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By R. A. Hill, J. D. Pitts

"Long distance slurry pipeline systems have been successfully trans-porting steam coal, iron concentrates, copper concentrates and limestone for an aggregate of nearly 60 years and nearly three billion ton-miles annually. A phosphate concentrate pipeline is nearing start-up and systems for coking coal, lead/zinc concentrates and fly ash are planned. The solids pipeline concept, first patented in the early 1900's and commercialized in the 1950's, is now a proven technology providing an economic, environmentally pleasing and safe method for long distance bulk transport.The challenge for larger tonnage and longer distance movements of solids, for traversing increasingly rugged terrain in grass roots mineral developments, for transporting untried materials and for shipping more economic forms of proven materials still remains. This has necessitated continued refinements and optimization of the technology. The economic and environmental incentives for slurry pipelines when compared to other transport alternatives has justified substantial activity in this area. The scope of planned systems in coal, iron and other minerals is indicative of the firm place slurry pipelines will have in worldwide mineral development.In this presentation, the state-of-the-art in long distance slurry pipelines is reviewed with a focus on major systems of interest to the mineral processing industry. An overview of new developments in the application of this concept is presented, and finally, a projection of the future of pipelines in mineral development both worldwide and in Canada is discussed."

Jan 1, 1978

By J. J. Bouillon

"Located in the northwestern portion of the Abitibi greenstone belt, Les Mines Selbaie is a copper-zinc (silver and gold) producer. Felsic tuffs host three are zones. Composed of sulphide veins and breccias, they are not exhalative massive sulphide deposits, except, probably, for the massive pyrite (fragments and lenses) localized close to the top of the volcanic sequence.The total of all (mineable and mined) ore in the three zones presently outlined on the property, is of the order of 29.9 x 106 tonnes with average grade of 0.63 g/t Au, 37.02 g/t Ag, 1.21% Cu, 1.91% Zn.The ore reserve calculations and the daily production procedures are computerized to various degrees. The data handling process is highly computerized.Various geophysical methods are, and have in the past, been successfully used on the property. The ""mise-à-la-masse"" surveys were used to perform the geological interpretation of an underground deposit (A-2 zone). Presently the PULSE-EM surveys are performed in all surface and underground exploration drill holes."

Jan 1, 1990

This paper discusses how elemental analysis of conveyed bulk flows in real time using PGNAA (Prompt Gamma Neutron Activation Analysis) technology has resulted in significant improvements in advancing plant control in mineral processing operations. Basis of operation of the PGNAA through belt, full stream, continuous, multi-elemental, real time analysis technology is explained. Capabilities and limitations in plant operating environments are discussed including comparison with other real time, online technologies through review of published information available for on-conveyor minerals applications. Case studies demonstrate benefits provided by this technology in two operating mines. An iron ore company had prior experience with the PGNAA analyser technology, implementing an early model in a minerals application in 2003 to develop confidence in its measurement performance and usefulness of the data. A number of analysers were subsequently included throughout the new plant design for extensive measurement and control of conveyed bulk flows, from conveying after mining through to rail load-out. A copper-gold company installed an analyser to address copper ore blending issues where an on-belt system was the only viable option for the degree of control required. Analyser suitability was evaluated through test work on site samples to determine customised multi-element precision guarantees for each application. Analyser results from normal operation are compared against laboratory analyses of multiple cross stream samples collected from conveyed flows to assess performance. Both projects discussed were able to realise benefits previously unattainable for plant quality control, surpassing capabilities of traditional technologies which were either unsuitable for measuring conveyed flows with adequate accuracy or in real time. Equipment payback within a few months was achieved for both sites. Through proven performance in these applications, the companies consider the technology an essential component in advanced plant control necessary in new process flow sheet designs, rather than an optimisation tool.

Jan 1, 2014

By Phillip J. Mackey, Gerardo R. F. Alvear F.

It is now 38 years since the publication of the first edition of "Extractive Metallurgy of Copper" by Professor W.G. Davenport and the late Professor A.K. Biswas. The text book would be issued in a total of five editions, soon became the standard for students and those in industry. Interestingly it was published at the onset of great change in the industry. This was a time when sulphur dioxide controls in the USA were introduced, and almost simultaneously, world oil shocks caused significant increases in the cost of fuel - all leading to efforts to adopt new technology and move away from the reverberatory furnace, which had no potential for sulfur capture and required large amounts of fuel. At the time of publication of the book, new processes such as the Noranda Process and the Mitsubishi Process were in their infancy, while flash smelting (which was first commercialized in 1949 at Harjavalta, Finland) had made significant in-roads in boosting copper output, with some 22 new flash smelters licensed by 1976. Computer technology and associated process control techniques were rudimentary compared to systems today, available physico-chemical data remained largely in tabulated format and needed to be manually entered into the main frame computers of the era, while modeling techniques common today had not yet been developed. The copper industry has therefore seen immense changes since the time of this textbook: world tonnage of smelted copper has risen from 7.34 million tonnes/year in 1976 to 16.7 million tonnes/year in 2012, over a two times increase. Technologies such as the El Teniente Process, or Top Submerged Lance technologies, typified by the ISASMELTTM, not commercialized at the time, now treat large tonnages of copper concentrate in a single furnace. Advanced heat and mass balance software and thermochemical modeling are now used extensively to examine chemical and physical characteristics for process design and to support process optimization. These changes in the industry have been chronicled in subsequent editions of the book. This paper looks at the issues and problems seen in 1976 and over the intervening years, and compares this with the challenges facing the industry with the available tools and technologies today. Some ideas looking into the future are also discussed in the paper.

Jan 1, 2014

By A. H. Winckers

"The San Nicolás deposit is a complex fine grained polymetallic volcanogenic massive sulphide deposit located in central Mexico.Metallurgical testwork was carried out in 1999 and 2000 to develop a flowsheet. During the course of the metallurgical testwork it became evident that it would be impossible to develop an accurate head gradehecovery based model for the projection of metallurgy. It was suspected that variability in mineral texture was a predominate factor affecting metallurgy. For this reason it was decided to evaluate the applicability of QEM*SEM technology to this test program.This paper summarizes the metallurgical testwork and describes the evaluation and subsequent application of the QEM*SEM technology to characterize the mineral texture throughout the deposit. Finally it is shown how the QEM*SEM data are used in the mine block model to project the metallurgy.The QEM*SEM (Quantitative Evaluation of Minerals by Scanning Electron Microscopy) is an automated image analysis system developed by CSRO which provides quantitative numerical estimates of the compositional and textural properties of the mineral suite which govern the concentration process.INTRODUCTIONThe San Nicolás deposit, was discovered in 1997 through the application of geological and geophysical techniques. The property lies 60 km southeast of the city of Zacatecas on the central Mexican plateau as shown in Figure 1."

Jan 1, 2002

By Peter A. Oshust

The EKATI Diamond Mine has successfully completed the development and commissioning of the Panda Underground Project to extract the ore below the Panda open pit. The dominant kimberlite domain of the Panda Underground, intermixed olivine-rich and mud-rich resedimented volcaniclastic kimberlite (RVK), is very similar to the kimberlite domain characterizing the open pit portion of the pipe. A sublevel retreat mining method is in operation underground at Panda; near-vertical blast rings are drilled and blasted into an open void. Broken ore is mucked with load, haul and dump trucks (LHD?s), sized underground then moved to surface through a conveyor system to the process plant. An advanced feasibility study is in progress at the Koala Underground Project. The Koala pipe is geologically more complex than Panda. The pipe below the open pit is comprised of at least five distinct kimberlite domains. There is a 100 m thick zone of incompetent low-grade mud-rich RVK between the open pit and the stable high-grade olivine-rich primary volcaniclastic kimberlite where mining will be initiated. As a result, it is proposed that Koala will employ sublevel caving; each blast ring shot would be confined and mucked in sequence to maintain the cave of the over-lying low-grade RVK ore. The sublevel cave method seeks to safely defer the mucking of the low grade ore to the latter part of the mine life.

May 1, 2006

By E. Douglas, V. M. Malhotra

"This report presents the results of a laboratory study undertaken to evaluate ground granulated blast-furnace slag from Algoma Steel Corporation, as a replacement for Portland cement in cemented mine backfill. The evaluation was based on the results of compressive strength tests performed on cylinders cast from mixtures made with a granulated blast-furnace slag and tailings from four different mines in the Sudbury basin. Various percent¬ages of slags and various binder to tailings ratios were used.Tests results show that, in terms of compressive strength, the slag performs as well or better than Portland cement as the binding agent in cemented mine backfill. Lime was found to be a poten¬tially good activator for ground granulated slag in mine backfill applications.A field test was conducted at one of the mines, consisting of placing a mixture of 95.5% ground granulated slag and 0.5% lime into a stope, and inspecting the floor slab as it hardened. The slag performed as expected, considering the backfill mixture proportions used and the preparation of the test site."

Jan 1, 1989

By G. Thom, A. Kay, J. B. Hyde

"This paper will initially outline the specific types of copper castings used in blast furnace iron production. Tuyeres, tuyere coolers and stack cooling plates will be discussed in some depth, with particular reference to their dimensional configuration and purpose.Following this, the failure of such cooling members will be discussed, with special focus on tuyeres. Tuyeres are the most common casting which often requires replacement. Reasons for failure and its effect on operations will also be included.The latter sections of the paper will deal with the strict specifications demanded by ironmakers regarding copper castings. The justification for such rigid demands will be explained through the material presented earlier in the paper.IntroductionThe modern 5000-tpd blast furnace, worth millions, is a large capital investment to be sitting idle. Consequently, ironmakers strive to minimize the loss in iron production and revenue from furnace downtime. Two major causes of downtime are furnace relines and shutdowns to replace damaged tuyeresor tuyere coolers. Copper ancillaries are important items in the production of iron. They are consumable items, therefore they contribute to the campaign life or to the downtime in blast furnace production.Several combinations of lining and cooling systems have been used with varying degrees of success. Blast furnace refractories have evolved from ceramic to carbonaceous linings. The Russians and Japanese have successfully introduced stave-cooled furnaces that may have several forms, such as evaporative cooling. Stelco has two furnaces which use the Russian-designed cast iron system. However, this paper will be confined to the use of copper cast cooling members in a conventional internally cooled furnace.Figure 1 is a schematic cross section of the E blast furnace. The three major sections of a blast furnace-the stack, bosh and hearth-are identified. Further, the locations of the various copper members are indicated. This furnace produces an average of 5300 tons of hot metal per day."

Jan 1, 1982

By Sergei Decterov, Peter Hayes, Evgueni Jak, Taufiq Hidayat, Denis Shishin

Copper smelter operations around the world are under sustained pressure to reduce costs and increase output. Through the provision of reliable data on fundamental properties of the chemical systems the opportunities for improvements of industrial copper production can be identified. Experimental and modelling studies on phase equilibria, thermodynamics of copper slag/matte/metal/gas systems have been combined into an integrated research program undertaken at PYROSEARCH. Experimental phase equilibria studies involve high temperature equilibration in controlled gas atmospheres, rapid quenching and direct measurement of compositions of equilibrium phases with electron probe X-ray microanalysis (EPMA). Thermodynamic databases development and modelling is undertaken using the computer package FactSage in conjunction with advanced models used for complex solution phases. Progressive development of these research methodologies has brought significant advances in characterisation of these complex systems, including a new optimised thermodynamic database for the copper-containing slag system Cu2O-Al2O3-CaO-FeO-Fe2O3-MgO-S-SiO2.The new data and models can now be applied to assist in improvements of the processes. The effects of changes to chemistry and process conditions within the smelting reactor can be examined. Several examples of new descriptions of the phase chemistry of the Cu2O-CaO-SiO2 and Cu2O-FeO-Fe2O3-SiO2 systems in equilibrium with metallic copper showing calculated and experimental data are presented.

Jan 1, 2014

Strategic Branding ? In 2006 Explore for More was created ? By industry, for industry ? Cohesive approach to career attraction ? Defines industry and employers of choice Career Kits ? Positive impression of mining: ? before seeing the brochure 31% ? after seeing the brochure 83% ? Likely to consider a career in mining: ? before seeing the brochure 12% ? after seeing the brochure 23% MARS ? Mining Attraction, Recruitment, and Retention Strategy (2007-2010) ? Attract, recruit and retain a skilled workforce ? Increase participation of under-represented groups ? Dispel myths and misconceptions of the mining industry ? Inform and educate Canadians of career opportunities ? Promote and build upon industry best practices in workforce planning

May 1, 2009

By Marius Jacobs

"Safety performance in many companies and even industries has stalled in the few years. Accidents rates are at a "plateau" and yet, serious accidents and fatality rates are not. In more dramatic cases, such as in the BP Texas disaster, organizations that have "exemplary" safety statistics, suddenly have a catastrophic or multi-fatality event occurring. Other classic examples are the Piper Alpha disaster and NASA’s Challenger and Columbia disasters and even the Chernobyl nuclear reactor. Looking into the root causes of these accidents provides an insight into the events and deficiencies that led up to the accident, but what are the common features in the organization’s mindset? What characterizes these organizations’ decision-making, their approach to safety and to risk and are there specific cultural features that can be delineated? The research and review presented in this paper covers a period since 1994, which started in the Australian resource industry and since then covered international events. The features of these organizations are summarized as the seven deadly delusions, based on extensiveresearch since 1997 of organizations that suffered unexpected disasters.The Piper Alpha Oil Rig disaster in the North Sea, on the 6 July 1988, was one of the most pivotal events in safety around the world. It changed the thinking and focus of governments, whole industries and led to numerous books and papers. It resulted in new legislation, textbooks and a critical self-examination by the oil and gas industry.In short, an explosion and fire occurred when a pipe started leaking gas and ignited. A temporary flange, with no safety valve, was used to block off this pipe during a maintenance operation the previous shift. The permit to advise operators not to start the pumps on this line was misplaced and lost. Several deficiencies, problems and system failures coincided. A key factor was that the water deluge system was inoperable at the time and failed to extinguish the large fire that erupted, followed by an even larger gas explosion. The accommodation unit, situated on top of the oil rig, was the main "killing field" - most men gathered here to await instructions (which never came) and where they died from smoke inhalation. Later, a large explosion of the gas pipeline from an adjoined rig exploded and killed more people.The line of events and the associated system failures are all very relevant in the analysis of the accident, and provide the only insight into what caused the accident. But much "further back", inside the organization’s culture, lurks more dark matter. The first inclination is to ask what was "deficient" in the culture. Did they not care enough about safety, or about their people? Did they have a culture where production decisions were more important than safety considerations? Did they ‘coerce’ supervisors and employees to ignore safety precautions or did they just "fly by the seat of their pants"?Not surprisingly, it is very difficult to identify the true nature of the culture in that business because in such an aftermath of death and destruction, no one dare say that the safety culture was a ‘positive one, focused and caring’ or that the company was doing well in safety and that this is only a freak accident. Indeed, there were many deficiencies in the organization’s systems and procedures, such as the permit to work system.But there are some niggling questions: How is then that the Piper Alpha Oil Rig was actually considered the company’s most productive and safest oil rig in the North Sea? Why would this rig win a safety competition, 6 months before the disaster, with the deficient permit as its most outstanding successful system?"

Jan 1, 2014

By Donald Leroux, Jean-François Wilhelmy, Ahmed Bouajila

"A young private R & D organization, COREM celebrated its 10th anniversary in 2009. Managed by a consortium of nine mining and metallurgical companies, COREM has the mandate of increasing the competitiveness of its member companies through the development and transfer of mineral processing innovations. Typical technologies developed by COREM include alternative mineral processing techniques, novel operating strategies, innovative sensors and process simulation software. Many of them were developed in partnership with universities and selected research organizations.Because the COREM membership base is broad (from ferrous to base, to precious metal industries), its R & D program must deliver technologies with applications within a wide spectrum of mineral processes. Comminution, physical (gravity and magnetic) separation, flotation, cyanidation, pelletizing and thermal processes are the main topics of the R & D program. A number of new technologies have been developed and transferred to the plants of member companies. These examples of transferable technologies demonstrate the capability of COREM to impact the bottom-line of its member companies.INTRODUCTIONCOREM is a private mineral-processing R & D organization that operates from its own laboratories and pilot-plant with its own staff of professionals and technicians (about 95 persons). It is managed and financially supported by a consortium of nine mining and metallurgical companies and three provincial and federal ministries. The very ‘down-to-earth’ mission statement of COREM is to increase the competitiveness of the mineral processing plants of its members. The eighteen plants upon which the COREM technology development program is currently based can be found in Québec, Newfoundland and Labrador, New Brunswick, the Netherlands and Botswana."

Jan 1, 2010

By Steve Wilson, Bern Klein, Jeff Drozdiak, Stefan Nadolski, Andrew Bamber

"In the adaptation of high pressure grinding rolls (HPGRs) to hard-rock mining applications, circuit development has mainly focused on placing HPGRs in the role of tertiary crusher in the preparation of ball mill feed. In order to take advantage of the potential energy benefits associated with HPGR and stirred mill grinding technologies, work was done to investigate the potential of and resultant energy benefits of placing the HPGR in series with a high speed stirred mill in replacement of the conventional cone crusher / ball mill arrangement. The proposed circuit was designed to operate with two stages of HPGR grinding followed by high speed stirred milling.Using a pilot-scale HPGR unit inline with a 20 liter stirred mill, the design criteria, circuit arrangement and consequent comminution energy requirements for a circuit processing copper sulphide ore from Teck Resources’s Mesaba Copper project were determined. These results were then compared to two other circuits, an HPGR / ball mill circuit and a conventional crusher / ball mill circuit processing the same ore.The testwork indicated at a proof-of-concept level that the HPGR / stirred mill circuit could potentially be appropriate for this duty and further work on the concept is merited. While the results indicate that the HPGR component of the circuit was more energy efficient than conventional cone crushing, the energy efficiency of the stirred mill was found to be lower than that of a comparable ball mill undertaking the same duty. However, increased energy efficiency of the stirred mill portion of the circuit is possible with further optimization and further work on the concept is indicated."

Jan 1, 2010

By G. H. Hope

"Pamour Porcupine Mines of Timmins, Ontario operates two 3,000 ton per day concentrators which receive ore from eight to ten mines in the area. The area milling concept was adopted by Pamour in 1971 and has been used successfully since then. The purpose of this paper is to discuss the advantages and the problems of area milling. Pamour Porcupine Mines Limited operates two 3,000 ton per day concentrators which receive ore from eight to ten mines in the Porcupine region. The area milling concept was adopted as the route to survival for Pamour and has worked successfully. HISTORICAL BACKGROUNDProduction of gold from the Porcupine region in Northern Ontario started in the early 1900's and has been continuous since that time. Some of the major operators were companies such as Mcintyre, Hollinger, Dome, Pamour, Paymaster and the list goes on. An underground survey would reveal hundreds of miles of drifts and raises. The main group of mines extends from South East of Timmins to the Pamour Mine, 15 miles to the east. During the lean years of the sixties when gold prices were low, labour costs were rising rapidly, and ore grades were dropping, many of the area mines closed down their operations."

Jan 1, 1981

By Jean M. Richardson, Steven R. Williams

"Geometallurgical mapping is a new team-based approach that documents variability within an orebody and quantifies the impact of geology (host rocks, alteration and structure) and mineralogy on grinding, metallurgical response and metal recovery processes. The quantitative, spatially constrained database that results can be readily integrated into 3-D block models and mine planning activities. Thus it is an important tool to reduce the technical risk associated with new mine developments or expansions.Normally undertaken during pre-feasibility or feasibility planning stages, the first step of geometallurgical mapping is to review the geology, mineralogy and other critical parameters and construct a geometallurgical matrix. This matrix provides an objective base to then guide sampling and/or compositing activity for physical property and metallurgical testing. Depending on the project, an array of testing techniques can be used to characterize the ore and feed data into the geometallurgical model.While the range and number of tests needed is project-dependant, the geometallurgical mapping approach allows the development of empirical models or interdependent relationships, grounded by spatially-constrained real data. Extensive data sets collected using less expensive testing methods can be meaningfully correlated with results from complex specialty tests to yield realistic metallurgical recovery data.With geometallurgical mapping completed, the resulting database can be integrated into an overall 3-D block mine model. This means that metallurgical response can be used to realistically forecast the recoveries of specific blocks, perhaps sampled only by drilling. Mine planning can then incorporate the forecasted metallurgical response and generate future project cash-flows; opening the door to economic optimization of the mine exploitation."

Jan 1, 2004

By Y. Nakamura, T. H. Okabe, S. A. Sunkar, K. Nose

Ruthenium (Ru) is known as a platinum group element with a very small production volume. In fact, the worldwide annual production of primary Ru metal is only about 20-30 tons, and it depends on the amount of Pt produced, since Ru is a byproduct from Pt smelting. The primary uses of Ru are as a material for resistors, as a catalyst for electrode materials, and as a sputtering target for hard disk memory storage. In order to recover Ru from the spent sand-blasting abrasive media generated during the cleaning of the apparatus used for Ru sputtering, a hydrometallurgical recovery process using hypochlorite solution has been investigated. In this study, various types of recycling processes that are presently under development are also introduced, and their features and differences are discussed. The advantages and disadvantages of the new recycling processes are evaluated, and the possibility of establishing new recycling processes in the future is discussed from a multilateral perspective.

Jan 1, 2014

By Julián M. Ortiz

Uncertainty quantification of a spatially distributed variable at any scale can be handled through geostatistical simulation. Large computation time, storage, and post-processing of the realizations are required to obtain a final assessment of block uncertainty. Multi-Gaussian kriging is a flexible alternative to simulation. The idea is to compute the conditional distribution of uncertainty after normal score transformation of the original samples. Under the multi-Gaussian assumption, all marginal and conditional distributions are Gaussian, hence fully defined by their mean and variance. The parameters of the conditional distributions are obtained by simple kriging and can be back-transformed to the original units of the variable of interest. An estimate and any summary of uncertainty can be easily retrieved. The main disadvantage of performing multi-Gaussian kriging is that change of support is not straightforward, that is, calculating uncertainty of block grades. We propose a methodology to overcome this limitation by considering a matrix simulation to generate multiple probability fields. Each probability field is used to draw spatially correlated point values from the point-support conditional distributions, and multiple realizations of the average can be obtained. This permits the calculation of the average over the block and its uncertainty. These blocks may correspond to selective mining units or to volumes from longer production periods relevant for engineering decisions. They can even be disjoint blocks, such as when several faces are mined at the same time. POSTMG, a Fortran program to perform these calculations, is described and a case study is provided.

May 1, 2004

By Hugo Julien

According to OSHA 29 CFR Part 1910.119 (Process safety management of highly hazardous chemicals), Canadian Provincial Safety Standard Acts or Federal laws, the owner of stationary equipment (Boilers, stacks, piping, pressure vessel, etc.) should follow a complete inspection plan and follow an Integrity Operating Windows (IOW’s) adapted for each of them. Some important variables shall be included in the inspection plan such as specific damage mechanisms versus alloy used in the original design, process conditions, etc. Some North American’s standards exist, like the one listed in ASME PTB-2-2009, to help owner to create such inspection program. The new API RP 584, Integrity Operating Windows (IOW’s) can guide Mining & Metals owners to find out their specific IOW’s. Inspection frequencies are often described in the Jurisdiction Safety Acts such as the British Columbia B.C. Reg. 104/2004, etc. but often left in the owner representative responsibility (Bill C-21, federal Act to amend the Canadian Human Rights Act). Every deficiency should be repaired according to recognized Codes such as ASME PCC-2, etc.. If flaws not conforming to the original Code of construction are found, this article will introduce the Fitness-For- Service assessment principles to be able to accept it without any repairs.

Jan 1, 2014

By J. Y. Hascoet, M. Marya, S. Marya

Additive manufacturing (AM) brings disruptive changes over the ways parts and products are designed, tested, qualified, assembled, inspected and marketed. With these changes come real technical challenges, therefore, concerns that mainly stem from the maturity and diversity of today’s AM processes, the feedstock materials, and the interactions between the numerous process parameters. Additive manufacturing is somewhat similar to laser and electron-beam conduction-mode welding and involves a multitude of dynamic physical events between projected feedstock and moving heat source that influence process robustness, thus deposit properties. For this paper, a simple air vent, selected for its size, thin-wall, hollow geometry and materials (specifically 316L) was made using a laser-based powder blown technology (Construction Laser Additive Direct or CLAD) and systematically inspected by optical microscopy, microhardness indentations, and electron-back-scattering diffraction (EBSD). This exemplary part is first analyzed to broadly discuss metallurgical phenomena in additive manufacturing, particularly solidification and texture development. The paper then expands to other alloys and continues correlating metallurgical observations to process parameters and part properties, also borrowing from past experience with the same 3D printing CLAD system.

Jan 1, 2014

By D. M. Wyslouzil

"Micron or sub-micron size gold inclusions in sulphide minerals cannot be extracted by conventional cyanidation. The gold surfaces must first be exposed by decomposition of the sulphides. As an alternative to roasting, pressure oxidation was found to be a most satisfactory method to decompose the sulphides and expose the gold particles. For cyanidation the pressure oxidation pulp has to be neutralized and stabilized with lime. It was found that thiourea can extract the gold directly from the pressure oxidation pulp without pH adjustment and oxidant addition. The main obstacles against the common use of thiourea are circumvented by a recently described control method of the emf during leaching.INTRODUCTION Amalgamation and cyanidation are the most conventional processes for gold extraction. The prerequisite for gold extraction is the possibility of gold surface to be contacted by mercury or by the cyanide solution. Encapsulated gold has little chance of contact and this leads to poor extractions.The reasons why a gold ore may be refractory are: micron or sub-micron size gold particles included in the sulphide minerals, such as pyrite and arsenopyrite; presence of and association with antimony, arsenic, tellurium and carbonaceous minerals. Some alternative processes, such as double oxidation(1,2), pretreatment with lime , H202, chlorine and Caro's acid, roasting and thiourea leach, have solved some of the problems of ""refractory"", but the sub-micron gold in sulphide minerals still shows little response. Roasting and the cyanidation of the calcine was one of the most successful approaches for this type of gold ore.There are two disadvantages to the roasting process: (1) the generation of sulphur, arsenic, antimony and mercury containing gases is undesirable and the control is extremely expensive; (2) gold particles might be fused in the roasted product and the oxidized base metals in the calcine might reduce the cyanidation efficiency. In recent years the pressure oxidation process has been considered as one of the more effective and satisfactory methods to decompose the sulphide minerals and expose the gold surface to the cyanide leach solution. The pollution problem could be better controlled and the gold extraction is more amenable."

Jan 1, 1985