By Ramoutar (Ken) Seecharran (MEIZ) Plant Superintendent, Superintendent, Frontier Sprl, Sprl, DR Congo
FIRST QUANTUM’s ACTIVITIES
PLACEFIX MAP SHOWING FRONTIER MINE
BRIEF HISTORY OF FRONTIER MINE
The deposit was known to exist, but not fully quantified The Japanese had shown interest during the Gecamine era but depressed copper price and high project cost curtailed project implementation First Quantum was operating the Lonshi mine to the south east of this deposit, in the Pedicle, so logistically, it was easy for us to carry out extensive drilling Once quantified, the board was committed to our development of copper deposits in this part of Africa, and approved the project The project was implemented at a cost of $US226m
For the valuable mineral(s) of an ore to be economically separated from the waste, host rock (gangue), the ore has to be broken down to a small enough enough size (comminu (comminuted), ted), to “liberate” “liberate” it (them) from from the rock matrix. Comminution represents the most expensive part of a plant’s circuit. At this point, we minerals engineers use our knowledge of the properties of the various minerals to exploit noticeable differences in those properties, between the valuable mineral(s) and their gangue counterparts. These differences in properties could be: • Diffe Difference rence in speci specific fic gravi gravity ty • Diffe Difference rence in solubilit solubility y in a suita suitable ble dissolvin dissolving g chemical chemical • Diffe Difference rence in in magnetic magnetic suscepti susceptibili bility ty or, • Diffe Difference rence in surfac surface e properti properties es BEFORE THESE DIFFERENCES CAN BE EXPLOITED, WE MUST PRESENT THE MINERALS TO THE SEPARATION STAGE IN AS UNIFORM A SIZE AS POSSIBLE, IDEALLY, ALL OF THE SAME GRAIN SIZE….OR AS NEAR TO THE SAME SIZE AS POSSIBLE
So once broken, the grains of minerals in the ore must be sized. Those which meet this size requirement progress to the separation stage, those failing, are given another chance to be broken. Therefore, the ideal sizing machine should be some form of a screen, however, the huge tonnages we treat these days will not make screening feasible. Hydrocyclones are used as a practical compromise. A hydrocyclone utilises drag force to remove the fines through the vortex finderr in the middle of the cone finde cone while the coarser coarser particle particles s are acted upon by centrifugal force and forced outwards towards the wall. They then migrate downwards under gravity and get another chance to be broken.
THE NEED FOR COMMINUTION
POLISHED SECTIONS OF FRONTIER’s ORE
FREE CuFeS2
LOCKED CuFeS2
POLISHED SECTIONS OF CONGOLESE/ZAMBIAN Cu ORES
Traditionally, comminution in a concentrator plant was carried out in a few well defined steps, namely: • Prima Primary ry crushing… crushing….under .underground ground or on on surface surface • Secon Secondary dary crushing… crushing….on .on surface surface in the concentrator concentrator plant • Tertiary crushing….in crushing….in the concentrator plant, and in closed closed circuit circuit with a screen • Ball milli milling…. ng….in in the concentr concentrator ator plant plant • Befor Before e the refinement refinement of tertiary crushing, crushing, this this function function was performed performed by rod milling. Rod mills became unfashionable in the early 1960’s as crusher bowl and mantle profile research r esearch enabled ore to be crushed successfully to a size below 10mm without causing metal to metal contact between the crushing faces. • Crushi Crushing ng and screening screening plants had numerous numerous operationa operationall problems problems such as Bogged crushers, Blinded screens and Blocked chutes. A phenomenon we plant operators called the three B’s!
FRONTIER’s OPEN PIT
In the past three decades, high maintenance costs, coupled with ever rising labour costs, led to the development of Autogenous (AG) and Semi-autogenous (SAG) mills. Such mills performed the functions of secondary and tertiary crushers and rod mills….in a single unit. Here, we utilise the ore to break itself, as opposed to squeezing it between metal plates, or hammering it with solid steel rods. It is obvious that rock-on-rock crushing is more efficient. The SAG mill is assisted in carrying out breakage by the addition of a small amount of large diameter, >100mm steel balls. Today, one will be considered to be an anachronism if one is to design a concentrator plant and not consider AG or SAG milling. HOWEVER, SUCH MILLS DO POSE TECHNICAL CHALLENGES TO PLANT OPERATORS.
This presentation addresses one such challenge.
PROCESS DESCRIPTION AT FRONTIER
• Run of mine mine ore is primary primary crushed crushed and stockp stockpiled iled • Crushe Crushed d ore at a size of 80% 80% smaller smaller than 112mm 112mm is fed fed to a Semi Autogenous Grinding (SAG) mill • SAG mill product is fed to a cluster of hydrocyclones (sizing device) device) where the fine product progresses to the separation stage and the coarse fraction is routed to a ball mill and given a second second chance to be broken, the ball mill product is fed to the same cluster of hydrocyclones, in order to close the circuit • Separation of valuable mineral mineral from waste waste is carried carried out using using froth flotation flotation • The copper minerals recovered recovered is thickened thickened and filtered to produce a solid solid cake containing about 9.5% moisture • The waste waste rejected rejected (tail (tailings) ings) is thicken thickened ed and disp disposed osed of of in a respo responsibl nsible e manner in a tailings impoundment • All water water used in the the plant is recycl recycled ed in the process process
GYRATORY CRUSHER AT FRONTIER
CRUSHED ORE STOCKPILE
SAG and BALL MILLS AT FRONTIER
INSIDE THE SAG MILL
SEPARATION UTILISING FROTH FLOTATION
People being exposed to SAG milling for the first time often underestimate the crushing forces present in these mills. They are normally of >32 feet in diameter. So the crushing force at the toe of the crop load, the cataracting end, is phenomenal. Some studies suggest pressures as high as 20 bars are present. Due to the shortage of suitable grinding media in Sub-Saharan Africa, we were forced to start our plant with sub-standard grinding media in October 2007. Time was of the essence as we were faced with an entirely new work force and on-the-job training had to be expeditiously carried out.
POROUS CAST BALL- INVITING DISASTER
POROUS BALLS AFTER 20 Hrs OPERATION
A WEEK LATER!!!!
EVEN THE BETTER QUALITY CAST BALLS HAD THEIR OWN PROBLEMS ….caused by the intense pressure in the SAG mill
BROKEN BALLS ARE DETRIMENTAL TO SAG MILLING OPERATIONS
They take up useful space that could be occupied by ore. Being streamlined, they do not get flushed out easily They are too light to have enough kinetic energy to break rock They add to power consumption, by their weight and being pinned to the mill shell, shell, they they get carried carried up to a point beyond which which a whole ball leaves the shell on its trajectory, thus adding more load to the motor They add to steel consumption, Fe ions are detrimental to flotation. Sulphide minerals get depressed When hammered against the discharge grate, they cause peening. Peening reduce the flow through a mill They act as a cushion against which rocks are hammered by balls, so breakage is less effective
Cast balls are not suitable for a SAG mill The outer crust is hard, typically in excess of Brinell hardness 450. This quickly diminishes towards the centre of the ball. It is for this reason, when you open a SAG mill, in which cast balls were fed, you see balls of predominantly one size.
DATA I COMPILED FOR ZCCM IN 1989 under a different incarnation BRINELL HARDNESS ACROSS BALL
500 500 450 450 400 400 350 350 300 300 250 250 200 200 150 150 100 100 50 0 1
2
3
4
5
6
7
8
PICTURE FROM INSIDE KANSANSHI’s SAG MILL NOTE BALLS OF ONE SIZE
EFFECTS OF SOFT CENTRE OF A CAST BALL uneven wear
EFFECTS OF SOFT CENTRE OF A CAST BALL dimpled ball
WHAT WAS THE SOLUTION?.....FORGED BALLS The bar from which a forged ball is made is cut into shots, reheated and hammered at least 30 times. Compacting and hardening it. The initial hot rolling of the bar followed by rapid cooling, reheating and subsequent hammering, also changes the microstructure from one resembling pearlitic to that closer to martensitic. The steel’s grain structure aligns and stretches in the forging process, thus creating a stronger, more compact steel. The balls are then quenched and reheated for tempering and stress relieving, respectively. We therefore then get a ball which is of uniform hardness hardness and weight. weigh t. A ball which can can withstand withstand the harsh harsh conditions conditions in a SAG mill mill.. The difference in weight between a well forged ball and an excellent cast counterpart could be about 9% Here networking helped us. ECLIPSE foundry in South Africa was helpful. I knew Humphrey, their Zambian representative for many years, and studied at university in the UK with Jacob, their ex MD in South Africa.
PHOTOMICROGRAPHS OF STEEL
PEARLITIC
MARTENSITIC
ECLIPSE FOUNDRY’s FORGED BALL
Although proved to be successful, the volume of forged balls we required, ECLIPSE was unable to supply.
So, armed with valuable information gathered from the ECLIPSE trial, we went to the European Union, from where our sister plant in Mauritania was sourcing their SAG mill balls.
Long lead time and transport logistics remain a nightmare.
I was informed that ECLIPSE has plans to expand their production capacity, we welcome such a symbiotic move.
FORGED MILL BALLS FROM SPAIN
HAND FORGING IN CHINA
MILLING INFORMATION SHOWN ON THE COMPUTER
EMPHASIS ON CENTRE HARDNESS
AERIAL VIEW OF THE PLANT
FINISHED PRODUCTION
CURRENT MILL BALL PROFILE
BENEFITS
• We have found found that throughput throughput has increa increased, sed, as grinding grinding is now more efficient • The quality quality of the ground ground ore has has improved improved • Steel consum consumption ption has has decreased, decreased, due due to less ball breakage and a more competent ball • Grind Grinding ing cost has decreased decreased,, steel consumpti consumption on dropped from about 550 g/tonne of ore milled to about 450. It is currently running at about $US 1.15 per tonne of ore milled we are confident that this will reduce r educe as the circuit is optimised
MILL BALLS WORLDWIDE
HARDNESS TESTING
BRINELL - A hardened BRINELL hardened ball of specified specified diamet diameter er is impressed impressed into the material’s surface, at a known pressure. The Brinell hardness number results from calculations involving the load and the spherical area of the ball’s impression.
ROCKWELL – This method ROCKWELL method involv involves es measuring measuring the the depth a hardened steel ball or a diamond penetrates into the material being tested, under controlled loading.
STEEL MANUFACTURING PROCESS microstructure
STEEL MANUFACTURING
HARDNESS OF MARTENSITE WITH CARBON CONTENT
FRONTIER’s FLOWSHEET
BEST EVER DAY’s RESULT
FIRST QUANTUM’s PRODUCTION PROFILE
15 YEAR COPPER PRICE
OROGENESIS, MINERALS FORMATION
Cu PRICE IN JAN 2006 cents
NORMAL RUNNING and OVERLOADED MILL
PEENING OF DISCHARGE GRATES
HYDROCYCLONES
HYDROCYCLONE OPERATION
