Sulfidation treatment of metal compounds involved in plating sludges

The present study is concerned with the sulfidation treatment of metal reagents of Cu(OH)2, ZnO or Ni(OH)2 as a means of metal resources recovery. The sulfidation was conducted separately for each reagent by contacting the reagent with Na2S solution (S2– to Me2+ molar ratio of 1.5) for 1 h to 7 d. The solid to liquid (S:L) ratio was varied from 1:250 to 1:25. In the case of ZnO and Ni(OH)2, the conversion was determined based on thermogravimetric analyses of the sulfidation products, while XRD analysis was used as the main evaluation parameter for the conversion of Cu(OH)2 to copper sulfide. As a result, it was found that Cu(OH)2 was completely converted to copper sulfide within 1 h at all S:L ratios. In the case of ZnO, a two stage phenomenon was observed. A high conversion rate was observed within the first 12 h at all S:L ratios followed by a lower conversion rate from 12 h to 7 d. The maximum conversion of ZnO to ZnS was 60.3%, which was obtained after 7 d at S:L ratio of 1:25. Meanwhile, the maximum conversion of Ni(OH)2 was only 32.4% after 7 d at the S:L ratio of 1:25.

The conversion of Cu, Zn and Ni to metallic sulfides was studied by contacting separately the reagents of Cu(OH)2, ZnO or Ni(OH)2 with Na2S solution at various S:L ratios for time period varying from 1 h to 7 d. After contacting the samples with the sulfidation agent, the samples were dried and subjected to XRD and TG analyses to evalu- ate the conversion of these metals to their sulfides. The main conclusions are summarized as follows: • It was found that Cu(OH)2 was completely converted to CuxSy (mainly Cu7S4) which had the molar ratio of copper to sulfur of about 2 to 1. The conversion was achieved within 1 h of contacting Cu(OH)2 with Na2S solution and was irrespective of S:L ratio. When the contact period was extended up to 7 d (S:L ratio of 1:50), a gradual conversion of Cu7S4 to CuS (covellite) was observed. • The conversion of ZnO to ZnS was found to be strongly dependent on pH of the reacting system, which resulted from an increase in solubility of ZnO as well as from an increase in pH of the reacting system owing to the formation of more soluble zincates. The conversion ratio of ZnO to ZnS increased from 21.5% at pH value of 12.95 (S:L ratio of 1:250) to 60.3% at pH value of 13.7 (S:L ratio of 1:25), after 7 d. Interestingly, when the pH value of the reacting system was further increased by using 1M NaOH, and ZnO was treated with Na2S at S:L ratio of 1:25, the conversion obtained after 7 d was only 51.3%. It is considered that at a pH value above 13.7, ZnO started to rapidly dissolve into the Na2S solution and Zn2+ ions present were instantaneously precipitated with S2– forming a thin and almost monolithic layer on ZnO particles. The subsequent dissolution of ZnO was thus prevented leading to a lower conversion. • The conversion ratio of Ni(OH)2 to nickel sulfide was kept at a low value of about 17.8% for a S:L ratio of 1:250 and increased to only 32.4% when Ni(OH)2 was treated for 7 d at a S:L ratio of 1:25. It is considered that a relatively low conversion of Ni(OH)2 to nickel sulfide resulted from the low solubility of Ni(OH)2 at high pH values. • Finally, based on the results separately obtained for the metal reagents of Cu(OH)2, ZnO or Ni(OH)2, the sulfidation treatment of real Treated plating sludge: CuS, ZnS, Ni -compounds Separation by means of flotation CuS ZnS Ni - compounds Fig. 9. Proposed separation procedure of plating sludge treated with Na2S.