Precious Metal Refining

Precious Metal Refining: Dore is a mixture of gold and silver typically containing less than 5% base metal impurities. The exact composition varies widely depending on its source and processing history. Dore producers, in deciding whether or not to refine their dore, can custom design a facility around a single feedstock. Refiners, in contrast, must handle a variety of feedstocks. The ratio of gold to silver and the type and relative amounts of impurities determine the most effective refining sequence.

The typical strategy adopted by large refiners is base metal removal followed by the separation of gold and silver (known as parting), culminating with fine gold production. Platinum group metals are recovered and separated after gold. Figure 2 shows how three types of dore bullion can be integrated into this general scheme. Since it is all encompassing, the processing path for low-silver high-copper dore is described below.

Smelting

After melting incoming dore for homogenization and sampling (discussed in a later section), most base metals are removed prior to parting gold and silver. This step is generally done pyrometallurgically, sometimes in the same equipment used for initial melting. In some cases, this step may be performed at the mine rather than at the refinery (for example, on-site retorting for zinc or mercury removal).

Typical of these pyrometallurgical operations is “dore furnace” refining as practiced by the copper producers. Decopperized tankhouse slime is placed in a small reverberatory furnace to form a slag layer which is skimmed off. Antimony is volatilized and collected as flue dust. Selenium and tellurium are partially volatilized, but are mainly collected into an alkaline slag.

A low-silver high-copper dore probably contains sufficient copper to warrant an oxidative smelt known as cupellation to form a copper oxide dross. This can be accomplished in a reverberatory furnace by oxygen lancing until the batch contains less than 10% copper. Dross consists principally of base metal oxides, but does contain enough precious metal to require further treatment, usually by leaching.

Silver Refining

Bullion from cupellation is cast into anodes for electrolytic refining. In this process, silver dissolves into a dilute nitric acid electrolyte and plates out on a cathodic surface (stainless steel or graphite) in a very dendritic, crystalline deposit. Gold and principal platinum group metals form slimes which are collected in anode bags. After drying, these slimes are sent to the gold refining area. Copper, the main electrolyte contaminant, is allowed to build to relatively high levels before solutions are removed and treated for silver recovery. Silver crystal is better than 0.999 fine and, after rinsing and drying, is cast into 1,000 ounce ingots for marketing.

Gold Refining

Two approaches have evolved for the treatment of gold containing residues. The first is high temperature chlorination of molten metal (Miller process) followed by gold electrowinning in an aqueous chloride solution (Wohlwill process). The second approach is hydrometallurgical involving aqua regia dissolution of granulated metal followed by silver chloride filtration and gold precipitation. In both cases, silver is parted from gold as silver chloride, which requires reduction to metal and electrorefining. Also, platinum group metals are retained in a chloride solution.

As a general rule, the Miller-Wohlwill process compares economically with aqua regia refining only for large throughputs. Small-scale refiners inevitably opt for dissolution due to lower capital and inventory costs.

Chlorination-Electrowinning

Chlorination is used to upgrade bullion to .95 fineness or better. Electrolytic, refining, known as the Wohlwill process, produces better than .9995 fineness, suitable for trading in international markets.

The Miller process is based on selective chlorination. The charge is melted under a borax/silica flux at about 1100°C and then chlorine gas is introduced through silica tubes submerged into the bath. Zinc, iron, tin and lead form volatile chloride species. Copper and silver, in contrast, form molten chlorides which report to the slag (Miller salt).

The Miller process is generally recognized to proceed in three stages. In the initial stage, chlorine gas is taken up slowly as the volatile species are formed. Stage II is characterized by rapid production of Miller salt. This salt is periodically skimmed from the surface. In the third and final stage, the remainder of copper and silver is removed. State III, again slow, is crucial to the operation since volatile gold chlorides can also form resulting in significant furnace losses. For this reason, chlorination is usually stopped near .99 fineness. The treatment of a single charge is generally complete in one shift.

Bullion from the Miller process, containing PGM along with some silver and copper, is cast into anodes and suspended in an electrolyte of dissolved gold and free hydrochloric acid. Cathode starter sheets are either titanium or gold strips. Gold is selectively plated at the cathode while silver forms an insoluble silver chloride slime. Copper, platinum and palladium are soluble in the electrolyte and are removed in a bleed stream.

The Wohlwill process is characterized by a high inventory cost which is minimized by operating at high current densities, sometimes exceeding 100 ASF. To achieve this, the electrolyte is heated and the soluble gold content is maintained at a high level. Air lifts are used to promote mixing and prevent stratification of the electrolyte.

Dissolution/Precipitation

The second gold refining process involves the dissolution of gold in aqua regia, a mixture of nitric and hydrochloric acids. This process is used in most small refineries and, to some extent, in any refinery employing a Wohlwill cell to make fresh electrolyte. Gold bullion in granular form is charged to the reactor. Aqua regia, in the ratio of one part nitric acid to four parts hydrochloric acid, is then metered in. Nitrogen oxide scrubbing represents a major cost for this process. Gold and PGM dissolve into solution leaving particulate silver chloride. After carefully filtering silver chloride away from solution, gold is precipitated, usually with a sulfur species such as sulfur dioxide gas or sodium metabisulfite. An important consideration is the form of the precipated gold. If it is too fine, problems arise in filtration and washing. If too coarse, solution entrapment can occur. These characteristics are controlled by the nature of the reductant and the rate at which it is added to solution. Precipitated gold is collected, washed and melted to yield a bullion that is at least .9995 fineness and, with proper precautions, can exceed .9999.

PGM Recovery and Separation

Platinum group metals provide a small but important source of revenue to most refiners. These metals include platinum, palladium, irridium, ruthenium, osmium and rhodium of which platinum and palladium are by far the most significant. Only these two elements will be discussed here.

Platinum and palladium are soluble in aqua regia, and for this reason follow gold through the refinery. Since they are less noble, gold is preferentially reduced, whether in the Wohlwill process or in the precipitation of gold from the aqua regia process. In either case, a solution containing platinum and palladium as chlorides must be processed for their recovery. In general, this is done by adding ammonium chloride to the solution, thereby precipitating platinum as ammonium chloroplatinate. This solid is filtered off and ignited to form platinum sponge. Palladium is then precipitated by the addition of dimenthyglyoxime which is in turn filtered off and ignited to form palladium sponge. Alternatively, by oxidizing palladium to its highest state with nitric acid or sodium chlorate (or even electrolytically), an ammonium chloro salt of palladium can be precipitated. Generally, multiple precipitations and washes are necessary to produce metal at the required purity.

Alternatively, platinum and palladium can be extracted and separated using solvent extraction or ion exchange.