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By M. L. Leming

Introduction In the decades since silica gel was first identified in material exuding from cracked concrete, a great deal of research has been conducted regarding the chemical reactions between the alkalies found in portland cement and silica found in aggregates. The reaction is complex and one that is not yet completely predictable, especially from the point of view of developing specifications that are appropriate to all situations. This paper is not intended to be a rigorous review of the research findings but is an attempt to provide a simplified review of the mechanisms of the alkali-silica reaction (ASR), so that one can better understand the implications of the specifications, test results and effects on structures. In addition, the contractual relationships between the aggregate supplier and one of their major clients, the concrete supplier, will be examined with regard to the ASR. ASR basics Silica. Silica (silicon oxide) may exist in naturally occurring aggregates in various forms and in combination with a number of other elements. When the silica is completely crystalline, such as in quartz, it is chemically and mechanically stable. Quartz silica is impermeable and reacts only on the surface of the crystal, where the silicon and oxygen bonds are broken. Because the surface area per unit volume of most quartz is low, the reactivity is also low. Completely amorphous (noncrystalline) silica is, on the other hand, more porous and very reactive. The less "crystalline" the silica is in the aggregate, the more reactive. Silica that has melted and cooled quickly without recrystallizing, creating a glassy material (such as in certain volcanic aggregates), has a very low state of crystallization and will be much more reactive in an alkaline solution. Crystalline silica that has been transformed by heat and pressure may have a large quantity of strain energy stored in the crystal lattice. The presence of this higher energy will make the silica more likely to react. The "strained quartz" found in many metamorphic aggregates means that these aggregates are potentially susceptible to deleterious alkali silica reactivity, although the rate of reaction is typically much slower than with aggregates composed of or containing glassy or amorphous silicas. Another problem may exist with aggregates in which the silica is primarily crystalline. In aggregates such as chert, in which the silica exists as very fine crystals (i.e., crypto- or microcrystalline), the very high surface energies between the crystals contribute to alkali sensitivity. Therefore, the potential reactivity of an aggregate is seen to be a function of both the degree of crystallization of the silica in the aggregate and the amount of energy stored in the crystal structure, whether due to a large quantity of microcrystalline silica, a high strain energy stored in the crystals or some combination of these factors. The surface area per unit volume of the reactive silica will also affect the rate of reaction, because a larger surface area of reactive silica will have more opportunity to react. Obviously, the reactivity of the aggregate is also affected by the silica content. However, in this case, the results are not quite so obvious. A discussion of the effect of silica content will be postponed until after a discussion of the contribution of the cement paste. Paste characteristics. Hydrated portland cement is a very alkaline material with a pore solution pH typically in excess of 12. The alkaline environment of moist concrete provides an ideal place for noncrystalline or cryptocrystalline silica to react. However, not all alkalies are equal in their effects. Calcium compounds react with glassy silica to form calcium silicate hydrate, commonly abbreviated C-S-H a poorly crystalline material that can occur in several forms and chemical compositions. C-S-H was at one time called tobermorite gel, because it was chemically similar to the naturally occurring crystalline mineral tobermorite and because it had a gel-like (noncrystalline) structure when viewed under an optical microscope. The formation of C-S-H is the basis for both portland cement hydration and reaction with, for example, fly ash. C-S-H is relatively stable. Although drying will cause some shrinkage and rewetting will cause some expansion, the volume stability of the C-S-H is very good compared to the volume stability of most alkali silica gels. Alkali silica gels with high sodium contents, for example, are nonstable compared to C-S-H.

Jan 1, 1997

By M. L. Leming

The mechanisms of Alkali-Silica Reactivity (ASR) are briefly reviewed, test methods are examined, and typical specifications are discussed. In addition, views on responsibility of the aggregate supplier regarding specifications are presented.

Jan 1, 1995

By Cheng Xiang, Nan Ye, Xinyuan Ke, Yang. Jiakuan, Huabin Wang

Bayer red mud is a highly alkaline residue produced by the alumina industry through the Bayer process. Its disposal is a worldwide problem. The high alkali content and poor pozzolanic activity is aconstraint for its utilization on construction materials. In this study, the red mud was activated through an alkali-thermal method to improve its activity so that it can be utilized in geopolymerization. The red mud was mixed with different amounts of sodium hydroxide, and calcined under 800 °C to produce geopolymer precursors, and then mixed with water directly to prepare one-part geopolymer. X-ray diffraction was conducted to study the phase transformation of red mud and the mechanism of its geopolymerization. Nitrogen Isothermal Adsorption tests were conducted to analyze the pore structure of the geopolymer binders. The results show that calcinations with alkali would promote the Al in red mud transforming into glassy SiO2 to form the N-A-S phase which is soluble in water. When the red mud was calcined at 800 °C with a Na2O addition more than 10%, the dissolving of Si and Al from red mud was greatly improved. The thermal-alkali activated red mud with 10% Na2O obtaining a compressive strength of 9.77 MPa after it reacted with water and cured for 7 days at 20 °C and 95% RH. But the strength decreased as the curing time extended to 28 days. The reason is suggested to be that the polymerization of Al-O and Si-O is unstable due to the low Si/Al ratio of red mud. The alkali in the structure dissolves again into the pore water and corrupts the pore structure. Therefore, the thermal-alkali red mud should be combined with other reactive aluminosilicate materials such as granulated blast furnace slag to absorb the alkali released by the thermal-alkali red mud at the latter curing days.

Jan 1, 2014

By R. E. Whitehead

"Alkali/alumina and MgO/alumina molar ratio plots graphically portray both chemical and mineralogical changes accompanying potassic, phyllic, argillic, chloritic, and alunitic alteration of quartz monzonites and granodiorites hosting porphyry (as well as lode and greisen) ore deposits. The molar ratio plots can be used to identify different types of alteration. In most cases, the identification based on molar ratios coincides with petrographic data. In those instances where the molar ratio and petrographic identifications do not agree, the mineralogy might need to be re-examined. Hydrothermal alteration studies using trace elements could benefit from the application of complementary alkali/alumina molar ratio plots.Le diagramme des rapports molaires alkali/alumine et MgO/alumine illustre clairement les changements chimiques et minéralogiques qui accompagnent les altérations potassique, phyllique, argillique, chloritique, and alunitique des monzonites à quartz et des granodiorites encaissant les gîtes de type porphyrique, ainsi que des veines et des greisens qui leur sont associés. Les diagrammes de rapports molaires peuvent être utilisés pour identifier divers types d’altération. Dans la plupart des cas, l’identification basée sur les rapports molaires est en accord avec celle basée sur les données pétrographiques. Là ou il y a désaccord entre les rapports molaires et les données pétro-graphiques, il peut être nécessaire de réexaminer la minéralogie. Les études de l’altération hydro-thermale accompagnées d’une étude des éléments traces auraient avantage à utiliser un diagramme des rapports molaires alkali/alumine."

Jan 1, 2009

By T. S. Heah, S. T. Bishop

"The Lorraine property is located in the northern Intermontane Belt, north-central British Columbia. The property is underlain by intrusive rocks of the Duckling Creek Syenite Complex, an Early Jurassic, alkaline phase of the Late Triassic-Early Cretaceous Hogem Batholith. Two significant zones of copper-gold mineralization have been explored to date; the Main zone (historical Upper and Lower deposits) and the recently discovered Extension zone. A combined geological resource was estimated in 1975 for the Main zone deposits at 10 million tonnes averaging 0.7% Cu, and from 0.1 g/t Au to 0.34 g/t Au. The Extension zone, still at an exploration stage, is estimated to be considerably smaller than the Main zone.Copper-gold mineralization at Lorraine Mountain exhibits characteristics suggestive of both magmatic and hydrothermal origins, and may be related to orthomagmatic-hydrothermal fluid flow contemporaneous with magmatism and development of migmatitic fabrics. A depth greater than the depth of formation of classical porphyries may explain many of the variations displayed by mineralization at Lorraine. These characteristics distinguish Lorraine from other members of the alkalic suite of porphyry Cu-Au deposits in the Canadian Cordillera.IntroductionThe Lorraine prospect is located in the Omineca Mountains, at the headwaters of Duckling Creek, 56 km northwest of Germansen Landing (Fig. 1). The property is centred at latitude 55°55'N, longitude l25°26'W on NTS mapsheet 93N/14W. The area is typified by mountains of moderate relief, with elevations ranging from 1150 m in the valleys to peaks of 2000 m. Valleys are U-shaped and blanketed by glacial till which give way to steep, talus covered slopes and sharp ridges. Outcrop is generally limited to elevations above 1600 m (Fig. 2)."

Jan 1, 1995

By Kenneth G. Thomas

This paper reviews the alkaline and acid gold autoclaving experiences gained at Barrick Gold Corporation. Alkaline autoclaving was operational at Barrick Mercur, Utah, U.S.A., on carbonaceous/sulphide refractory ore for eight years commencing in 1988. Acid autoclaves have been operational at Barrick Goldstrike, Nevada, U.S.A., on sulphide refractory ore since 1990 and now process 17,500 stpd of ore through six large autoclaves. Information is presented on operating costs, materials of construction, reagent requirements and flow sheet developments that reduced capital and operating costs.

Jan 1, 2000

By A. Robles

The main goal of the present work is to propose a viable alternative for selective arsenic elimination from copper smelter flue dust without the need of a global leaching scheme for all the individual components of the flue dust. Using this alternative, the inherent copper recovery problems are avoided. Arsenic precipitation is also considered for regenerating the solution and completing the process cycle. The experimental results for arsenic extraction and precipitation are satisfactory. Concentration of NaOH used was relatively low. The main reason for succeeding in high arsenic extraction and low NaOH consumption is due to the previous water washing of the flue dust. Water washing dissolves much of the iron and copper that are present in certain percentage as sulphate.

Jan 1, 1999

By Kenneth E. Skiff

INTRODUCTION United Nuclear Corporation and Homestake Mining Company are partners in a major uranium mining and milling venture near Grants, New Mexico. The partnership was formerly called Homestake-Sapin Partners and was recently renamed United Nuclear-Homestake Partners (UN-HP). The mill, originally built in 1958, had a capacity of 1,650 tons per day. With subsequent improvements and the addition of the adjacent Homestake - New Mexico Partners mill, the UN-HP mill is currently rated at about 3,500 tons per day. An alkaline type leach process is utilized by the mill. Milling consists of five basic steps: A. Ore handling and preparation B. Extraction C. Solid-liquid separation D. Precipitation and purification E. Product preparation MILLING Ore Handling and Preparation The primary source of ore for the UN-HP mill is from its four

Jan 1, 1983

By Xiaolong Bi, Shaohua Luo, Longjiao Chang, Shiyuan Cao, Jianan Liu

iliary agent was proposed. The mineralogical transformation and silicon extraction kinetics of laterite nickel ore in alkaline hydrothermal treatment were studied. The effects of reaction temperature (175 ~ 250℃), molar ratios of sodium hydroxide to ore (1.05 ~ 1.2:1), and laterite nickel ore particle size (44 ~ 178 μm) on the extraction ratio of silica were investigated by using the shrinking core model. The results show that the extraction ratio of silica increases with the increase of reaction temperature, sodium hydroxide to ore molar ratio, reaction time, and the decrease of raw ore particle size. The kinetic analysis shows that the separation of silica in laterite ore can be described by internal diffusion model. The apparent activation energy (Ea) of the reaction was 8.87 kJ mol− 1, and the kinetic modeling was established.

Dec 2, 2021

ALKALINE LEACHING; SODIUM HYDROXIDE U.S. 4,091,071 -In the alkaline digestion of bauxite containing goethite, boehmite and/or diaspore, the process is accelerated by adding to the digestion mass an optimum proportion of calcium oxide and divalent manganese ions, advantageously in the form of a calcium carbonate-containing manganese oxide ore, whereby goethite is transformed into hematite, the alumina yield increased, and loss of caustic soda values minimized. J. Boros, assigned to Femipari Kutato Intezet, Aluterv Alumiumipari Tervezo Vallalat and Almasfuzitoi Timfoldgyar. May 23, 1978. 7 pp. Filed: 12-23-75 (643,670). Priority: Hungary, 12-24-74 (AU 330). Same: British 1,528,146, dated Oct. 11, 1978.

Jan 1, 1979

US 4,133,866-Selective recovery of the bound sodium content of red mud obtained in the production of alumina by the Bayer process The red mud is mixed with aqueous ferric sulfate, the resulting suspension stirred intensively at a temperature of 15-85°C and adjusted to a pH value of from 4.4 to 4 6, and the solids are removed. The clarified solution of sodium sulfate can be used directly for the leaching of additional bauxite T Lakatos, M Miskei, J Szolnoki, F. Toth, and L Revesz, assigned to Aluminiumipan Tervezo es Kutato Intezet, Almasfuzitoi Timfoldgyar, and MTA Geokemiai Kutatolaboratorium. Jan. 9, 1979. 3 pp (423-208). Filed: 10¬28-77 (846,588) Priority: Hungary, 10-29-76 (AU 364). US 4,145,398-Continuous digestion of bauxite with aqueous sodium hydroxide, with higher throughput in existing plants or the construction of smaller plants with equivalent capacity The ore suspension at a solids concentration of 1-6% by weight and temperature of 200-300°C is flowed through a digestion tube or nest of tubes at a velocity of preferably from 2 to 5 m/s and under a pressure of from 10 to 200 atm Heat is supplied by a fluid heat carrier flowing in countercurrent in a concentric outer tube. Preferably, some of the red mud is recycled to the digestion operation. L. Plass, assigned to Vereinigte Aluminium-Werke A.G Mar 20, 1979. 5 pp. (423-121) Filed: 2-27-76 (754,749) Same: British 1,437,548, dated May 26, 1976 Same Canadian 1,008,253, dated Apr 12, 1977. US 4,146,573-An aqueous slurry of red mud, formed in the Bayer process production of alumina from bauxite, can be solidified by treating the red mud with a described quaternary ammonium salt, continuing mixing until the red mud has set up to a rigid mass, and recovering the solidified concentrated red mud J Kane, assigned to Nalco Chemical Co Mar 27, 1979 4 pp (423-82). Filed 8-2-78 (930,251).

Jan 1, 1980

US 4,190,436 - In the cyanide leaching of gold ore in tailing or ore dump piles, or gold ore in vats or on pads, the ore is agitated and oxygenated by introducing timed bursts of air, along with cyanide in a cyclic or regulated manner. The leach liquor is drained from the material by providing a continuous flow of air. Fines are homogenously mixed, thereby reducing channelling In the ore bed. Yield is improved and cyanide consumption lessened. R M. Hughes Feb. 26, 1980. 7 pp. (75-105). Filed. 12-20-77 (862,335).

Jan 1, 1982

US 4,201,749-In the Bayer process production of alumina wherein aluminum hydroxide is precipitated from a sodium aluminate solution for use as seed, sodium oxalate and caustic components are removed from the precipitate by washing it first with a washing liquor which is saturated with sodium oxalate but has little content of caustic, and secondly with hot water. Substantially no sodium oxalate is recycled, yet loss of sodium aluminate is minimal. K. Yamada, T Harato, and H Kato, assigned to Sumitomo Aluminum Smelting Co Ltd May 6, 1980 8 pp. (423-127). Filed. 5-8-79 (37,145) Priority. Japan, 5-8-78 (53/54,803). Same¬British 2,020,642, dated Nov. 21, 1979.

Jan 1, 1982

By S. N. Rossovsky

"A new method for the hydrometalfurgical treatment of refractory gold arsenosulphide concentrates is presented. Themethod involves a combination of' finegrinding, intensive agitation, flotation, and cyanidation, and consists of leaching of -10u pyrite and arsenopyrite particles in alkaline solution under intensive agitation without the application of costly high-pressure and roasting techniques. Consistent extractions of gold, in excess of 96%, are achieved.IntroductionConventional methods of alkaline leaching of gold arsenosulphide concentrates in an autoclave» and under electrochemical oxidation( 2) require expensive equipment as well as high-cost energy and reagents. In other cases, processing of gold concentrates inelude: ultra-fme grinding, calcination at 200°C to 250°C with the addition of an oxidant (NH4NOJ followed by alkaline leaching of the resulting calcine with hydrogen peroxide, the latter being a complicated method yielding poor extractions of gold (89% to 90%). In this paper, a hydrometallurgical method is described for processing refractory gold arsenosulphide concentrates in which alkaline leaching is followed by cyanidation of the residue."

Jan 1, 1993

By Spencer PA, Winborne DA

The Porgera mineral deposit in Papua-New Guinea contains gold as finely divided, probably submicroscopic (1 um) particles dispersed in a pyrite sulphide matrix. Only 25-30% of the gold in pyrite can be extracted by direct cyanide leaching and fine grinding the pyrite does not increase gold extraction significantly. This work is concerned with the application of alkaline leaching at atmospheric pressure to oxidise the pyrite prior to gold recovery by cyanidation. The paper presents and discusses results of a comprehensive set of testwork using sodium carbonate, sodium hydroxide and calcium hydroxide alkaline media. Best results, at 60-70% gold extraction, were obtained with sodium hydroxide systems.

Jan 1, 1984

By P. N. H. Bhakta, X. P. V. Lei

The U.S. Bureau of Mines developed an alkaline oxidative pretreatment to increase the recovery of gold from refractory sulfide ores containing arsenopyrite (FeAsS) and/or pyrite (FeS2). Pretreatment of a low grade ore containing 0.4% FeAsS, 3.5% FeS2, and 2.74g/mt Au with 1.2m NaOH, 40 psig o2, 100° C, and 4 hr in a 2-L autoclave resulted in 94 to 96% sulfide oxidation. cyanidation of the pretreated residue resulted in 88% Au extraction compared to <20% Au extraction for untreated ore. Cyanide consumption was 0.25 kg NaCN per metric ton of ore. A flowsheet for treating sulfidic low-grade gold ore is proposed.

Jan 1, 1990

"In conjunction with Banick Mercur , Wright Engineers Limited designed the first multi-stage aqueous pressure oxidation system in an alkaline environment in North America. A three-stage slurry heating and slurry pressure let-down with heat recovery was an innovative approach to the problem of high energy cost with the low sulphur content of the ore. The design of this system required solving a number of problems associated with heating and pumping of slurries at pressures up to 3230 kPa and temperatures in excess of 220°C. Wright received an Award of Merit in October 1989, from the Canadian Consulting Engineering Awards Committee in the Process Category for its design of the Mercur pressure oxidation system.The Mercur Mine is located at the south end of the Oquirrh Mountains in Tooele County about 65 miles by road from Salt Lake City, Utah, U.S.A. The original mill was designed to process 5000 tpd of oxidized gold ore in a carbon-in-leach (CIL) circuit. A new ore treatment system was required to treat the substantial quantities of refractory ore reserves which could not be treated economically in the existing mill. The new pressure oxidation circuit makes the refractory ore amenable to gold recovery by oxidation of the sulphides contained in the ore .Wright Engineers Limited was commissioned by Barrick Mercur Gold Mine Ltd ., a wholly-owned subsidiary of American Barrick Resources, to design the pressure oxidation facility. This followed completion of a feasibility study by Wright which showed that the project was technically and economically viable.Work on the feasibility study began in 1986, detail design commenced in February 1987, and was completed in December 1987. Mechanical installation was completed in January 1988, and slurry feed began in February 1988. The plant has operated since June 1988 without major interruption . Wright was responsible for engineering, procurement and construction management for the addition of the 750 tpd pressure oxidation circuit to the existing mill and also had commissioning responsibility.Major unit processes involved in the pressure oxidation facility are ore thickening, three-stage slurry preheating, pressure oxidation and three-stage slurry cooling with heat recovery, stearn generation, propane and oxygen storage and a video graphic display process control system were included in the scope as process anciliaries.The pressure oxidation increases gold recovery from the refractory ore by conventional CIL from as low as 35% to more than 86070. A patent has been granted for the process."

Jan 1, 1989

By P. T. O'Kane

"INTRODUCTION Wright Engineers Limited is active and has extensive experience in the design of pressure oxidation plants for the treatment of refractory gold ore and concentrate. In addition to the Mercur oxidation facility, similar projects undertaken during the last few years include the 3000 ton per day acid oxidation plant for the Getchell Mine, Nevada, a concentrate treatment autoclave for Asamera in 1984 and a feasibility study of a 50-tons per day autoclave plant for Consolidated Professor's Duport property in Ontario. The Equity Silver antimony and arsenic precipitation autoclaves, and the low pressure autoclaves at Key Lake Uranium in Northern Saskatchewan were also engineered by Wright in recent years.In August 1986, Wright was retained by Barrick Resources (U.S.A.) Inc. to add a 750 tpd pressure oxidation circuit to the existing mill at their Mercur Gold Mine, Utah. This paper describes this facility and outlines the start up in the spring of 1988 and subsequent successful operation. MERCUR OPERATION The Barrick Mercur Gold Mine is located at the south end of the Oquirrh Mountains in Tooele County, Utah. The carbon-in-leach (CIL) process plant camnenced operation in 1983 with a design throughput of 3000 tpd and has since been expanded to handle rrore than 5000 tpd. 'IWo ore types are mined, an oxide ore and a refractory ore, which is by Barrick's definition ""an ore from which less than 60% gold extraction is achieved by a standard CIL amenability test."" Typical of many sulphidic/carbonaceous ores, the refractory gold does not respond well to the cyanidation process. As a result, refractory ore has been stockpiled since the start of mining. As of June 1987, Mercur had about 570,000 tons of refractory ore stockpiled and 5.7 million tons of proven and probable reserves, grading 0.06 oz/t including approximately 3.5 million tons of tailings from the previous operation grading 0.55 oz/t. In 1986, Barrick decided to build a 750 ton per day pressure oxidation plant to process these ores. The process, for which a patent was granted, comprises pressure oxidation of the sulphides in an alkaline environment. It was developed in 1981 but was set aside in preference of a CIL plant treating only oxide ores by the previous owner of the property, Getty Oil. This process is capable of increasing gold recovery from the refractory ore from as low as 35% by conventional CIL processing to more than 85%. In 1986 Wright Engineers was engaged to undertake a feasibility study based on batch testing. This lead to a decision by Barrick Mercur later that year to build the commercial pressure oxidation plant. Wright was awarded the contract for engineering, procurement, construction managernent and commissioning in March 1987. The plant was mechanically complete in January 28, 1988, and was in operation by February 1988."

Jan 1, 1989

By Peng Qian, Xiao Li, Guoyan Fu, Peiwei Han, Yongliang Wang, Shufeng Ye

"Arsenic removal from high arsenic gold concentrate is conducive to increase the mineral value and realize the mineral comprehensive utilization. This study investigated the alkaline pressure oxidation leaching (APOL) of an arsenical gold concentrate in lower temperature to separate the arsenic into filtrate. The effects of temperature, NaOH concentration, solid-liquid ratio, reaction pressure and stirring speed on the leaching efficiency were investigated, and the characteristics of the gold concentrate and leaching residues were evaluated using XRD and SEM analysis methods. The obtained results showed that by using lower temperature (<60?) it is possible to avoid the production of elemental sulfur during the leaching process, which is benefit for the oxidation of arsenopyrite; When the reaction was carried out under appropriate conditions, more than 98% of the arsenic will be extracted into the filtrate as the form of arsenate or thioarsenate. INTRODUCTION In recent years, there has been considerable interest in the recovery of gold from refractory arsenical gold ore. The arsenic in this kind of ore usually exists as the form of arsenopyrite, and the gold surrounded by arsenopyrite exists as the form of fine particles, which will give many difficulties to metallurgical work. When the content of arsenic in the gold ore is more than 0.5%, it will seriously penalize the smelting efficiency (Ruiz et al., 2014). So pretreatment of the refractory gold ore is necessary to obtain the largest economic value and environmental benefits (Balaz et al., 2000). The oxidation pretreatment processes mainly include roasting oxidation, pressure oxidation, chemical treatment and biological treatment (Ann et al., 1992; Kou and Chen, 2012; Tongamp et al., 2010; Fernandez et al., 1995; Lin and Zheng, 1996). Among them, the roasting oxidation and pressure oxidation are the more ordinary processes in industry. The roasting oxidation process is the mature and adaptable technology, but the production of waste gas containing sulfur and arsenic will pollute the environment. The pressure oxidation method possesses certain metallurgical and environmental advantages, and can efficiently decompose the mineral. According to the different solution media, the pretreatment can be divided into alkaline and acidic pressure leaching pretreatments (Papangelakis and Demopoulos, 1990; Hiskey and Sanchez, 1995). The acidic pressure oxidation method has been used in most smelting plants for high arsenic gold ore pretreatment, while the alkaline pressure oxidation is still not widely applied. Compared with the acidic pressure oxidation, the alkaline pressure oxidation can be realized under lower temperatures."

Jan 1, 2018

By V. G. Papangelakis, A. Dani

"Alkaline pressure oxidation, particularly in the presence of trona as additive, can be used to oxidize high carbonate refractory gold ores as it prevents the formation of CO2 in the autoclave. However, the presence of silica in the ore can lead to the encapsulation of pyrite due to the solubility of silica in alkaline solutions and its subsequent reprecipitation on the reacting pyrite surface, leading to the formation of a passive layer. This fundamental study investigates the chemical composition and thickness of the passive layer on a rotating pyrite disk using XPS and SEM, as a function of process conditions at 230°C, in an aqueous slurry containing silica sand, sodium carbonate and calcium carbonate under 100 psi of oxygen overpressure.INTRODUCTIONAlkaline pressure oxidation (POX) has been used successfully to treat high carbonate refractory gold ores, which contain carbonates in addition to sulphides, most notably at Mercur and Goldstrike mines operated by Barrick Gold. There are several advantages to using alkaline POX over its acid counterpart, especially due to high carbonate content. First, it eliminates the production of CO2 in the autoclave without resorting to pre-acidification, which allows for high oxygen utilization. Second, alkaline POX suppresses the formation of elemental sulphur and intermediate sulphur oxyanions (such as thiosulphates and dithionates), which reduce gold recovery. Third, the use of sodium hydroxide or salts prevents the formation of anhydrite scale by forming soluble sodium sulphate. Other advantages are the use of cheaper materials of construction and the formation of non-acidic residue that do not require further treatment prior to cyanidation (Thomas, 2005).Sodium carbonate in particular is a promising reagent for alkaline POX because it was shown to significantly accelerate the kinetics of pyrite oxidation by molecular oxygen, when compared to other reagents such as sodium hydroxide, sodium borate and calcium hydroxide. This has been attributed to the following factors: a) the faster oxidation kinetics of Fe2+-CO3 complexes compared to Fe2+ oxidation, b) the buffering effect of carbonate ions on the acidic pyrite surface, and c) the increased solubility of oxidizing Fe(III) species due to carbonate complexation (Caldeira, 2010)."

Jan 1, 2012