Recovery of Cr(VI) from vanadium precipitated solution by precipitation with PbSO4

The recovery of Cr(VI) from vanadium precipitated solution obtained by adding Ca(OH)2 into chromium-containing vanadate solution and ball milling was investigated by precipitation with PbSO4. It was found that Cr can be effectively precipitated from the solution by PbSO4. The precipitate is mixture of PbSO4 and PbCrO4.PbO. The concentration of Cr remained in the solution decreases from 2.360 g/L to 0.002 g/L by adding PbSO4 into vanadium precipitated solution according to PbSO4/Cr molar ratio 3.0 under pH 11.5 and stirring for 120 min at 30 °C. After filtration, the precipitates were leached in H2SO4-Na2SO4 system solution to obtain Na2Cr2O7 solution and the filtrate can be reused to leach the precipitate contain PbSO4 and PbCrO4.PbO.


Introduction
Vanadium and chromium are very important industrial elements and strategic metals which are widely used in the fields of metallurgy, chemical industry, material preparation and so on [1]. Due to the similar nature of vanadium and chromium, they are often in the form of symbiosis existing in minerals [2]. During the smelting process of vanadium bearing titano-magnetite, vanadium and chromium are mainly enriched in converter slag, which is usually called vanadium slag as well as the main material of V2O5 production [3].
After extraction of vanadium from the vanadium slag, wastewater containing Cr(VI) and a small amount of V(V) was generated. The Cr(VI) and V(V) in aqueous are harmful to lives, and sometimes can be fatal [4]. The wastewater is mostly treated by reduction with sodium pyrosulfite, followed by neutralization to form V-Crbearing reducing slag [5]. V-Cr-bearing reducing slag is dangerous solid waste, and several methods have been proposed to handling of it including direct leaching using NaOH+H2O2 [6]and NaOH leaching under electric field strengthening [7]. The treatment process of V-Cr-bearing reducing slag has recently made new progress, however it is still cumbersome and uneconomical [8]. On the basis of recovery of Cr from the hydrolyzed solution by copper salt precipitation, the modified process of V 2 O 5 production with vanadium slag is proposed, see Fig.1

Experimental
Chromium-containing vanadate solution was provided by Pangang Group Vanadium & Titanium Resources Co., Ltd.. The V precipitated solution was obtained by adding Ca(OH) 2 and ball milling, and its composition is listed in Table 1. The compositions of experimental samples were determined by chemical methods and inductively coupled plasma emission spectroscopy (ICP) with a PS-6 PLASMA SPECTROVAC, BAIRD (USA) [9]. The Xray diffraction (XRD) patterns were recorded on a Rigaku Miniflex diffractometer with Cu Kα X-ray radiation at 35 kV and 20 mA. The pH was determined with ORION-230A which was made in USA, the degree of accuracy is 0.01.   Table 1 as well.

Effect of initial pH value of solution
The effect of initial pH on the precipation of chromium was studied and the experimental results obtained by adding PbSO 4 into vanadium precipitated solution according to PbSO 4 /Cr molar ratio 3.0 under different pH and stirring for 120 min at 30 ℃. As seen in Fig. 3, Cr concentration decreases from 0.476 g/L to 0.003 g/L with the increase in pH from 8.0 to 11.5, and it is almost unchanged with the further increase in pH. However, Cr concentration sharply increases from 0.002 g/L to 0.645 g/L with the further increase in pH from 12.5 to 13.5. This indicates that formed lead chromate and lead hydroxide are dissolved into the solution at pH >13, and the chemical reactions can be expressed by equations (1) and (2). Therefore, the initial pH of 11.5 was chosen in the following experiments.

Effect of stirring time
The effect of stirring time on the precipation of chromium was studied and the experimental results obtained by adding PbSO 4 into vanadium precipitated solution according to PbSO 4 /Cr molar ratio 3.0 under pH 11.5 and stirring for different time at 30 ℃. It is observed from Fig. 6 that Cr concentration decreases from 0.680 g/L to 0.003 g/L with the increase in stirring time from 20 min to 100 min, and it is almost unchanged with the further increase in stirring time. Therefore, the stirring time should be ≥ 100 min.   Fig. 7 is the experimental results obtained by adding PbSO 4 into vanadium precipitated solution according to PbSO 4 /Cr molar ratio 3.0 under pH 11.5 and stirring for 120 min at different temperature. It can be seen that the effect of reaction temperature on precipitation efficiency of chromium was insignificant, and the Cr concentration in the solution was all about 2 ppm. Therefore, the precipitation of chromium should be carried out at 30 ℃.

Conclusion
A process of vanadium precipitation with calcium salt followed by chromium precipitation with lead salt was adopted to extract vanadium and chromium from chromium-containing vanadate solution. From the study results, several conclusions are made as following: 1. The initial pH of V precipitated solution and PbSO 4 addition had big influence on the precipitation efficiency of chromium.
2. Chromium in vanadium precipitated solution can be effectively precipitated with PbSO 4, and the precipitate is the mixture of PbSO 4 and PbCrO 4 ꞏPbO. The concentration of Cr remained in solution decreases from 2.360 g/L to 0.002 g/L by adding PbSO 4 into vanadium precipitated solution according to PbSO 4 /Cr molar ratio 3.0 under pH 11.5 and stirring for 120 min at 30 ℃.