Australia hosts the XVIIIth International Mineral Processing Congress.
The papers were presented in parallel sessions over four days, the Wednesday being a public open day for the accompanying exhibition. The conference organizers took advantage of this gap in the proceedings to organize educational symposia on the mining industry for local schoolchildren. Invitations were sent to New South Wales schools to send delegations of pupils to learn at first hand what the mining industry in Australia is all about. The major Australian mining companies laid on a series of presentations and lectures. This initiative could usefully be copied at other mineral industry meetings worldwide.
The need to educate the public on the importance of mining and its contribution to national wealth continues and there are signs that the balance of power is being redressed. Australian governments (state and federal) are less antagonistic to the industry than in the past and are beginning to take a more positive approach. A recent upset however, has been the supreme court decision in the "Mabo" case upholding the concept of "native title" to land. The decision puts into question of secure title to mining rights in many parts of the country and fresh legislation will be required to define the issue. Nevertheless, exploration is active and the next few years should see a number of major metals projects come on stream, notably MacArthur River, Centenary, and others around Mount Isa.
Investment is also going into coal, Australia's greatest mineral export both by volume and value. Many of the big Queensland open-cut mines are reaching the limits of economical stripping and going underground. The coal measures dip gently at 4-5 |degrees~ and there comes a point where open-pit mining is no longer viable.
One of the questions posed at the conference was "Why is the mineral processing field behind in the application of new technology?" There is no shortage of ideas but very few make it to the operating level. This is probably true of much purely academic research and the pace of change may seem slow to many in the industry but to the outsider progress looks substantial. The last decade has witnessed the wide-scale application of autogeneous and semi-autogeneous grinding, unprecedented advances in control and instrumentation, almost universal acceptance of column flotation, and a huge rise in the number and efficiencies of solvent-extraction operations. These are just a few of the obvious developments.
Dr. Frank Lederman, senior vice president - technology for Noranda Inc. and Richard Mozley, managing director of Richard Mozley Ltd., looked at two separate aspects of this problem in their keynote addresses.|1,2~
Lederman ascribed part of the reason to the difference in attitudes between R&D personnel and mine managers. "R&D departments have been complaining for years that they are the key to the future but are treated as just another staff function. We are treated as a function because we behave that way" he said. "We expect top business management to understand R&D when we make little effort to appreciate the subtleties of financial management. The onus is on R&D to relate to upper management."
He pointed out that mineral processing was at the center of the ore to metals chain but that because of the large volume of materials processed, incremental technology investments pay off better at the mill than elsewhere. What is needed he said, was "the proper establishment of technology strategy and more effective technology transfer."
Noranda's approach is to prioritize business needs, identify the critical technologies required, and then ask: who are the world leaders? How is the technology likely to evolve? What is, and should be, our strategic position relative to the world leader (lead, equal, follow)? And how can we change the payoff-risk balance? This approach leads to a ranked list of programs which are then subjected to a "tollgate system" of phased implementation to improve the chances of successful technology transfer. The next stage, he said, should never be started unless it is believed the transfer will be successful. In conclusion, management of technology is as or more important than inventing it.
Richard Mozley was unable to attend but sent a video presentation of his talk. He described the difficulties of launching a new product in the mineral industry and called for greater partnership between mines and companies. Innovative companies, he said, have ideas, personnel, and expertise in a particular area, but usually lack cash while mineral processors with a problem need the expertise and have the money. The self-interest of both is best served if a working partnership can be developed to solve the problem. Mozley described his own experience with the Multi-Gravity Separator (MGS), the arrangements made with Wheal Jane for its development, and proposed similar partnerships for other companies and developments.
Typically, the innovative company would put in the idea and the partner the money, management being contributed by both sides. At the conclusion the mineral processor would keep the development but leave the rights to the original idea with the innovator. Royalties would be split between both parties.
Mozley pointed out that there is a natural resistance to change in all walks of life and that it is usually only when all else fails that we seek innovation. When a crisis strikes there are three options: to do nothing, which leaves the company completely at the mercy of external forces; to opt for conventional technology at reasonable expenditure and reasonable payback, which may appear to be the safe option; or to buy novel equipment from an innovation company. In the last case the costs are the same but a much higher level of efficiency and faster payback can be expected.
However, one effect he and others have often noticed is the "Steamboat Effect", so dubbed because of the advances in sailing technology which took place when sailing ships were faced with the threat of steam power. The speed and capacity of sailing ships made unprecedented strides in the late nineteenth century until improvements in steam technology finally caught up. So it can be in the mineral industry. The crisis promotes testing of the new equipment but the general influx of new ideas surrounding the testing causes the company to improve its process anyway, thus moving the goal posts. The company then tends to turn around and say "we don't need your equipment because we are making those recoveries and grades already." The truth is that in most cases the synergistic effects of the new equipment on top of the company's own efforts often lead to even greater financial gain.
This happened to a degree with the MGS project at the Wheal Jane tin mine in Cornwall. Major improvements to conventional tin flotation were achieved but the strength of the partnership was such that the MGS was eventually used for final cleaning of tin flotation concentrates with a payback period of less than five months.
The search for more efficient comminution methods continues. Mazurkiewicz and Galecki of the University of Missouri-Rolla|3~ described their recent work with high-pressure water jets in their Double Disk Mill. A jet at 70 MPa pressure has a velocity of around 400 m/sec giving a high-energy impact which enters the tips of microcracks and thereby propagates disintegration. Most of the work was done with coal on a laboratory scale and the results show that double-disk mills have the potential to become one of the most economical coal-milling devices ever built.
The double-disk mill consists of two horizontally mounted 182-mm-dia, independently driven disks. Maximum speed tested was 4,000 rpm. The two flat-cone disks rotate with a gap between them at their outer edges. In the center of the chamber is a rotating high-pressure water nozzle. Coal is fed through a central feed pipe into the chamber and thrown out towards the gap between the disks by centrifugal force. Comminution takes place through the impact of the water jet on the packed coal, and to a lesser extent through the relative motion of the two disks. The best energy consumption on breaking 8-mesh feed coal to minus-75-microns was 62.38 kWh/st. For a given gap, energy consumed is a function of feed size, jet pressure, and disk speed.
Koivistoinen|4~ et al described the Outokumpu-type, or "Outogenius" autogeneous grinding pilot circuit at the Technical Research Center of Finland and the use of the Ball Mill Unit (BMU) concept in calculating scale-up factors.
Outokumpu autogeneous grinding is well-proven and differs from the North American practice of using large diameter to length mills. In "Outogenius Comminution" the primary mill has a dia/length ratio of 1:1.1-1:1.5, consumes 25-40% of total grinding energy, and transforms the feed into non-critical size pulp. Discharge pebbles are either crushed and recirculated or used as pebble feed for the larger secondary mill (this two-mill approach has similarities to the Boliden autogenous-grinding technique developed in Sweden). The "Outogenius" advantage is that ore feed rate can be kept much more constant since variations in hardness manifest themselves only as variations in the circulating load of critical material.
Having proved the method at full-scale, Outokumpu has built a pilot autogeneous grinding circuit to effectively designing the scale-up procedure in reverse. The 1,650-mm-dia x 1,700 mm primary mill has variable pebble ports and is controlled by measuring the power draw, weight, and volumetric load. Discharge is screened and the critical size fraction crushed in a gyratory crusher, the power draw and feed rate of which are also measured. The 1,800-mm-dia x 1,600 mm secondary mill is also controlled by power draw, weight, and volumetric loading. Additional measurements are cyclone overflow volume and on-line particle size analysis. Scale-up factors are based on the BMU which is derived from the well-known mill power equations. A grinding mill's active size in BMUs can be specified by the formula:
BMU = |D.sup.2.5~L,
where D and L are the diameter and length of the mill in meters measured inside the liners.
In the first stage of a test run, the primary mill's maximum capacity is measured by increasing feed rate gradually while recording power draw, weight, and volume filled. Maximum throughput is the highest throughput with these components constant. By dividing this throughput by the BMU a specific throughput capacity in mt/hr/BMU is obtained. Dividing maximum power draw by BMU gives specific power draw in kW/BMU. Power consumption and circulating load through the crusher enable the total kWh/mt to be calculated.
In the second stage, throughput of the secondary mill is increased gradually and power draw controlled at its maximum value by addition of pebbles screened from the primary mill. When the fineness of the cyclone overflow is in the target value, measurements are taken and similar calculations made for the secondary mill.
Examples of how these BMU-based figures are used for scale-up are given in the paper. Correlation between test results on ores from operating mines and the full-scale plants was extremely good.
Miettunen|5~ et al described process control improvements at the Pyhasalmi mine in Finland where two novel instruments have been installed: a Particle Size Indicator (PSI) and Mill Level Monitor (MLM). The PSI has proved the key to enhanced performance and the MLM has shown that even a slight rise in volumetric filling may permit a significant throughput increase.
The PSI-200 particle size indicator was on show at the IMPC exhibition. The instrument is in a sense a throw-back to the times when mill operators habitually felt the pulp with their hands to determine fineness of grind. A slurry sample fills a channel between two measuring heads, one a fixed anvil and the other a cylinder which moves up and down at a rate of approximately one cycle/sec. At every stroke the heads capture grains from the slurry between their smooth surfaces and the distance between the surfaces is measured. Statistics of at least 120 successive measurements are collected and used to calculate the particle size distribution. At Pyhasalmi, correlation between PSI readings and laboratory results were 0.84 for the plus-149-micron fraction and 0.88 for the minus-74-micron fraction.
The MLM uses the power draw oscillation caused by the mill lifters. Each lifter causes a peak in the power draw, the frequency of the oscillation depending on the number of lifter rows and the mill speed. In a fixed time interval, the oscillation peaks will change if the level of the mill charge changes. Because the method depends on the phase shift of the power draw curve, the exact time of the power peaks must be determined. Initially, the device is calibrated by measuring a reference curve of the power draw and then stopping the mill to determine the actual volumetric filling. Continuous measurements are then started and the average power curve of the last 10 revolutions compared with the reference curve to determine the possible phase shift and hence change in volumetric filling.
Norrgran and Merwin of Eriez Magnetics discussed advances in magnetic separation techniques made possible by rare-earth permanent magnets.|6~ Neodymium-boron-iron magnets have reached energy levels approaching 40 MGOe (Mega Gauss-Oersteds) and have led to the development of high-intensity magnetic separators which operate virtually energy free. The Rare-Earth Drum magnetic separator has a peak magnetic field intensity of approximately 7,000 Gauss which is sufficient for removing many paramagnetic materials. Additional advantages are low capital and operating costs compared with electromagnetic separators, and the ability to treat a much wider size range of particles compared with cross-belt or induced magnetic roll separators.
The Rare-Earth Drum consists of a stationary shaft-mounted magnetic circuit completely enclosed by a rotating drum. The magnetic circuit consists of segments of alternating rare-earth magnets and steel pole pieces which span an arc of 120 |degrees~. Ore is fed directly on to the drum. The 15-in.-dia drum can be up to 60-in.-wide and in one particular example a precious-metal bearing mafic rock of 75-mm top size is being treated at 25 st/hr/ft drum width. Up to 5 st/hr/ft drum width is typical for minus-65-mesh industrial minerals.
High temperatures have an adverse effect on the field strength of permanent magnets, significant decreases being experienced at temperatures up to 100 |degrees~ C. However recently, temperature stable Ne-B-Fe magnets have been developed which show only a slight decrease in magnetic field strength up to 275 |degrees~ C.
The use of mercury for amalgamating gold concentrates is an environmental hazard. Russian scientists reported on a process for recovering gold from gravity concentrates which eliminates the hazard and increases gold recovery.|7~ Two stages of standard magnetic separation are followed by final clean-up in a Magnetic Fluid Separator. The process is in commercial use at nine plants in Siberia with overall gold recovery of 98.6% compared with 96.6% with amalgamation.
In the first stage an MSZ-1 separator with a magnetic field of less than 0.26T (Tesla) recovers magnetite and other strongly magnetic materials. In the second stage an SSM-1 separator with a magnetic field of 2.0-2.6T removes medium and weakly magnetic materials. The non-magnetic fraction of the second stage is fed to a SMZh-1 magnetic fluid separator which recovers liberated gold. Gold recoveries were 99.5% at feed rates of up to 100 kg/hr, 60 kg/hr, and 12 kg/hr for each stage respectively.
A novel method of cleaning iron ore by combining flotation and magnetic separation in a single flotation cell was described by Yalcin.|8~ Magnetic particles are prevented from reporting to the froth while non-magnetic particles are floated. The advantage of magnetoflotation is that it can produce a final concentrate in a single step. Laboratory tests on a 32.9% Fe ore, of which 87.6% was magnetic, produced a final concentrate assaying 69.1% Fe at an overall recovery of 84.8% and magnetic iron recovery of 96.4%. At least four cleaning steps were required to achieve similar results using conventional magnetic separation and flotation. Producing a high-grade concentrate in a single step by magnetoflotation should lead to a simplification of plant flowsheets with the consequent benefits of lower capital and operating costs.
The laboratory magnetoflotation cell has a large flotation compartment and a small froth-product compartment. Between the two is a cylinder containing rotating permanent magnets. Froth from the flotation compartment flows over a weir onto the cylinder and any entrained magnetics are swept back towards the flotation compartment by the rotating magnets.
Numerous papers were devoted to column flotation. Kawatra and Eisele discussed the advantages of baffled columns for coal flotation.|9~ The baffles consisted of a series of horizontal plates with open area of 30-40%. Improved washing efficiency is achieved by making the flow patterns in the column more closely approach plug flow and better selectivity obtained through reduced back-mixing and entrainment. Tests were deliberately run on poor quality coals. On a coal containing 38% ash and 3.23% total sulphur, the baffled column recovered 84% of combustibles while rejecting 84% of the ash and 54% of the pyritic sulphur. Without baffles, the column recovered 88% of combustibles while rejecting only 76% of the ash and 45% of the pyritic sulphur.
A novel design of flotation cell also involving packing is the Sheet Flotation Cell proposed by Meloy and Williams of West Virginia University.|12~ This is a two-dimensional column cell using a sheet of bubbles rising through packing contained between two vertical walls. Like a shaking table, a sheet flotation cell produces continuous multiple products. Feed enters at the lower left and product exits along the top and right hand edges. High-grade products exit at the top left, middlings at the top right, and tailings at the bottom right. The vertical motion is provided by the stream of bubbles and the horizontal motion by the slanted packing.
Lakefield Research conducts dozens of column flotation tests every year and Huls and Williams produced a welcome discussion on their limitations.|10~ Column cells are not a panacea in all cases. The authors stress the importance of testing column cells in an in-circuit configuration.
Whenever entrainment is a minor issue compared with particle locking, column flotation will not improve concentrate grades. Columns may therefore not be desirable in scavenger-cleaner applications. Pulp viscosity is important in reducing entrainment. Viscous pulps lead to very poor draining froths. Selectivity can also be a factor. An example of the flotation of sphalerite from pyrrhotite is given where acceptable results were obtained in the laboratory cell but pilot testing of columns in two different circuit configurations gave poor results due to the build-up of a circulating load of pyrrhotite. A conventional arrangement of three stages of mechanical cleaners proved superior.
A particularly dramatic example of Lakefield's experience with right or wrong locations for columns in a circuit was the reverse flotation of quartz from hematite. Using a column as a final cleaner as normally practiced resulted in only a very minor increase in iron concentrate grade. It proved much more satisfactory to use the column for a pre-float to produce a silica concentrate sufficiently low in hematite that could be discarded.
Rubinstein and Gerasimenko described column flotation machines developed in Russia.|11~ These complex designs allow tailings to repeatedly return to the collection zone, employ alternate co- and counter-current slurry and air flows, and are supplied with pneumohydraulic and stepped airlift spargers to control bubble size. Advantages claimed are the ability to combine different flotation stages in a single unit, improved metallurgy, and reduced power consumption.
The recovery of silver from zinc plant residues has long been problematic. Eropkin et al of Russia's Mekhanobr Institute described a new process capable of silver recoveries from residues in excess of 90%.|13~ In the zinc cake studied the main silver-bearing phase was copper sulphides present as less than 3-micron sized edgings on zinc silicate and oxide grains. Standard flotation gave very poor recoveries so to help liberate the sulphides the pulp was preconditioned with sodium chloride at 90 |degrees~ C. During the experiments an interesting phenomenon was noted. After conditioning in a glass or enameled vessel flotation results were worse than with the initial cakes. But if conditioning took place in an aluminum vessel both recoveries and grades were incomparably greater.
It appears that during conditioning oxidation of the sulphides proceeds and metal ions report to solution. Despite the negligible solubility of silver chloride, silver ions immediately react with the aluminum and precipitate on it, effectively a cementation reaction. Due to the constant removal of silver and other metal ions from solution, decomposition of the solid sulphides proceeds to completion. The experiments were repeated using aluminum powder in conditioning and a silver concentrate of 2,600 g/mt at a recovery of 70% was made from an initial cake assaying 200 g/mt silver.
A variation of the technology was then tried. The silver was first leached from the cake. The leach removed about 40% of the silver which was then precipitated by zinc dust to a product assaying about 2% silver. The residue was then subjected to the above process to produce a concentrate assaying up to 1,200 g/mt silver. Overall silver recovery using the combined process was up to 94%.
Apart from the magnetic fluid method of recovering gold from gravity concentrates mentioned above|7~, Russia's Irgiredmet Institute has developed specific leaching equipment.|14~ Cyanide leaching is undertaken in an inverted cone with solution fed at the apex to provide segregation of gold grains in an upward flow. Coarse grains concentrate at the bottom and are washed by the highest velocity solution. Dissolution of both large and small grains therefore terminates approximately simultaneously. Flow rate is chosen so that a clarified layer of solution is always present at the top. Part of the pregnant is recirculated and the rest transferred to the stripping section.
At one mine a 2 mt/d plant using twin 2-|m.sup.3~ leaching cones is in operation. Leaching is performed in two stages with 3-5 g/l cyanide. Each stage lasts 12-18 hr and recovery of gold into a pregnant solution assaying 500-1,000 g/mt gold is 94-98%. Leach residue is returned to the milling circuit.
Henkel Corp. in conjunction with Mintek of South Africa reported on the development of a new range of solid and liquid ion-exchange extractants for gold based on guanadine.|15~ The new LIX 79 is a strong extractant for auricyanide up to pH 11.5 and can be readily stripped at pH 14 using 1M sodium hydroxide. The pregnant strip solution produced by the use of LIX 79 or resins based on it are suited for gold recovery by conventional electrowinning processes.
The kinetics of extraction are extremely fast, typically 98% stage efficiency with a mixer retention time of 2 min in a conventional SX plant. The guanadine-based resin extracts gold faster than fresh carbon and significantly faster than a typical regenerated carbon from an operating plant. Selectivity for gold over other metal-cyanide complexes, particularly copper, is excellent. Furthermore, loading capacities of guanadine-based resins are significantly greater than those of commercially available weak-base and strong-base resins, and equal to or greater than that of activated carbon.
The authors point out that there are significant advantages of solvent extraction over solid ion-exchange systems. The main disadvantage is the need for leach solutions to contain less than 50 ppm solids. The greatest potential for the use of the liquid extractant is seen in heap-leach solutions, particularly those carrying significant quantities of copper.
Corrans et al gave a paper on the Activox process for sulphide concentrates, which based on the limited data available to date, could be an economic alternative to pressure oxidation or roasting.|16~ Concentrates are finely milled to activate the sulphide minerals. Grinding to 80% finer than 15 microns is usually necessary and power consumptions range from 80-180 kWh/mt feed. Various types of stirred ball mills have been found suitable. The finely ground concentrates are then oxidized in an autoclave at a temperature of 80-100 |degrees~ C and under an oxygen overpressure of 400-800 kPa. Retention times range from 0.5-4 hr.
The acidic solutions produced can be treated by various conventional methods. For example, copper can be recovered directly by solvent extraction and electrowinning. Nickel and cobalt can be recovered by raising the pH and adding ammonia to convert the cations to ammine complexes. These can then be extracted with organic solvents and precipitated from pure solutions. Gold can be recovered as from a pressure oxidation product. A 5 kg/hr pilot plant has been installed at AMMTEC's laboratories in Perth and examples of the above extractions are given. Capital and operating costs of the Activox process are estimated at 60-80% of those for pressure oxidation.
1. "Why is the Mineral Processing Field Behind in the Application of New Technology?" F.L. Lederman, Snr. VP-Technology, Noranda Inc.
2. "Resistance to the Introduction and Testing of New Technology in Mineral Processing" R. Mozley, Richard Mozley Ltd.
3. "Coal and Minerals Comminution with High Pressure Water Jet Assistance" M. Mazurkiewicz & G. Galecki
4. "Autogeneous Grinding and Design of Comminution Circuits" P. Koivistoinen et al.
5. "New Methods for the Control of Autogeneous Grinding Circuits" J. Miettunen et al.
6. "Industrial Applications of the High Intensity Rare-Earth Drum Magnetic Separator" D.A. Norrgran and R.A. Merwin.
7. "Commercial Equipment for the Recovery of Gold from Gravity Concentrates using Magnetic Fluids" R.D. Smolkin et al.
8. "Iron Ore Concentration by Magnetoflotation" T. Yalcin
9. "The Use of Horizontal Baffles to Improve Column Flotation of Coal" S.K. Kawatra & T.C. Eisele
10. "Limitations in the Application of Column Flotation" B.J. Huls & S.R. Williams
11. "Design, Simulation, and Operation of a New Generation of Column Flotation Machines" J. Rubinstein & M.P. Gerasimenko
12. "Sheet Flotation - Two Dimensional Packed Columns" T. Meloy & M. Williams.
13. "Silver Recovery from Zinc Cakes Obtained as By-Products in Metallurgical Zinc Plants" J.I. Eropkin et al.
14. "New Technology and Equipment for Gold and Silver Leaching from Gravity Concentrates" V.M. Mullov & G.B. Rashkowski.
15. "New Solid and Liquid Ion Exchange Extractants for Gold" G.A. Kordosky et al.
16. "The Recovery of Nickel and Gold from Sulphide Concentrates" I.J. Corrans et al.
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|Author:||Suttill, Keith R.|
|Publication:||E&MJ - Engineering & Mining Journal|
|Date:||Aug 1, 1993|
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