Experience in solving the problem of filtering and washing unstructured sediments of hydrometallurgical processing of polymetal sludge

The article aims to select a filter design and mode of washing cake in the hydrometallurgical processing of polymetallic sludge in order to minimize the volume of washing water and obtain a high-quality cake—iron oxide pigment precursor. An analysis of the chemical composition of the subject matter–pulp following hydrochloric acid treatment of polymetallic sludge—was performed using atomic absorption spectrophotometry and X-ray fluorescence spectrometry, while the particle size distribution was studied via laser diffraction. The analysis of the chemical composition revealed that the liquid phase contains, g/dm3 : MgCl2 90.5–105.6; AlCl3 35.8–37.6; NiCl2 8.1–9.0; FeCl3 24.5–27.1; CrCl3 5.9–6.5; MnCl2 2.1–2.4; HCl 6.5–7.7. The solid particles present in the pulp are represented by two distinct groups of particles having the following sizes: 0.1–1 µm and 1–120 µm. It was found that pulp separation using a chamber-membrane filterpress with the vertical arrangement of plates minimizes washing water volume up to S:L 1:1, returns the mother liquor captured by the cake to the process cycle, and helps to obtain high-quality washed cake with a minimum content of residual salts (0.5 wt%). The optimal temperature of the pulp entering the filtering process is 80℃, while that of washing water is 60℃. Cake washing was shown to be the limiting step of the pulp separation process, which can be realized via the displacement method given the use of acidified water with pH 1.5. It was found necessary to use filter cloths made of 100% polypropylene, with a specific weight of the fabric of 540 g/m2 ± 10%, air permeability of 6 L/dm2 per minute ± 30% at 200 Pa, and a maximum operating temperature of at least 90ºC. Thus, the specific capacity of the filter press was calculated and a standard machine with the vertical arrangement of plates and a filtering surface of 500 m2 was selected for pulp separation following hydrometallurgical processing of wet polymetallic sludge (8000 tons/year).

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