TECHNICAL FIELD

The present disclosure generally relates to systems and methods for reducing formation of CaSO4and/or Fe2O3containing deposits in a pressure oxidation autoclave during pressure oxidation of gold-containing ore and more particularly relates to systems and methods utilizing a scale inhibitor for reducing formation of the CaSO4and/or Fe2O3containing deposits.

BACKGROUND

In a broad sense, hydrometallurgy is a branch of metallurgy concerned with the science and art of extraction of metals from their ores by processes involving solutions in water. The processing of various types of ore and other materials can be conducted in various environments to produce a desired product. Although pressure oxidation of gold ores and concentrates does not directly recover gold in a strict sense of the above definition, it enables the gold to be recovered and accordingly is part of hydrometallurgy.

In hydrometallurgical processes for the extraction of metals from ores or secondary materials there are three basic procedures: (1) dissolution of a compound and/or metal from an ore or concentrate into a leach solution; (2) purification and/or upgrading of the leach solution, and (3) subsequent recovery of values from the purified solution. Besides the three basic procedures, there are processes in hydrometallurgy that are used, for example, as pretreatment steps. Such is the case in the pressure oxidation of refractory gold ores.

Within the gold industry, the term pressure oxidation is synonymous with pressure hydrometallurgy. Pressure oxidation refers to the oxidation of sulfides, such as pyrite [FeS2] and marcasite [FeS2], at elevated temperature and pressure. Oxidation releases encapsulated gold grains from the sulfides and makes the oxidation residue of the ore or concentrate more amenable for gold recovery by cyanidation in a subsequent leaching step. Pressure oxidation of gold ores is therefore a pretreatment leaching step to enhance gold recovery.

Pressure oxidation heats the gold-containing ore in an aqueous solution in closed vessels with sulfates being formed instead of sulfur dioxide gas. Within the gold industry, pressure oxidation of whole ores can be carried out using both alkaline and acidic processes and acidic pressure oxidation of sulfide concentrate are most commonly utilized. In these processes, slurries of ore from a grinding circuit are typically pumped to a series of thickeners and then typically to a series of acidulation tanks. Sulfuric acid is typically added to the slurries to destroy sufficient carbonate prior to entering a pressure oxidation autoclave. Process air is also typically injected into the acidulation tanks to aid in carbon dioxide removal. Then, the slurries are typically advanced from the acidulation tanks through a series of direct contact heater vessels before feeding into the pressure oxidation autoclave. The slurries are typically preheated as they pass through the heater vessels. The slurries typically enter the heater vessels at the top and cascade down the internal baffles, while flashed steam typically enters a lower section of the vessel and rises, contacting the slurries for direct heat transfer. The discharge from these vessels is then typically pumped into the pressure oxidation autoclave. The slurries then typically discharge from the autoclave and pass through a series of pressure letdown stages called flash vessels. Pressure and temperature are gradually let down to atmospheric pressure and temperature. The objective of the pressure-oxidation/autoclaving process is the destruction of the sulfides, such as pyrite, marcasite, or arsenopyrite, thereby liberating the occluded gold. The gold is then amenable to recovery by the cyanidation process.

Some of the oxidation and hydrolysis reactions that occur during this process are set forth below:

Oxidation Reactions
2FeS2+7O2+2H2O=2FeSO4+2H2SO4(1)
4FeSO4+2H2SO4+O2=2Fe2(SO4)3+2H2O  (2)
4FeS2+15O2+2H2O=2Fe2(SO4)3+2H2SO4(1)+(2)=(3)
4FeAsS+11O2+2H2O=4HAsO2+4FeSO4(4)
HAsO2+2FeSO4+H2SO4+O2=Fe2(SO4)3+H3AsO4(5)
4FeAsS+13O2+2H2SO4+2H2O=2Fe2(SO4)3+2H3AsO4+2HAsO2(4)+(5)=(6)
Hydrolysis Reactions
Fe2(SO4)3+3H2O=Fe2O3(hematite)+3H2SO4(7)
4FeS2+15O2+8H2O=2Fe2O3+8H2SO4(oxidation+hydrolysis)  (3)+(7)=(8)
Fe2(SO4)3+2H2O=Fe(OH)SO4(basic iron sulphate)+H2SO4(9)
3Fe2(SO4)3+14H2O=2H3OFe3(SO4)2(OH)6(hydronium jarosite)  (10)
3Fe2(SO4)3+M2SO4+12H2O=2MFe3(SO4)2(OH)6+6H2SO4(M=Ag+,NH4+,K+,½Pb2+)  (11)
Fe2(SO4)3+2H3AsO4=2FeAsO4+3H2SO4(ferric arsenate)  (12)
2FeAsS+7O2+2H2O=2FeAsO4+2H2SO4(oxidation+hydrolysis)  (6)+(12)=(13)

During these processes, Fe/As sulfides in the gold-containing ore are released and oxidized, thus producing sulfates. During oxidation, the reaction is often quenched with process water which includes CaOH2. Process water containing lime is also intrinsic to slurry fed into the reactor. This combination of reactants produces undesirable CaSO4scale that deposits onto on the equipment. Moreover, undesirable Fe2O3and other Fe-containing scale is also produced during the pressure autoclave oxidation process and also accumulates on the equipment. This accumulation requires periodic shut-down and cleaning of the equipment thereby leading to delayed production and increased costs.

Accordingly, it is desirable to provide systems and methods for reducing formation of CaSO4 and/or Fe2O3 containing deposits in a pressure oxidation autoclave during pressure oxidation of gold-containing ore. Furthermore, other desirable features and characteristics will become apparent from the subsequent summary and detailed description and the appended claims, taken in conjunction with the foregoing technical field and background.

SUMMARY

This disclosure provides a method for reducing formation of CaSO4and Fe2O3containing deposits in a pressure oxidation autoclave and/or adjacent circuits during pressure oxidation of gold-containing ore, wherein the gold-containing ore is combined with water to create an aqueous slurry that is heated and introduced into the pressure oxidation autoclave.

The method includes the step of providing a scale inhibitor that is free of an organic polymer and includes:

an inorganic phosphate according to formula (I), (XPO3)m(I), wherein X is Na, K, H, or combinations thereof, and m is at least about 6,

an inorganic phosphate according to formula (II), Yn+2PnO3n+1(II), wherein Y is Na, K, H,

and n is at least about 6,

an organic phosphonate (III); or combinations thereof.

The method also includes the step of combining the scale inhibitor and at least one of the gold-containing ore, the water, and the aqueous slurry to reduce the formation of scale in the pressure oxidation autoclave and/or adjacent circuits.

This disclosure also provides a composition for reducing CaSO4and Fe2O3containing deposits in a pressure oxidation autoclave and/or adjacent circuits during pressure oxidation of gold-containing ore. The composition includes an aqueous slurry of the gold-containing ore and water and the aforementioned scale inhibitor.

This disclosure further provides a system for reducing formation of CaSO4and Fe2O3containing deposits in a pressure oxidation autoclave and/or adjacent circuits during pressure oxidation of gold-containing ore. The system includes a substrate having a surface in contact with at least one of the gold-containing ore and/or an aqueous slurry of the gold-containing ore and water and the aforementioned scale inhibitor, such that the scale inhibitor exhibits a reduced formation of the CaSO4and Fe2O3containing deposits on the surface as compared to a system free of the scale inhibitor.

