The feeding material , which are normally Alluvial and crude gold particle from leaching process after rough refinery at 60-90% purity, is crude gold or partially alloyed gold,it needs to be tested for impurity content,differentiate and purify the material according to feeding material property, bulk alloy gold in this process can very be finely processed by centrifugal tableting, which can improve the dissolution efficiency. The crude gold is firstly dissolved in aqua regia, and then leached to obtain a chloroauric acid solution. After hydrolysis and reduction, sponge gold powder is obtained. The gold powder is smelted and splashed, then dried and weighted, and quickly cast ingots with casting machine. In general, 90% gold undergoes a wet refining process, and the purity of its gold powder can reach 99.95%. However, the purification of some high-quality alloy gold with high impurity content is unstable. The smelting ingot can reach 99.99%, and the waste gas produced by it is discharged up to the standard after being treated by the waste gas equipment, and the waste water generated is subjected to acid-base neutralization to precipitate most of the metal slag and then discharged to the waste water treatment
Recovery of crude silver from fractional gold slag：Heat the chloroauric acid solution, and quickly add sodium sulfite solution while stirring, until the gold is completely reduced to gold powder precipitation, and then the solution is left to stand until it is colorless and transparent, and then filtered. The waste gas produced by this process includes NOX, HCl , sulfide and other waste gases, the waste water may contain a small amount of precious metals, which can be replaced and reduced by zinc wire, and the reduced waste water is pretreated by the waste water and then enters the waste water treatment system.
Gold Reduction：the chlorogold acid solution is heated in stirring process, and quickly add sodium sulfite solution until all the gold is reduced to gold powder precipitation, and then leave the solution until the solution becomes colorless and transparent before filtering the solution, this process produces exhaust gases including NOX, HCl, sulfide and other exhaust gases and wastewater may contain a small amount of precious metals, which can be extracted by adding zinc wire replacement to restore these precious metals, in this process, the wastewater is pretreated and then enters the wastewater treatment system.
Purification：The gold powder precipitation obtained by filtration may also adsorb some iron and silver compounds and other impurities, which must be removed to improve gold powder purity, hot concentrated nitric acid is added to the gold powder for soaking process, the waste nitric acid is collected in a storage tank after filtration and used for the next aqua regia liquid making.
Washing and Drying: The purified gold powder, after being washed repeatedly with hot distilled water until the solution is pH value meet 7 (almost no Cl-), is to be placed in full rest after filtration and drying ; this process will produce a certain amount of high temperature heat and baking exhaust, where the washing solution can be discharged, or could also be transferred to the purification process and hydrochloride acid for recycling application
Granular and Ingot Casting: the dried gold powder is sent to the bead sprinkling machine for granulation, gold particles after drying is sent to the ingot casting machine for rapid ingot casting into 1 kg standard Ingot which can be sold as end products, a full set of smelting furnace can be prepared for flexible processing of low-grade alloyed gold.
Extract Gold from Remaining Aqua Regia : in general, 90%~99.5% gold after a wet refining can reach to 99.99 grade, some impurity content is high in the purification of alloy gold, the product is not stable, if production cannot be up to standard request, the material can be purified once again. Gold powder after casting into ingots can be sold as finished products. For the reduction of a small amount of gold in the liquid, it can be extracted by zinc powder for reduction or activated carbon adsorption;
(1)Raw material pretreatment
This process contains gold material grade of 90% to 99.5%, taking into account the different impurity content and properties of the material affect the leaching efficiency, it needs to first detect and distinguish coarse gold size, such as large pieces of alloy gold can be with treated with centrifugal equipment into small partices because the small particles could be more easily dissolved by aqua regia and other solvents.
(2) Aqua Regia Dissolution
The process of dissolving crude gold in aqua regia, under the action of nitric acid, chloride ions and gold reaction, the formation of chloride complexes into solution.
Reaction is as follows.
Au + HNO3 + 4HCl = HAuCl4 + NO↑ + 2H2O
The process of gold dissolution in aqua regia is roughly as follows: feed the gold-containing materials into the acid-resistant reactor (titanium, glassware)
in ventilated condition, add 3~4 times the weight of metal in aqua regia, and dissolve under self-heating or later heating.
(3) Heating to drive out the nitrate
After the dissolution of the chlorogold acid for the next process to improve the direct receipt of the need to heat evaporation to about 1/5 of the original volume, so that the gold completely dissolved, and then add hydrochloric acid to drive out the free nitric acid to no red-brown gas occurs, take off the cooling, filtering to remove insoluble debris, water washing and collection.
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