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By R. J. Rice, K. F. Denning

Effect of iron on the removal of arsenic, mercury and selenium from gold mining effluents. Arsenic, mercury and selenium are common trace elements in gold tailing effluents that require removal before the water is either recycled or returned to a receiving body. One of the better methods of removal of these elements is the use of iron salts whereby, the metal is trapped on the freshly precipitated hydrolyzed iron oxides either by adsorption or enmeshment of the fine particulate and hence, removed from solution. The metal removal is effected by iron to metal ratio, pH, temperature etc. Pilot plant tests have shown that mercury can be reduced to < 5 ppb Hg, while full scale tests on arsenic and selenium have achieved levels of < 50 ppb As and < 10 ppb Se in well controlled systems.

Jan 1, 1993

By J. R. Dartez

The use of CCD (counter current decantat.io;1) thickeners, or; clarifiers, in the gold mining industry is quite common. Although there are a number of technologies in the industry which utilize these types of tanks, the fundamental requirement is to settle leach tailings and eventually separate gold and silver "values" from the "barren" (no values) solution. The thickener separation process varies depending upon the raw ores found in any specific mine. In mines where rudimentary, rocky ores are most common the settling process is fairly straight-forward and uncomplicated. But, for those mines with complex carbonaceous ores, usually identified by high silting, clay components, the ability to monitor and control the settled materials is quite laborious and costly. The task of monitoring and maintaining control of settled residue in clarifiers and thickeners, of any design, is difficult in any industry, but due to the added desire in the gold mining industry of leaching the values from the liquid slurry, the problem is multiplied. Other common activities taking place in the thickeners are exaggerated internal flow rates, settleable solids activity due to rake movement, and the settling activity generated by flocculents being added to the liquid medium. All of this action within the liquid column causes the continual monitoring of the interfaces within the tank close to impossible.

Jan 1, 1993

By W. D. Gould, . W. Koren, P. Bedard, P .. B. Marchant

Ammonia and nitrate in metallurgical operations arise from several sources: (1) the use of ammonium nitrate based explosives, e.g. ANFO; (2) the use of cyanide for gold extraction which is degraded to ammonia by most cyanide destruction techniques; and (3) the use of nitrogenous reagents in the mill for neutralisation, flotation and extraction. The concentrations of ammonia and nitrate in effluents vary from 10-50 ppm and 25-300 ppm respectjvely. A number of environmental problems are associated with both ammonia and nitrate. Both of these forms of nitrogen can contribute to the eutrophication of receiving waters. Ammonia and organic nitrogen can also cause high dissolved oxygen demands in receiving waters. Ammonia is also toxic to fish, particularly at elevated pH values. Several methods are in use for the removal of ammonia and nitrate from municipal wastewater, industrial effluents and agricultural runoff. To date, no treatment systems have been developed specifically for the removal of nitrogen from mine and mill effluents. This paper will report on some removal techniques that have been tested using actual and synthetic mine effluents.

Jan 1, 1993

By Nancy L. Jackson, Leslie C. Thompson

Acid rock drainage is one of the most complex issues facing regulators and mining companies for management and remediation of mining wastes. All mining waste units, including open pits, underground workings, tailings, waste rock and leached spent ore have the potential to generate acidic leachate based on the acid generation or neutralization potential of the material. This paper uses a case study of the Walker Mine in northern California to give an overview of the available control and mitigation methods. Wetland design is emphasized as a low-cost alternative treatment option for wastewater remediation. The Walker Mine is located on the eastern flank of the Sierra Nevada, 20 miles north of Portola in Plumas County, California. The Walker Mine was an active copper mine between 1915 and 1941. During active mining, tailings were discharged into an area 1 mile west of the site at the confluence of Dolly Creek and Grizzly Creek. In addition, there were two main sources of metal-rich, acid waters impacting Dolly Creek: discharge from the mine tunnel portal, and seepage and surface runoff from waters contacting mine waste downslope from the portal area. A mine portal seal was installed in 1987 which prevented the majority of mine water drainage from further contaminating the area. Minor drainage from the portal and seeps discharge into a holding pond which overflows into Dolly Creek when elevation exceeds 6,200 feet. A site characterization study was performed to evaluate the potential water quality threat to Dolly Creek from mine tunnel discharge and waste rock seeps. Based on the results of this study several remedial options were investigated, both on a design and cost basis, to determine which abatement alternatives could be realistically implemented. The remedial options considered include both in situ containment (landfill, waste pile) or removal (backfilling into underground workings and/or sinkholes) of solid wastes. Waste water remedial options include long-term, seasonal treatment of the portal mine water, or long-term, passive treatment of portal discharge and waste pile leachate.

Jan 1, 1993

By Gary Krix

The Amdel Thickener Interface Gauge (TIG) provides continuous on-line measurement of the level of the liquid/solid interface in thickeners and Counter Current Decantation (CCD) washers. Unlike other interface level measurement systems, the TIG also determines the span of the transition zone. The transition zone is the region between the clear liquid overflow zone and the compressed solids zone. The Thickener Interface Gauge determines the level of the interface by measuring the natural radioactivity of the solids in the thickener tank using an array of up to twelve detectors mounted vertically in a stainless steel probe. All solids contain low levels of natural radioactivity while the liquid phase generally does not. The concentration of natural radioactivity at each level in the thickener therefore depends on the concentration of solids. The radiation emitted by the solids is converted to electrical pulses by the detectors in the probe and the pulse-rate provides an accurate measurement of the density. A typical count-rate profile is presented in Figure 1. The vertical axis indicates the depth in the tank referenced to the surface of the liquid. The horizontal axis indicates the count-rate measured by the TIG probe, this count-rate is proportional to the density and can be used to determine the settling rate. The count-rate at the top of the graph is relatively low, this region corresponds to the clear overflow zone which contains virtually no solids. Moving down the vertical axis the count-rate increases dramatically at the level of the transition zone where the solids content increases with the depth in the tank. In this region the settling conditions of the solid particles change from free settling to compression. Moving further down the vertical axis the count-rate reaches a plateau where the count-rate remains relatively constant although there are often small fluctuations, this region corresponds to the solids compression zone. The liquid/solid interface as measured by the TIG corresponds to the depth at which the count-rate crosses the target count-rate which is generally selected to be midway between the liquid count-rate and the compressed solids count-rate. The length of the TIG probe is selected to cover the typical operating range of the liquid/solid interface in the thickener or washer. If, for example, the minimum bed level is 3.0 metres below the surface of the liquid then the probe length is selected to extend 30 metres below the surface. The TIG probe can include up to twelve detectors and the se are spaced over the measurement range. For small measurement ranges of around one metre 4 to 6 detectors would normally be used while for larger measurement ranges of three metres or more twelve detectors would be supplied.

Jan 1, 1993

By John Neale, Robert Huberts, Tony Pinches

A major R&D programme for the application of bacterial oxidation to the pretreatment of refractory sulphide gold ores was started at Mintek in the early 1980s. This arose from an established expertise base in the bacterial leaching of base-metal sulphide ores. The refractory gold programme has been supported by a number of South African gold-mining companies, but major support has been provided by the Anglo American Corporation of South Africa (AAC) and the AAC Vaal Reefs Exploration and Mining Company Limited · (Vaal Reefs). The Mintek-AAC collaboration has been formalized as a joint-venture technology agreement for the comme;cialization, as well as continued development and improvement, of the bacterial oxidation process (MINBAC). From a process standpoint, bacterial oxidation involves the contacting of slurries of finely ground sulphide concentrate with suitable strains of iron-sulphur oxidizing bacteria, which act as the catalyst to promote sulphide·· oxidation by oxygen from air. Process engineering is simplified in that this reaction is carried out at low temperatures and ambient pressures. The view was taken that the successful development and implementation of the MINBAC technology should be based on parallel R&D aimed at an understanding of the process fundamentals, the process engineering and scale-up requirements, as well as the ancillary requirements for the gold recovery-processes and environmental management. This has represented a major R&D effort at a total cost to date of around US $10 million.

Jan 1, 1993

By Rob Henson, Te-Lin Yau, Darryl Amick

The process industry faces tightening environmental regulations. In order to comply with these mandates, environmentally safe chemicals, such as oxygen and hydrogen perox­ide are becoming attractive oxidants. These oxidants appear benign but must be respected when involved in harsh environments. It is therefore important to address both the safety and suitability of materials selected for these environments. Otherwise, severe prob­lems, such as equipment failure or fire and explosion can occur. Laboratory and field experience will be used to discuss the perfor­mance of titanium, zirconium and Ti-45Nb alloy in piping systems for handling these environ­ments. Sealing methods for piping, pumps and valves of these metals will also be reviewed. For comparison, information on the performance of stainless alloys will be given.

Jan 1, 1993

By Jack Barrett

The oxidants which participate in the bacterially catalyzed oxidation of minerals are dissolved oxygen and some aqueous iron(III) complexes. Speciation studies indicate that these complexes consist of iron (III) -sulfate and iron (III) -arsenate ions. The complexes have different oxidizing powers as indicated by their calculated standard reduction potentials. The thermodynamics of the oxidations of pyrite and arsenopyrite are related to the mechanisms of the reactions. The mechanisms are discussed in terms of primary processes occurring on the mineral surfaces and the secondary reactions of the primary products. The importance of the conclusions to plant operation is discussed.

Jan 1, 1993

By Q. Yuan, B. Yarar

Steric stabilization is a phenomenon observed when relatively high doses of flocculants are added to fine particle suspensions. Experiments were conducted to establish conditions under which medium viscosity and the sedimentation properties of black sand components in a slurry could be controlled so as to enhance the selective sedimentation of gold grains. It was shown that black sand concentrates containing as low as 2.65 g/mt gold could be upgraded by selectively hindering the sedimentation of non-gold components in an aqueous slurry. This paper presents data describing the theory and practical implementation of this cyanide or mercury-free novel approach where gold could be concentrated by as much as 15 times with relatively high recoveries. The relationships of solids content of slurries, type and dose of polymers with the recovery process are documented. Key words: Gold, black sand, selective flocculation, steric stabilization.

Jan 1, 1993

By Dean J. Acuff

An overview of new size reduction technology involving the use of two-mass natural frequency vibration applied to a totally enclosed elongated drum. Material moves with a drum-like rotary motion with exposure to constant, aggressive, high frequency agitation for faster reduction and enhanced product quality. A proven, patented design with few moving parts, a vibrating mill drum provides the desirable environmental benefits of a totally enclosed rotary drum but without the same interfacing difficulties. Natural frequency design has reduced energy demand which is met by low horsepower, self-contained, motorized drives to dramatically reduce the cost per ton of finished product.

Jan 1, 1993

By Ian Corrans, Gary Johnson, John Angove

The Activox™ process is a new method for the pre­treatment of refractory gold ore concentrates prior to cyanidation. The process has been developed in Western Australia, and relies on a combination of fine grinding and pressure oxidation to achieve the destruction of sulphide minerals and the liberation of encapsulated gold. The process operates under much more moderate conditions than those that would typically be used for conventional pressure oxidation of similar concentrates. This results in significantly lower capital and operating costs being achievable.

Jan 1, 1993

By Ludovico Sebastiao Costa, Luis Albano Tondo, Bonzi Yokomizo Santos, Nelson Roberto Garcia Munhoz

This paper focus on the productivity, cost efficiency and training programmes that have enable RPM to rank as a world class operation in production efficiency and safety achievements. The first step was to commit all senior managers to implementing the NOSA Industrial Safety System based on Management by Objectives. This focused the workforce in a specific, results-driven direction which greatly promoted subsequent performance improvements. The early operations concentrated on increasing throughput via process simplification and removal of bottlenecks. Efforts were simultaneously directed to environment, health and safety as a means to improving the quality of operations. In the last two years, improvements in mining and in-tailings dam management have further improved the cost competitiveness of RPM such as to assure its future in treating the remaining CTBl ore reserves. Promissing progress is being made in developing a viable process to exploit a remaining large resource of B2 mineralisation.

Jan 1, 1993

By Karl O. Toppel, Jim Flaherty

The desire to minimize gaseous effluent in smelting and maximize gold yields from refractory ore are driving the nonferrous and gold refining industries to demand more and more oxygen. This, coupled with an increasingly competitive marketplace, forces the investigation of alternate methods of oxygen supply. Current progress in both the non-cryogenic and cryogenic fields is making the use of low-purity oxygen much more attractive. The benefits of on-site production are now available to most oxygen users. Historically, these on-site advantages could only be achieved by relatively large plants using high-purity cryogenic technology. The evolution of low-purity cryogenic and non­cryogenic oxygen production has added a new method of delivering on-site production. Specifically, the advent of oxygen Vacuum Swing Adsorption (VSA) technology has brought these benefits to smaller-volume customers by cost-effectively supplying oxygen requirements as low as 20 tons per day, (tpd). Low-purity cryogenic plants have also made an impact on the market, yielding cost reductions for larger-volume customers with oygen requirements greater than 70 tpd. This paper reviews the advantages and disadvantages of current supply methods for users having varying volume, purity and other requirements.

Jan 1, 1993

By Warwick Hoffmann, Robin Batterham, David Conochie

CRA has been actively involved in developing a bacterial oxidation process for the treatment of low grade, refractory gold resources. Success in this application is dependant on the design of large, energy efficient reactors. It was identified at an early stage, that the conventional, bottom sparged bioreactor is inherently inefficient because air must be compressed to a pressure above that at the bottom of the tank. This inefficiency increases as the size of the reactor increases. CRA has developed a novel approach to the aeration problem. In this, venturi aeration is used to aspirate air into a liquor stream. The stream is then reintroduced at the top of a draft tube bioreactor. The advantages found with this technique include · low energy requirements and high oxygen utilisations. From results at the 30 m3 scale, we predict an energy usage of -500 kWh/t sulphur oxidised at a scale of 1000 m3.

Jan 1, 1993

By Satid Therdkiattikul, Donald A. Dahlstrom

Many gold ores are known to be refractory so that conventional cyanide processing will result in serious losses of gold and silver values. A significant percentage of these refractory ores contain aresenopyrite and the gold and silver is usually encapsulated within this matrix as well as the pyrite matrix. Normally the values can be recovered as flotation concentrates because of the sulfide compounds with recoveries of over 90% and at very good grades. In order to free the gold and silver, the sulfide matrix must be destroyed. This can presently be achieved by (1) pressure­temperature leaching (2) roasting and (3) bacterial leaching. All of them have advantages and disadvantages as well as an obviously increased cost for the each processing method. However, the increased gold and silver recovery should and must more than offset this cost. It is believed that gold recovery should be well over 90% from the concentrate so that overall recovery from the ore should be in the 90-95% region. All three processes must consider pollution control of both sulfur and arsenic to prevent contamination of air, water or land through improper waste disposal. This paper will concern only bacterial leaching and its necessary control of effluents and solids. Sulfur does not appear to be a problem as the element is oxidized to sulfate and is precipitated with an acceptable solubility. Other important improvements that must be controlled are arsenic and iron which are present as significant values (1-12 g/1 of arsenic, 20-40 g/1 of iron). In addition much smaller values of base metals, aluminum and magnesium can be present as well as other elements depending on the ore and these must be low enough after treatment to meet EPA regulations.

Jan 1, 1993

By A. Anderson, G. May, C. Kubank, S. Meik

The Kori Kollo operation is owned by Empresa Minera Inti Raymi S.A., a Bolivian mining company. Inti Raymi is an eighty-five percent owned subsidiary of Battle Mountain Gold Company (BMGC). The remainder of Inti Raymi is held by Zeland Mines S.A., a Bolivian mining company. The project is located near the village of La Joya, approximately 220 km south of La Paz, and 40 km north west of Oruro on the Bolivian altiplano. Inti Raymi started a heap leach operation in 1985. By 1988, following several expansions, 4,000 tonnes per day of oxide ore were being treated. At this time drilling was started to delineate the larger sulfide orebody. A feasibility study for the development of this deposit was completed in 1990, and following project approval, the engineering design was started later that year. During this period orders were placed for the major items of mechanical equipment. In July 1991, Minproc Engineers Inc. was awarded a firm price contract to design and construct the 14,000 tonne per day sulfide ore process facility.

Jan 1, 1993

By George Hope, Jacques McMullen

Recent process advances at Lac Minerals Est Malartic Division are discussed. The Est Malartic mill treats massive sulphide, copper/gold ore from the Bousquet 2 mine, to produce a gravity gold product, a copper flotation concentrate and a gold precipitate. The mill operation has been improved by the application ofKnelson concentrators to replace jigs, the application of Outukumpu high grade tank cells and optimizing the cyanide leach to minimize cyanide consumption.

Jan 1, 1993

By Hans von Michaelis

This paper examines the process options available for treatment of "refractory" gold ores. As more and more of the easily leached oxidized gold ore zones are exhausted, the pressure is on to find more economical ways of treating refractory gold ore deposits. Historically, refractory ores were generally concentrated, e.g. to make a flotation concentrate that could be roasted. A major development is taking place in the USA in which ores from which gold cannot be concentrated well by flotation are being treated using whole ore oxidation processes prior to cyanidation. Refractory gold ore treatment processes are generally expensive to install and operate. Thus, for a refractory gold ore project to be economically viable, the grade must generally be higher than for free milling gold ores.

Jan 1, 1993

By H. Mercado G., J. R. Parga T

Mexico continues to be one of the main producers of silver, lead and zinc; although this leadership is ending because of the use of obsolete technologies, which is reflected in low recoveries of these processes. The country therefore requires to develop on new technologies in the field of nonferrous metallurgy, particularly be'cause it is the second most important source of foreign exchange. A novel method, presented in this paper, demostrates that refractory gold and silver can be extracted from a complex sulphide concentrate by a one stage pressure oxidation in an alkaline cyanide solution, this is achieved by simultaneous liberation_ and solubilization of the precious metals. Both gold and silver extractions are greater than 97%. Factors affecting recovery efficiency, such as: pH, lixiviation time in the reactor, temperature, particle diameter, pressure, % of NaCN and pulp density, were investigated. A summary of actual plant operations is also be included.

Jan 1, 1993

By Frederick W. DeVries

EPA announced, when they proposed to set a potable-water limit on cyanide, in August, 1990, that they had never encountered cyanide in a :sample of potable water, but they felt this was a sufficiently toxic and common material that it should be covered with a specification. The writer worked with both the American Mining Congress and the Aluminum Association in responding to their proposed spec. of 0.2 ppm max. of "cyanide" in· potable water. Although there are many subcategories of "cyanide", they failed to specify which type they meant.

Jan 1, 1993