Zinc plant residue of National Iranian Lead and Zinc Co.’s (NILZ) after brine leaching and water washing was used for all tests. The metal contents of feed were determined by using an Atomic Absorption Spectrometer (Perkin-Elmer, 300). The leachates were acidified with nitric acid to prevent precipitation of metals. Merk hydrochloric acid was used to acidify the brine solution. In cementation stage, aluminum powder with 99.5% purity was used.

All the tests were carried out in the 1 L glass beakers to optimize the parameters. The plastic material shaft was used to avoid cementation of lead. To prevent evaporation during the tests, top of the beakers was covered. All solutions for cementation were obtained by leaching of secondary zinc plant residue (obtained after filtration of acid leaching and water washing of zinc plant residue) in the 300 g/L NaCl brine solution at time 30 min and temperature 30˚C. Aluminum powder was added suddenly to the solution and the effects of cementation temperature (40˚C, 50˚C, 60˚C, 70˚C), time of cementation (10, 20, 30, 40, 60, 90 min), initial pH of solution (1, 3, 4, 5, 6), aluminum content (1, 1.5, 2 stoichiometry × Pb), stirring speed (50, 100, 150 rpm) and initial lead content of brine solution (5.63, 11.77, 16.36 g/l) were investigated.

The results of aluminum addition at the initial pH = 4 on the lead recovery are shown in Figure 1. With addition of aluminum powder, pH of solution decreases.

The results show that the rate of lead cementation increases with raising the amount of Al at the same time and lead content of solution remained negligible after 90 min.

According to Equation (4), increasing of aluminum causes reaction progress.

With addition of aluminum, contact surface reaction increases that leads to increasing of lead cementation.

The results of increasing temperature on the recovery of lead are shown in Table 1 and Table 2.

The results show that increasing temperature has small effect on the lead recovery.

Small increase in the cementation recovery is because of increasing in the diffusion of reduced metal on the formed film of oxidized metal surface (Table 1, Table 2).

Some exams were carried out to examine the effect of stirring velocity on the cementation of lead. Table 3 shows the effect of stirring velocity on the lead cementation at T = 30˚C.

According to lead cementation by adding the aluminum powder, in fewer stirring velocity, the remained aluminum powders were surrounded by the precipitated lead particles, so contact surface decreases for the cementation reaction.

In high stirring velocity, according to Equation (6) oxygen leads to decrease in the cementation recovery (Table 3).

Pb + 2H₂ + 1/2O₂ = Pb²⁺ + H₂O (6)

The results of initial pH of the solution before addition of Aluminum powder on the lead recovery are show in Table 4.

The results show that increasing of pH from 4 to 6 decreases the cementation efficiency.

Increasing initial pH of solution before cementation, causes in precipitation of some lead as hydroxide.

It should be mentioned that with increasing of initial pH of solution, some Aluminum precipitates as hydroxide without its roll in lead cementation.

At initial pH of the solution less than 4, cementation process delays because of releasing Hydrogen ion according to Equation (7).

Pb + 2H⁺ = Pb²⁺ + H₂ (7)

As increasing of lead content in the solution, causes in reduction of hydrolyze pH, cementation recovery reduces. By the way, it should be prevented of increasing pH before cementation (Table 4).

The results show low cementation recovery about a solution with initial pH = 3 in comparison with a solution with initial pH = 4.

Some tests carried out to determine the initial lead content on the cementation recovery. So it was prepared some solutions with different initial lead content.

The results show that with increasing initial lead content, cementation recovery reduces.

According to Equation (8), increasing of lead content, increases driving force of cementation.

Pb²⁺ + 2e → Pb E₀ = −0.126 E = E₀ + RT/ZFln[Pb²⁺] (8)

Table 5 shows the initial lead content on the lead recovery.

Investigation by XRD shows Massicot (PbO), Lead (Pb), Halite (NaCl), Litharge (PbO) in the cementation product.

Table 6 shows chemical analyze results that the obtained product has a high purity and hence it can be used as Caldo furnace feed. Final product also is suitable because of its valuable elements such as Silver, Antimony.