Dense medium separation(DMS)
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Dense medium separation (DMS), also refereed to as heavy medium separation (HMS) is applied to the pre-concentration of minerals, i.e. the rejection of gangue prior to grinding for final fin al liberation. It is also used in coal preparation to produce a commercially graded end-product, clean coal being separated from the heavier shale or high-ash coal. Heavy liquids of suitable density are used, so that those minerals lighter than the liquid float, while those denser than it sink. A thick suspension, or pulp,of some heavy solid in water is used in industrial separations since most of the liquids used in the laboratory are expensive and/or toxic. The process is most widely applied when the density difference occurs at a coarse particle size, as separation efficiency decreases with size due to the slower rate of settling of the particles.
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Dense medium separation (DMS), also refereed to as heavy medium separation (HMS) is applied to the pre-concentration of minerals, i.e. the rejection of gangue prior to grinding for final fin al liberation. It is also used in coal preparation to produce a commercially graded end-product, clean coal being separated from the heavier shale or high-ash coal. Heavy liquids of suitable density are used, so that those minerals lighter than the liquid float, while those denser than it sink. A thick suspension, or pulp,of some heavy solid in water is used in industrial separations since most of the liquids used in the laboratory are expensive and/or toxic. The process is most widely applied when the density difference occurs at a coarse particle size, as separation efficiency decreases with size due to the slower rate of settling of the particles.
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Any substance used for media must have the following characteristics:
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Hardne Hardness: ss: It must must not not easi easily ly brea break k down down or or abrad abradee into into a slime slime unde underr working conditions.
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Chemi Chemical cal Stab Stabili ility: ty: It It must must not be chem chemica ically lly corr corrosi osive ve or or liable liable to to react react with the ore minerals undergoing treatment.
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Slow Slow settle settleme ment nt at reaso reasonab nable le visc viscosi osity: ty: It must must form form a fairly fairly stab stable le pulp pulp without having to be ground very fine, otherwise the medium will be too viscous.
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Specif Specific ic grav gravity ity:: It must must have have high high enou enough gh spec specifi ificc gravi gravity ty to to give give the required bath density at low % solids, again to minimise the viscosity. viscosity.
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Regene Regenerat ration ion:: The dens densee medi mediaa must must be be easy easy to clean clean for for recy recycli cling. ng. Losses of up to 0.5 kg/t could lead to high operating costs.
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Price Price and and avail availabi abilit lity: y: The soli solid d should should be readi readily ly avai availab lable le and and cheap cheap.
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Separating vessels may be classified into gravitational ("static-baths") and centrifugal (dynamic) vessels depending on the separation forces employed.
Gravitational units comprise some form of vessel into which the feed and medium are introduced and the floats are removed by paddles, or merely by overflow. The Wemco cone separ ator is widely used for ore treatment, since it has a relatively high sinks capacity. The cone, which has a diameter of up to 6 m, accommodates feed particles of up to 10 cm in diameter, with capacities of up to 500 tph. The feed is introduced on to the surface of the medium by free-fall, which allows it to plunge several centimetres into the medium. Gentle agitation by rakes mounted on the central shaft helps keep the medium in suspension. The float fraction simply overflows a weir, whilst the sinks are removed by pump or by external or internal air lift.
Wemco cone separator. (a) Singl e-gravity, two-pr oduct system wi th tor que-fl ow-pump sink r emoval; (b) Si ngl e-gravity, two-pr oduct system with compressed-air sin k removal
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Dr um separator s are built in
several sizes, up to 4.3 m diameter by 6 m long, with maximum capacities of 450 tph, and can treat feeds of up to 30cm in diameter. •
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Separation is accomplished by the continuous removal of the sink product through the action of lifters fixed to the inside of the rotating drum. The lifters empty into the sink launder when they pass the horizontal position.
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The float product overflows a weir at the opposite end of the drum from the feed chute. Longitudinal partitions separate the float surface from the sink-discharge action of the revolving lifters.
Dr um separator : (a) side view, (b) end vi ew
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The Dr ewboy bath separator is used widely in the cleaning of coal. The coal is fed into the bath at one end and the floats scraped from the opposite end while the sinks are lifted out from the bottom of the bath by the vanes of a slowly revolving inclined wheel. The medium is fed into the bath at two points - at the bottom of the vessel and with the raw coal- the proportion
being controlled by valves. •
The Drewboy bath has a high floats capacity and handles a feed from 12.7-600 mm at up to 820 t/h for a 4 m diameter bath.
Dr ewboy bath
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In the Norwalt washer raw coal is introduced into the centre of the annular separating vessel, which is provided with stirring arms. The floats are carried round by the stirrers, and are discharged over a weir on the other side of the vessel, being carried out of the vessel by the medium flow. The discard sinks to the bottom of the vessel and is dragged along by scrapers
attached to the bottom of the stirring arms, and is discharged via a hole in the bottom of the bath into a sealed elevator, either of the wheel or bucket type, which continuously removes the sinks product.
Norwalt washer
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These separators provide a high centrifugal force and a low viscosity in the medium, enabling much finer separations to be achieved than in gravitational separators. Feed to these devices is typically de-slimed at about 0.5 mm, to avoid contamination of the medium with slimes, and to minimise medium consumption. The most widely used centrifugal DM separator is the cyclone whose principle of operation is similar to that of the conventional hydrocyclone. The largest cyclones now exceed 1 m in diameter and are capable of throughputs in coal preparation of over 250 t/h. The ore or coal is suspended in the medium and introduced tangentially to the cyclone either via a pump or it is gravity-fed. The dense material is centrifuged to the cyclone wall and exits at the apex. The light product "floats" to the flow around the axis and exits via the vortex finder.
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The Vorsyl separator is used in coal-preparation plants for the treatment of small coal sizes up to about 50 mm at feed rates of up to 120 tph. The feed to the separator, consisting of de-slimed raw coal, together with the separating medium of magnetite, is introduced tangentially the top of the separating chamber, under pressure. Material of specific gravity less than that of the medium passes into the clean coal outlet via the vortex finder, while the near gravity material and the heavier shale particles move to the wall of the
vessel due to the centrifugal acceleration induced. •
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The particles move in a spiral path down the chamber towards the base of the vessel where the drag caused by the proximity of the orifice plate reduces the tangential velocity and creates a strong inward flow towards the throat. This carries the shale, and near gravity material, through zones of high centrifugal force, where a final precise separation is achieved.
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The shale, and a proportion of the medium, discharge through the throat into the shallow shale chamber, which is provided with a tangential outlet, and is connected by a short duct to a second shallow chamber known as the vortex tractor. This is also a cylindrical vessel with a tangential inlet for the medium and reject and an axial outlet. An inward spiral flow to the outlet is induced, which dissipates the inlet pressure energy and permits the use of a large outlet nozzle without the passing of an excessive quantity of medium. Vorsyl separator
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The L ARCODE M S (L arge Coal D ense M edium Separator) was developed to treat a wide size range of coal (-100mm) at high capacity in one vessel. It has also been used in concentrating iron ore. It consists of a cylindrical chamber which is inclined at approximately 30 o to the horizontal. Feed medium at the required relative density is introduced under pressure, either by pump or static head, into the involute tangential inlet at the lower end. At the top end of the vessel is another involute tangential outlet connected to the vortex tractor. Raw coal of 0.5-100mm is fed into the separator by a chute connected to the top end, the clean coal after separation being removed through the bottom outlet. High relative density particles pass rapidly to the separator wall and are removed through the top involute outlet and the vortex tractor.
LARCODEMS
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Laboratory testing may be performed on ores in order to assess the suitability of dense medium separation and other gravity methods, and to determine the economic separating density. Liquids covering a range of densities in incremental steps are prepared, and the representative sample of crushed ore is introduced into the liquid of highest density. The floats product is removed and washed and placed in the liquid of next lower density, whose float product is then transferred to the next lower density and so on. The sinks product is finally drained, washed, and dried, and then weighed, together with the final floats product, to give the density distribution of the sample by weight. After assaying the fractions for metal content, the distribution of material and metal in the density fractions of the sample can be tabulated.
Schematic of Heavy liquid testing
Heavy liquid test results performed on a tin ore.
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From columns 3 and 6 of the table, if a separation density of 2.75 was chosen, then 68.48% of the material, being lighter than 2.75, would be discarded as a float product, and only 3.81% of the tin would be lost in this fraction. Similarly, 96.19% of the tin would be recovered into the sink product, which accounts for 31.52% of the original total feed weight. The smaller throughput will lower grinding and concentration operating costs, the impact on grinding energy and steel costs often being particularly high. Against these savings, the cost of operating the DMS plant and the effect of losing 3.81% of the run-of-mine tin to floats must be considered. The amount of recoverable tin in this fraction has to be estimated, together with its subsequent loss in smelter revenue. If this loss is lower than the saving in overall milling costs, then DMS is economic. The optimum density is that which maximises the difference between overall reduction in milling costs per tonne of run-of-mine ore and loss in smelter revenue.
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In coal preparation, heavy liquid tests are important in order to determine the required density of separation and the expected yield of coal of the required ash content. Since coal is lighter than the contained minerals, the higher the density of separation the higher is the yield. Results of heavy liquid tests performed on a coal sample are shown in the following table. The coal was separated into the density fractions shown in column 1, and the weight fractions and ash contents are tabulated in columns 2 and 3 respectively. The weight per cent of each product is multiplied by the ash content to give the ash product (column 4). The total floats and sinks products at the various separating densities shown in column 5 are tabulated in columns 6-11.
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To obtain the cumulative per cent for each gravity fraction, columns 2 and 4 are cumulated from top to bottom to give columns 6 and 7 respectively. Column 7 is then divided by column 6 to obtain the cumulative per cent ash (column 8). Cumulative sink ash is obtained in essentially the same manner, except that columns 2 and 4 are cumulated from bottom to top to give columns 9 and 10 respectively. The results of the table are plotted in (cumulative ash % float vs. cumulative % float yield, and specific gravity vs. cumulative % floats yield) to give washability curves. Suppose an ash content of 12% is required in the coal product. It can be seen from the washability curves that such a coal would be produced at a yield of 55% (cumulative per cent floats), and the required density of separation is 1.465.
Results of heavy liquid tests performed on a coal sample.
Washability curves for coal
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The efficiency of separation can be represented by the slope of a Partition or Tromp curve. It describes the separating efficiency for the separator whatever the quality of the feed and can be used for estimation of performance and comparison between separators. The partition curve relates the partition coefficient or partition number, i.e. the percentage of the feed material of a particular specific gravity which reports to either the sinks product (generally used for minerals) or the floats product (generally used for coal), to specific gravity. It is similar to the classification efficiency curve, in which the partition coefficient is plotted against size rather than specific gravity.
Partition curve
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The ideal partition curve reflects a perfect separation in which all particles having a density higher than the separating density report to sinks, and those lighter report to floats. The area between the two curves is called the "error area" and is a measure of the degree of misplacement of particles to the wrong product. Many partition curves give a reasonable straightline relationship between the distribution of 25 and 75%, and the slope of the line between these distributions is used to show the efficiency of the process. The probable error of separation or the Ecart probable ( ) is defined as half the difference between the density where 75% is recovered to sinks and that at which 25% is recovered to sinks: =
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The density at which 50% of the particles report to sinks is shown as the effective density of separation, which may not be exactly the same as the medium density.
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The lower the , the nearer to vertical is the line between 25 and 75% and the more efficient is the separation. An ideal separation has a vertical line with an = 0 whereas in practice the usually lies in the range 0.01-0.10.
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The partition curve for an operating dense medium vessel can be determined by sampling the sink and float products and performing heavy liquid tests to determine the amount of material in each density fraction. The results of heavy liquid tests on samples of floats and sinks from a vessel separating coal (floats) from shale (sinks) are shown below:
Coal-shale separation evaluation
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Columns 1 and 2 are the results of laboratory tests on the float and sink products and columns 3 and 4 relate these results to the total distribution of the feed material to floats and sinks which must be determined by weighing the products over a period of time. The weight fraction in columns 3 and 4 can be added together to produce the reconstituted feed weight distribution in each density fraction (column 5). Column 6 gives the nominal specific gravity of each density range, i.e. material in the density range 1.30-1.40 is assumed to have a specific gravity lying midway between these densities - 1.35. The partition coefficient (column 7) is the percentage of feed material of a certain nominal specific gravity which reports to sinks, i.e. 4 5
∗ 100%
