Abstract
Recently, huge amount of waste has been recycled in cement plant, and many kinds of aversive substances (chlorides, alkalies, sulfates and heavy metal salts) have been brought into cement manufacturing process with the wastes as raw materials. These substances are usually removed by chlorine bypass system from cement kiln-preheater, called as “K powder”. Since cement plant is expected to use more wastes, “K powder” constituents should be separated with each other as recycled resources.
In this study, we tried sulfidized flotation to separate lead components from “K powder” with the recovery of more than 80% by changing the addition order of sulfuric acid and NaHS in flotation conditioning, and also clarify the reaction mechanisms. As a result, we found the optimum conditions of the new process in case of 150 g-“K powder”/L pulp density as follows. 1) Adding NaHS (molar ratio of NaHS/Pb in “K powder”=1.0) into the pulp, and mixing 15 min for sulfidization, 2) Adjusting the pulp pH under 4.0 by sulfuric acid, 3) Mixing the pulp 30-90 min for aging of gypsum formation at pulp pH under 4.0, 4) Adding the total PAX of 1.5 g/kg-“K powder” in 2 or 3 stages, and carrying out 40 min flotation. We also found that the K2Pb(SO4)2 formation which causes the decrease of lead recovery can be suppressed by adding NaHS prior to sulfuric acid addition. Additionally we noted that the new process can decrease the addition of NaHS and PAX to four fifth and half respectively, and can extend the acceptable range of pulp density, gypsum formation pH, gypsum aging time, and flotation pH for good separation compared with the conventional process, then, the process control could become simplified. As the result, we could separate lead components in “K powder” with the recovery over 80 % as a lead concentrate over 40 wt% Pb by this new sulfidized flotation process.
