE SIZE
Input D80 =1380 microns
273 microns
Output D80= 620 microns
RATE
CONTINUOUS/BATCH/INTERMITTENT
24,434 tonnes / day
MOISTURE/VOL Slurry ATILE COMPONENT
OPERATING RANGE
SURGE
Continuious
DESIGN
SLURRY DENSITY
1500 kg / m 3
TEMPERATURE
INLET
Ambient
OUTLET
Ambient + heat from crushing (minimal)
PRESSURE
OPERATING
Atmospheric
OUTLET
Atmospheric
FED FROM
Slurry Storage and mixing tank
Atmospheric (1 atm) atm)
DESIGN
Atmospheric + 10% = 1.1atm
MATERIALS OF CONSTRUCTION
Steel mill shell, Rubber mill lining,
DISCHARGING TO
Hydrocyclone seperators
High Chrome Iron Lifters
FIRE / EXPLOSION PROTECTION
Grounding Cables. Humidity controlled environment prevents build up of static charge on mill shell.
FUEL
Electricity for motor drive provided by Gas fired CHP Units
FLOW RATE
510 tonnes / hour of solids 1018 tonnes / hour slurry
PRINCIPAL DIMENSIONS (IF AVAILABLE)
Internal dimensions of mill: 3.6m x 3.2m diameter See Appendix … for Mechanical Drawings
MOUNTING
Steel supports Mill shell supported by bearings
SAFETY REQUIREMENTS
Static Electricity
ANCILLARY EQUIPMENT
Slurry Storage Tank & Feed Overflow discharge grate Slurry Pump Hydrocyclone
SPECIFIC REQUIREMENTS/REMARKS none
REQUIRED GUARANTEE Linings: no replacement required before 3 months (6 months is recommended) Mill Drive & Motor: Same as motor guarantee
Page 43 of 44
Appendix Q – Plant Wide P & ID Appendix R – Mechanical Drawings
Page 44 of 44
