Brass is alloy containing copper and zinc metals with different proportions for targeted applications and to cater requirements of physical properties for its end uses as bathroom fittings, aesthetics, corrosion resistance and many more. Brass is manufactured in smelting units in widely accepted form of billets ranging its diameter, proportion, and length as per requirements. Copper has higher melting point (1085o C) whereas zinc evaporates in form of zinc oxide at 907o C.
Because of this limitations and differences in melting and evaporation temperatures of copper and zinc, common practice is opted to melt and purify copper and zinc is added at the end of operation to maintain ratio of copper and zinc to achieve low melting loss upon zinc evaporation. This purification process generates lumps of slag which floats over molten alloy and removed by available physical means. Removal process is never ideal and separation of slag always takes away molten brass along with slag removal.
Slag generated by brass industries fall under schedule IV category of pollution control board and strictly advised to dump in certified TSDF sites. This occupies major spaces in TSDF sites whereas material loss is major aspect associated with it. That is why recovery of precious metals is mandatory for economical and environmental reasons. Grade and percentages of copper and zinc in slag are prime factors to chose the right way for recovery of precious metals. Pulverization is mandatory prior to chose the recovery process as it will not only make it easy for recovery but also gives nearly precise sampling and accurate analysis of copper and zinc.
When copper and zinc contents are low, about 5 – 7 % copper and 15 – 25 % zinc, hydrometallurgy is followed for metal recovery. Hydrometallurgy involves mixing of pulverized residual brass slag with acids like sulfuric aci to form copper sulphate and zinc sulphate. Hydrometallurgy is more of a targeted practice where amount of acid to be used is directly related with amount of metals existing in slag. So regardless of handling cost, utility cost will be same for any number of copper and zinc concentrations. Repeated extraction is advisable for maximum recovery. When extraction is precise and controlled; very less amount of heavy metals will be left in residual sludge and can be used for land dumping, cement mixing, or concrete mixing. This is economically viable and also environmental friendly.
When copper and zinc contents are marginally high as 9 – 50% copper and 25 to 55% zinc, more effective pyrometallurgical recovery process is followed. From high grade brass slag, we can recover brass ingots, copper ingots and zinc oxide powder. When slag is melted in closed chamber furnace, slag will again float and we will have more dense molten brass at bottom and slag is removed by separation means. Copper ingots and zinc oxide powder can be recovered using rotary furnace which separates metals by difference in density and difference in evaporation temperatures as zinc evaporates at 907o C, bag filter followed by cooling system will collect evaporated zinc oxide from opening of rotary furnace where as all which is left will be some more residual slag floating over densed molten copper at bottom of furnace. Slag is remover to obtain 98-99.5% pure copper ingots. This slag can be pulverized up to 2 to 5 mm particle size and used for cleaning with sand blasting of rusty metal surfaces. Recycling of slag to achieve economical and environmental stability is a practice that everyone should follow to make world a better place.