**3. Magnetic iron seeding and separation**

Han et al. [25, 31–32] combined seed induced crystallization and magnetic separation, and proposed a novel magnetic seeding and separation process, as shown in **Figure 5**. Before the iron is precipitated as goethite, fine-grained maghemite or magnetite particles are added to the leaching solution to make the goethite precipitate and grow on the surface of the magnetic particles, thereby avoiding mixing with the calcium sulfate precipitation in the solution. The iron precipitates on the surface of the goethite to form large magnetic particles with a core-shell structure, and the precipitates are efficiently settled and separated by magnetic separation. The results show that the iron content in the dry iron residue is more than 52% and the Ni content is less than 0.6%, which can be used in industrial applications to deal with a large amount of iron precipitation. After the calcium sulfate precipitation is roasted, 99% of S and As can be removed, and the roasting residue can be respectively used as raw materials for ironmaking and building materials.

**Figure 5.**

*The process of iron precipitation on the magnetic seeds and the magnetic flocculation in magnetic field [3, 25].*

*Iron Ores*

loss of valuable metals.

industrial applications.

**38**

**Figure 4.**

**Figure 3.**

*(2 g/L limonite seeds, pH 2.1–2.5, 85°C) [30].*

*The specific surface area and the nickel grade of iron precipitates with limonite seeds in different size ranges* 

*(a) SEM images of the goethite precipitate at different pHs and (b) pH effect for the nickel loss, the* 

*crystallinity, and the specific surface area of the precipitate [23, 25].*

filtration difficulties and metal loss are composed of amorphous iron phase, six-line ferrihydrite, poor crystalline goethite, solid solution jarosite phase and silica [2, 22]. Therefore, the crystallinity, size and content of the goethite particles can be controlled by adjusting the pH, thereby improving the separation performance and the

Yue and Han [23] study that as the pH value decreases from 5.0 to 2.0, as shown in **Figure 3**, the crystallinity of goethite decreases, the goethite particles tend to agglomerate, the particle size increases significantly, and the filterability of the precipitate improves. Nickel is lost in the iron precipitate by being incorporated into the crystal lattice and adsorbed on the surface of the goethite particles, and the nickel adsorption loss are related to the specific surface area of the goethite particles. When goethite is in an intermediate transition state at low pH (2.5–3.3), which is between the crystalline state and the colloidal state, the loss of nickel is the least. However, the improvement by only adjusting the pH of the goethite precipitation process is minimal. Chang et al. [24] carefully reduced the pH from 4.0 to 2.5, and the loss of nickel is only reduced by about 10% in the iron precipitation. Moreover, it is not realistic to achieve such detailed condition control in actual

Yue et al. [31] applied magnetic iron seeding and separation to separate goethite from calcium sulfate in zinc leaching with maghemite fine particles as carrier. As is shown in **Figure 6**, the magnetic goethite-maghemite aggregates were separated effectively from calcium sulfate precipitates by magnetic drum separator, and 90% of Fe and Ca is respectively recovered in two corresponding products. Roasting goethite precipitate with coal powder under the optimum conditions removed 99% of S and As. Goethite products can be directly used in the ironmaking industry, and calcium sulfate precipitation can also be used to produce cement and building materials.

Yue et al. [32] establish the surface complex and precipitation model of goethite on magnetite and maghemite magnetic nanoparticles, as shown in **Figure 7**. The formation of Fe (III) surface complexes are directly related to the nucleation and

**Figure 6.**

*Schematic illustration of magnetic separation and production of desired goethite and gypsum product [31].*

#### **Figure 7.**

*Surface precipitation model modeling (a) of Fe3+ adsorption/precipitation on magnetite and maghemite with corresponding magnetic separation of goethite, images of the suspensions in a magnetic field with 2 g/L (b) magnetite and (c) maghemite NPs, and SEM images of goethite precipitates with (d) magnetite and (e) maghemite NPs [32].*

**41**

**Figure 8.**

*Magnetic Separation of Impurities from Hydrometallurgy Solutions and Waste Water Using…*

precipitation of goethite on the solid surfaces of the two magnetic nanoparticles. The more polynuclear surface complexes produced on the particle surface, the more precipitation of heterogeneous forms. Fundamentally, it is possible to screen out the best material as the crystal nucleus to separate goethite from calcium sulfate or

The Cr-bearing electroplating sludge is produced from the treatment of Cr wastewater and metallurgical processes [33–36]. It contains excessive amounts of heavy metals, such as Cr, Fe, Ni, Cu, Pb and Zn, or potential dioxin pollutants [37–38], therefore must be treated before stacking. Many methods have been applied to recover Cr from the acid leaching solution of electroplating sludge, such as electrochemical precipitation (ECP) [39], selective extraction [35, 40], adsorption or biosorption [41–44] and Cr-Fe coprecipitation [45–48]. Compared with other methods, recovering Cr by Cr-Fe coprecipitation is simple, economical and practical for industrial applications. In addition, the advance coprecipitation of

Fe and Cr can avoid their interference on the recovery of Ni, Cu and Zn.

Yue et al. [49] use the novel magnetic seeding and separation process 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. The active hydroxide radicals on the surface of magnetic seeds induce the nucleation and growth of goethite, which results in enhanced Cr (III)-Fe(III) coprecipitation. As shown in **Figure 8**, 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. The results of the two-stage coprecipitation showed that the total recoveries of Cr and Fe were 96.17 and 99.39%, respectively, and the grades of Ni, Cu, and Zn in the precipitates were 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

*SEM images of the Cr(III)-Fe(III) coprecipitates without maghemite fine particles (a) and with maghemite fine particles (b), respectively; scheme (c) of the formation of* γ*-Fe2O3/Crx Fe1-xOOH with core-shell structure [49].*

**4.1 Recycling Fe and Cr in Cr-bearing electroplating sludge**

*DOI: http://dx.doi.org/10.5772/intechopen.93728*

other heterogeneous precipitation.

**4. Application and prospect**

development of resources and environment.

*Magnetic Separation of Impurities from Hydrometallurgy Solutions and Waste Water Using… DOI: http://dx.doi.org/10.5772/intechopen.93728*

precipitation of goethite on the solid surfaces of the two magnetic nanoparticles. The more polynuclear surface complexes produced on the particle surface, the more precipitation of heterogeneous forms. Fundamentally, it is possible to screen out the best material as the crystal nucleus to separate goethite from calcium sulfate or other heterogeneous precipitation.
