Soda slags

Tellurium is stUl recovered in some copper refineries by the smelting of slimes and the subsequent leaching of soda slags which contain both selenium and tellurium. The caustic slags are leached in water and, using the controlled neutralization process, tellurium is recovered as tellurium dioxide. 

An economic analysis of the HRD flame reactor technology was performed by the U.S. EPA using 12 separate cost categories. Based on the assumptions made in the economic analysis, the estimated cost for treating secondary lead soda slag (SLS) ranges from 208 to 932 per ton. 

A somewhat similar product, Roechling phosphate, uses a soda slag that is a byproduct from the steel industry as the source of sodium. Also, the naturally occurring minerals, trona (sodium sesquicarbonate) or natron (sodium carbonate), may be used. Experiments have shown that a similar product can be made by sintering potassium carbonate with phosphate rock and silica to produce a product grade of 0-25-25. The phosphate compound in this product is presumed to be CaKP04. 

Although soda slags provide a ready means of capturing sulfur and provide a low viscosity slag/ matte at relatively low temperature for the efficient separation from bullion, there are significant associated problems, which may be listed as follows ... 

Soda slags are very aggressive to chrome magnesite brickwork and to steel if exposure occurs. 

Feed is composed of battery materials, other lead-bearing residues and scrap, fine coke or coal, and fluxes. The fluxes used depend on the type of slag and can be soda ash (sodium carbonate) and shredded iron scrap for the production of soda slags or limestone, silica and iron for the production of silica slags. Separate bins and weighfeeders for each component are used to continuously blend the feed, which is fed by conveyor into the kiln. 

Rotary kiln smelters are operated at the Harz-Metall plant at Goslar, Germany and at the Pedricktown plant of NL Industries in New Jersey, USA. The Harz-Metall kiln of 3.1 m internal diameter by 40 m long had an average capacity of between 130 and 300 t/d of lead bulhon depending on the nature of the feed material processed. Performance details are reported as in Table 11.1 on the basis of the production of a soda slag. 

Short rotary furnaces have been used for processing whole batteries as well as mixed components and separated pastes and metalUcs. The furnace has been widely used for processing pastes with the production of soda slags. A typical sulfur balance for a short rotary furnace is given in Table 11.3. 

Sulfur distribution in a rotary furnace with soda slags. 

The problems associated with soda slags detailed above have led to a shift to desulfurisalion of pastes and the reduction of desutfurised pastes with the production of lime silica slags. This has necessitated furnace operation at higher tanperature and the use of oxygen enrichment of the gas burner. 

Table 11.4 illustrates performance of a short rotary furnace of 3.5 m diameter for various feeds and with the production of a soda slag. 

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