Smart Processing of Brass Smelter Residues

In this study a industrial survey was performed on the secondary products generated in the Dutch metal industry. From this industrial survey it was concluded that the Dutch metal industry is already far ahead with closing the materials cycle as for almost every product is a processing route to recover its metal content. The residues generated at LDM BV, a secondary brass producer located in Drunen, are currently being processed by a different company; however in their goal to have a zero footprint on the environment LDM BV (hereafter referred to as LDM) is looking for opportunities to process the secondary products on site. Two different kinds of products are generated at LDM; a filter dust containing around 80% ZnO, 8% C, 4% Cu and 1.5% Pb and a slag containing various amounts of copper ~45% and zinc ~40% in metallic and oxide form. A study was performed on the processing techniques available to process zinc containing residues and from this study several treatment options were designed and tested in the lab. The main focus was put on the recovery of zinc from the LDM filter dust as this is the first goal the company has set itself, however since the nature of the two materials is similar a combined treatment of both materials was investigated as well. First the pyrometallurgical reduction by utilizing the carbon contained in the LDM filter dust was tested at 1200oC. It was found that a maximum of 97% of the zinc could be recovered; a higher recovery at 1200oC was not possible as the remaining zinc was bound to gahnite (ZnAl2O4). The residue still contained 15% ZnO and did not form a slag. A combined reduction of both the filter dust and melting slag resulted in the recovery of 96.8% of the copper, 99.9% of the zinc (as ZnO) and 64.9% of the lead. This test was performed at 1400oC and it was found that at this temperature the gahnite would be reduced. Also by addition of lime and silica a suitable slag was formed which contained 2.18% CuO and 0.16% ZnO. Both the copper and zinc oxide produced had a purity of over 98%. Hydrometallurgical treatment was tested by using two different leaching agents, NaOH and H2SO4. It was found that the best leaching conditions for NaOH at a liquid solid ratio of 10 were 90oC using 320 g/L NaOH. After 15 minutes the maximum amount of zinc was already leached and total of 86% was leached. Lead was leached for 68% and Copper for 5%. Using 200 g/L H2SO4 at 65oC resulted in 96% of all the zinc to be leached and 90.5% of all the copper. Both residues contained mainly gahnite which is impossible to leach under atmospheric conditions. After an iron removal step for the acidic solution both leach liquors were purified with Zn dust cementation. A zinc product of 99.7% purity was produced through electrowinning for both solutions; however the power consumption for the NaOH solution was significantly lower at 2.2 kWh/kg opposed to 3.04 kWh/kg zinc produced Finally a conceptual flow sheet is given for both a pyrometallurgical and a (sulfuric acid) hydrometallurgical processing facility in a brass smelting plant.