Cementation of Lead From Lead Sulfate Slurries. II. Reaction-Mechanisms With Zinc Precipitant

Abstract

The cementation of lead from aqueous slurries of lead sulfate with zinc precipitant in stirred-bed reactors is characterized by two main reaction profiles. The mechanisms giving rise to both these profiles involve dissolution of lead sulfate and subsequent reaction of lead ions with zinc metal. Dissolution is slower than deposition when the latter step is unimpeded, and limits the overall rate. When mechanism 1 is operative, the reaction proceeds initially at a rapid rate but, after a relatively short period, slows significantly and may even stagnate. This behaviour arises from the lead depositing mainly on the outer surface of the lead/zinc agglomerates which impedes subsequent transport of liquor ions to and away from reaction sites. Mechanism 2 involves a slower initial rate but the reaction proceeds to completion at a steady rate. The absence of inhibition in this regime is associated with the formation of basic zinc sulfate crystals within the lead/zinc agglomerates. It is considered that this process keeps the agglomerates porous and allows continuous free access of ions of the liquor to the zinc precipitant. Thus, the relative rates of the dissolution of lead sulfate and the development of bzs determines which mechanism is operative in each situation. The rate of diffusion of sulfate into the agglomerates is an important factor in the formation of the bzs.

The implications of these findings for the treatment of industrial residues are briefly discussed.