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The separation of Cr(III) from the polymetallic leaching solution is a typical problem encountered during the recycling of electroplating sludge. In this paper, a novel method was proposed to recover Cr(III) and Fe(II) synchronously by forming the Cr(III)-Fe(III) coprecipitates on the surface of maghemite (γ-Fe2O3) fine particles with Fe2(SO4)3 solution as the supplemental Fe source. The results showed that high Fe(III) to Cr(III) ratio in solution promoted the hydrolysis and precipitation of Cr(III). In addition, the maghemite particles, served as the crystal nuclei, could induce the formation of the core-shell structured Cr(III)-Fe(III) coprecipitates on its surface and accelerate the sedimentation of the coprecipitates in the magnetic field. By using a two-stage Cr(III)-Fe(III) coprecipitation process, the recoveries of Cr and Fe reached 96.17% and 99.39%, and the grades of Ni, Cu, and Zn in the coprecipitates were at 0.41%, 0.38%, and 0.22%, respectively. The obtained coprecipitates can be recycled as the feed material of chromium smelting after heat treatment. This method is simple and efficient for high-concentration Cr3+ solution treatment, which is beneficial for the sustainable development of resources and environment.

Respirable sintering dust was an abundant and hazardous waste arrested by electrostatic precipitators from iron and steel industry. In this study, this waste was first found to contain 0.3% Rb, existing in the form of chloride and making it a promising Rb extracting material. Meanwhile, 95.24% of Rb and 93.43% of K in it can be easily recovered by water leaching (S/L = 1:5), resulting in a solution of 740 mg/L Rb and 33.85 g/L K. Then this leachate was extracted with 4-tert-butyl-2-(α-methylbenzyl) phenol (t-BAMBP) in sulphonated kerosene to separate Rb from K after purification. The optimized solvent extraction conditions were 1.0 mol/L t-BAMBP, phase ratio of 3:1, 1.2 mol/L alkalinity in aqueous. Moreover, the organic phase was treated with deionized water (O/A = 5:1) for scrubbing and 1.2 mol/L HCl (O/A = 5:1) for stripping. Under optimal conditions, a single Rb extraction and back-extraction efficiencies were 91.80% and 89.93%, respectively. Finally, a RbCl with 99.5% purity and a total Rb extraction rate of 58.26% were achieved by a multistage continuous countercurrent extraction. These results did not only indicate that respirable sintering dust is a new tremendous potential Rb source but also provide a promising way to utilize Rb from this dust.

The goethite process has gained renewed attention due to the generation of environmentally acceptable sludge in hydrometallurgical processing of nickel. Goethite precipitation at low pH (2.1 to 2.5) contributes to minimizing the loss of nickel, but results in iron precipitates with poor filterability. In this paper, crystallization induced by natural limonite seeds was used to improve the filterability of iron precipitates. The X-ray Diffraction, Scanning Electron Microscopy and Laser Particle Size Analysis show that limonite seeds could induce the growth of goethite crystals and reduce the generation of amorphous iron phases at low pH. The limonite particles of size range − 74 + 37 μm were the most suitable for inducing goethite crystallization. A smaller specific surface area of goethite particles by limonite seeding and hot acid washing contributed to a decrease in the adsorption on to the precipitates.

The goethite process was conducted in nickel hydrometallurgy to examine the effects of pH levels (2.0–4.3) on the performance of iron removal and associated rate of nickel loss. The goethite precipitate was characterized by its crystallinity, particle size and microtopography using X-ray diffraction, laser particle size analyzer and scanning electron microscopy. Nickel adsorption was analyzed to study the nickel loss adsorbed on the goethite surface, and energy dispersive spectroscopy was used to detect the nickel distribution on the surface and interior of the goethite particles. Results show that as the pH decreases, the crystallinity of goethite lessens, goethite particles tend to agglomerate, the particle size increases significantly, and the filterability of the precipitate improves. Nickel is lost in iron precipitates by incorporation into the lattices and adsorption on the surface of the goethite particles. The nickel loss to the goethite lattices increases sharply with pH, and the nickel adsorption loss could be determined by the specific surface area of the goethite particles. This study demonstrates that a lower reaction pH causes the particle size of the precipitate to increase, improves the filterability, and results in a lower loss of nickel.

In nickel hydrometallurgical operations, the removal of iron from process liquors was critical, with the method of choice commonly being crystallization (precipitation). In this paper special attention was given to the magnetite precipitate. Potential/pH diagram for Fe–H2O system was derived at 100 °C, and indicated that the iron ion may precipitate as magnetite at the lower oxidation potential. And the crystallization experiments proved that the iron ion partly precipitate as magnetite by slow oxidation at pH 2.0–2.2 and temperature 90–100 °C. The precipitates could be efficiently separated from the solutions by magnetic flocculation and separation. Further, analyses with Environmental scanning electron microscopy (ESEM) and X-ray Diffraction (XRD) provided a fundamental understanding of the magnetite precipitation process.

The application and development of goethite process were hindered by the poor filterability of the precipitate and the great loss of nickel in nickel hydrometallurgy. In this study, the effects of pH on the goethite precipitation and the mechanism of the nickel loss were investigated. The goethite precipitation at a lower pH (2.4 to 3.0) guaranteed a higher recovery of nickel at the cost of the filterability of the precipitate. Then a novel process, magnetic seeding and separation, was proposed to improve the filterability of iron residues and simplify the iron removal process. The iron precipitated on the surface of the magnetic seeds to form a large particle with core-shell structures and the magnetic core endowed the particles magnetism. Efficient magnetic separation offseted the poor settleability and filterability of the precipitate. After magnetic separation of iron residue, the nickel solution was qualified and the dried iron residue contained more than 52% Fe and less than 0.6% Ni, indicating a promising alternative process for treatment of bulky residues of iron precipitate. Further analyses with Environmental scanning electron microscopy (ESEM) and X-ray Diffraction (XRD) provided a fundamental understanding of the new process.

The industrial application of the goethite process has been restricted due to the low recovery of zinc and the great loss of lead and silver in zinc calcine with iron residues. In this study, we examined the properties of + 35 μm zinc calcine and assessed its feasibility for serving as a neutralizing agent in the goethite process. Due to an insoluble fragment or porous structure, the particle size of + 35 μm zinc calcine only decreased slightly at pH 2–3 and 80–90 °C. Large differences between iron precipitate and leach residues in the particle size and specific gravity favored the separation, as proven by using the terminal settling velocity deduced from Stroke's law. A combined analysis of environmental scanning electron microscopy, X-ray diffraction, energy dispersive spectrometry, and laser particle size analysis provided a fundamental understanding of the properties and behaviors of different particles. This study proved that in the goethite process, + 35 μm zinc calcine was a suitable neutralizing agent that could work efficiently and be separated easily from iron residues in the goethite process. Moreover, it ensured the high recovery of valuable metals such as zinc, lead, and silver.