A section of the article on gold recovery techniques in the May issue of the magazine had to be cut due to space constraints. It covered the paper Basic iron sulphate – a potential killer for pressure oxidation processing of refractory gold concentrates if not handled properly presented at this year’s SME annual meeting by Chris Fleming of SGS. He suggests investigating the use of a hybrid Hot Cure/Lime Boil process.
He notes “refractory gold concentrates often contain sub-microscopic gold that is encapsulated within the crystal matrix of iron sulphide minerals such as pyrite, pyrrhotite and arsenopyrite. To recover the gold, the host mineral must generally be broken down chemically, by oxidative processes such as roasting, pressure oxidation or bacterial leaching, which expose the gold for subsequent recovery by leaching in cyanide solution. The focus of attention in these pretreatment processes is usually the oxidation of the sulphides to elemental sulphur, sulphur dioxide gas or sulphate ions. Less attention is paid to the deportment of iron and the changes in its oxidation state, although this can have a profound effect on gold and silver liberation, as well as down stream operating costs.”
His paper dealt specifically with basic iron sulphate; the conditions under which it is formed in an autoclave, the problems that are caused by its presence in the feed to a cyanidation plant, and possible remedial strategies that can be adopted, both in the autoclave and downstream.
He concluded: “Basic iron sulphate is the product of a hydrolysis reaction that occurs when pyrite and other iron sulphide minerals are oxidised to ferric sulphate and sulphuric acid in an autoclave. The proportion of iron in the autoclave feed that is converted to basic iron sulphate increases with increasing ferric ion and sulphuric acid concentrations in solution, and with decreasing temperature in the range 180 to 250°C.
“The alternative and much preferred hydrolysis product is hematite, which is favoured at lower acidity in the autoclave solution (10-20 g/litre) and higher temperatures (>200°C). In practice, the production of a hematite autoclave residue with minimal basic iron sulphate formation is difficult to achieve without a significant capital cost penalty, and the formation of basic iron sulphate is a reality in all commercial autoclave operations. It can cause serious operational, economic, environmental and health and safety problems in downstream cyanidation plants for gold/silver recovery.
“Basic iron sulphate is only moderately stable under atmospheric conditions, and can be decomposed either under alkaline conditions, which converts it to ferric hydroxide and gypsum precipitates, or under acidic conditions, which converts it to ferric sulphate in solution. Basic iron sulphate is only truly stable in an autoclave, at high temperatures (>140°C) and in the presence of reasonably high acid concentrations in solution (>30 g/litre).
“If the autoclave discharge is to be leached with cyanide for gold recovery, it is very important to destroy most of the basic iron sulphate in the residue prior to cyanidation. If this is not done, it is very difficult to maintain a pH of >10, which is needed to maintain cyanide in the free cyanide form rather than the toxic, gaseous HCN form. The pH constantly drifts downwards due to the slow consumption of lime by basic iron sulphate, which leads to the formation of HCN gas, and creates an unsafe working environment for the gold plant operators.
“One option is to neutralise the residue with lime at pH >10 prior to cyanidation, to convert the basic iron sulphate to ferric hydroxide and gypsum before the acid and cyanide have a chance to react. But this process is slow (up to 24 hours), consumes vast amounts of lime (up to 200 kg/t is not uncommon) and produces slurry with very poor rheology, owing to the presence of the fine precipitates.
“The much preferred option is to break down the basic iron sulphate under acidic conditions in the Hot Cure process. This process is somewhat faster (typically six to 12 hours) than high pH neutralisation, requires no additional reagents, and most importantly, allows all the iron and acid associated with basic iron sulphate to be neutralised with limestone, at a fraction of the cost of lime. If the solid residue and solution phases are separated by CCD or filtration prior to cyanidation, the precipitates of ferric hydroxide and gypsum that are formed during neutralisation with limestone can be kept out of the cyanidation feed, greatly improving slurry rheology in the gold plant (leach and Merrill Crowe, CIP or CIL).
“A potential drawback of the process is that silver recovery by cyanidation decreases somewhat after hot curing. This is thought to be due to the slow formation of a silver jarosite compound during hot curing, and the effect can be very minor or it can be very significant. In all cases, the operating cost and operational benefits afforded by the process have to be weighed against the loss of revenue due to lower silver recovery.
“In those cases where loss of silver revenue is significant to the economics of the project, a possible flowsheet option to investigate would be to operate a hybrid Hot Cure/Lime Boil process. By operating Hot Cure before lime boiling, most of the sulphate in the autoclave residue could be decomposed and separated from the solids prior to lime boiling, which should significantly lower lime consumption in the lime boil process.”