**4. Application and prospect**

*Iron Ores*

materials.

**40**

**Figure 7.**

**Figure 6.**

*and (e) maghemite NPs [32].*

*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* 

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

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

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

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

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 development of resources and environment.

#### **Figure 8.**

*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].*

### **4.2 Removal of As in arsenic alkali residue**

Arsenic (As) is contained in most metal deposits, and therefore a large amount of arsenic-containing wastewater, flue gas and residues will be produced in mineral processing and smelting, posing a huge threat to the environment [50–52]. Commonly used methods for removing arsenic from solution include precipitation, electrocoagulation, ion exchange, membrane technology and adsorption [53–56]. In order to remove arsenic and recover valuable metals at the same time, these methods all require acid leaching of the waste, which will produce highly toxic and deadly arsine gas [54, 57]. As is shown in **Figure 9(a)**, Yue [58] developed a safer alkaline leaching method - oxidation alkali leaching of the wastes to transform arsenic compounds into arsenate ( <sup>3</sup><sup>−</sup> *AsO*<sup>4</sup> ) and subsequently recycling the alkali solution after arsenate removal, to treat the arsenic bearing wastes at a lower risk level.

There are a large number of reports that iron oxides have excellent adsorption and precipitation effects on heavy metal ions impurities in aqueous solutions, such as CrU and As. Garcı́a-Sanchez et al. [59–60] found that goethite has a special adsorption effect and capacity for As ions. Wei Jiang [61] considers that arsenic [ <sup>3</sup><sup>−</sup> *AsO*<sup>4</sup> ] absorbs on the surface of goethite by forming a bidentate-binuclear complex, and that pH and other metal ions in the solution will affect the distance and coordination number of As/Fe. His et al. [62] found that Uranyl can be adsorbed on goethite, amorphous ferric oxyhydroxide, and hematite sols at 25°C, and the adsorption effect on amorphous iron oxide is the strongest. Yue et al. [58] synthesized a series of high-concentrated ferric oxyhydroxide gels (HFGs) at different supersaturation to adsorb arsenate at high alkalinity, achieving zeroconsume of the alkali resources. As is shown in **Figure 9(b)**, using HFG(I) that synthesized under the lowest super-saturation condition as the sorbent to treat the oxidation alkali leaching solution of the copper slag from real industry, the residual concentration of arsenic (As (V)) could decrease from 2084 to 71.8 mg/L, which fully meet the requirements for high-concentrated arsenic stabilization at high alkalinity and alkali resource recycling. To further improve the efficiency of filtration and separation, magnetic seed sowing and separation technology can also be introduced to make this process more complete. Related research is underway.

#### **4.3 Removal of phosphate and starch in wastewater**

Phosphorus and starch reportedly are the main wastewater contaminants that are difficult to remove efficiently [63–64]. When the phosphorus concentration in water exceeds 0.02 mg/L, phosphorus becomes a polluting element and causes eutrophication of water bodies [65–67]. Starch is a commonly used and cheap material, widely used in many chemical and material industries, but it produces high concentration of organic wastewater, which will affect the environment [68–70]. Therefore, phosphate and starch removal from wastewater has become the focus of many studies. The main phosphate and starch removal methods are similar, such as chemical precipitation [71–74], biological methods [75–79] and adsorption techniques [80–83]. Among them, Chemical precipitation and adsorption technology is commonly used in wastewater treatment due to the simple operation with low cost and large processing capacity compared to other methods [84–86]. However, chemical precipitation inevitably produces a large amount of fine precipitation and suspended solids, which seriously affect the sedimentation and filtration efficiency [84, 87]. And the adsorbents currently used in adsorption technology, such as activated carbon [70, 88], silica gel [89–90], membranes [91–93], etc., have

**43**

**Figure 10.**

**Figure 9.**

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

high production costs and poor adsorption performance, which greatly limits the

Magnetic flocculation is an effective way to remove ultrafine suspended solids in water treatment [94–95]. It adds magnetic seeds to the aqueous solution to form magnetic flocs with the ultrafine suspended solids in the wastewater, and then passes through a magnetic separator to achieve rapid precipitation and separation [3, 95–96]. The combination of magnetic flocculation and chemical precipitation can make up for the shortcomings of ultrafine suspended solids and low separation efficiency of chemical precipitation. Magnetic flocculation has been widely used to treat wastewater with high pollution concentration [71], high turbidity [96] and high chemical oxygen demand (COD) [97]. It is worth noting that in many studies, iron-bearing minerals have shown the characteristics of removing phosphorus from aqueous solutions [98–99]. The iron-bearing minerals can be coordinated with phosphate and therefore have the potential to be used as adsorption materials for

*(a) Flow diagram of the comprehensive treatment of the arsenic alkali residue and (b) arsenic removal from* 

*arsenic alkali solution with different HFG samples synthesized at pH 3(I), 7(II), and 11(III) [58].*

*The chemical precipitation and magnetic flocculation of removed hydroxyapatite contaminants [103].*

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

adsorption effect and industrial applications.

phosphorus and starch in wastewater [100–101].

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

high production costs and poor adsorption performance, which greatly limits the adsorption effect and industrial applications.

Magnetic flocculation is an effective way to remove ultrafine suspended solids in water treatment [94–95]. It adds magnetic seeds to the aqueous solution to form magnetic flocs with the ultrafine suspended solids in the wastewater, and then passes through a magnetic separator to achieve rapid precipitation and separation [3, 95–96]. The combination of magnetic flocculation and chemical precipitation can make up for the shortcomings of ultrafine suspended solids and low separation efficiency of chemical precipitation. Magnetic flocculation has been widely used to treat wastewater with high pollution concentration [71], high turbidity [96] and high chemical oxygen demand (COD) [97]. It is worth noting that in many studies, iron-bearing minerals have shown the characteristics of removing phosphorus from aqueous solutions [98–99]. The iron-bearing minerals can be coordinated with phosphate and therefore have the potential to be used as adsorption materials for phosphorus and starch in wastewater [100–101].

#### **Figure 9.**

*Iron Ores*

level.

underway.

**4.2 Removal of As in arsenic alkali residue**

Arsenic (As) is contained in most metal deposits, and therefore a large amount of arsenic-containing wastewater, flue gas and residues will be produced in mineral

Commonly used methods for removing arsenic from solution include precipitation, electrocoagulation, ion exchange, membrane technology and adsorption [53–56]. In order to remove arsenic and recover valuable metals at the same time, these methods all require acid leaching of the waste, which will produce highly toxic and deadly arsine gas [54, 57]. As is shown in **Figure 9(a)**, Yue [58] developed a safer alkaline leaching method - oxidation alkali leaching of the wastes to transform arsenic compounds into arsenate ( <sup>3</sup><sup>−</sup> *AsO*<sup>4</sup> ) and subsequently recycling the alkali solution after arsenate removal, to treat the arsenic bearing wastes at a lower risk

There are a large number of reports that iron oxides have excellent adsorption and precipitation effects on heavy metal ions impurities in aqueous solutions, such as CrU and As. Garcı́a-Sanchez et al. [59–60] found that goethite has a special adsorption effect and capacity for As ions. Wei Jiang [61] considers that arsenic [ <sup>3</sup><sup>−</sup> *AsO*<sup>4</sup> ] absorbs on the surface of goethite by forming a bidentate-binuclear complex, and that pH and other metal ions in the solution will affect the distance and coordination number of As/Fe. His et al. [62] found that Uranyl can be adsorbed on goethite, amorphous ferric oxyhydroxide, and hematite sols at 25°C, and the adsorption effect on amorphous iron oxide is the strongest. Yue et al. [58] synthesized a series of high-concentrated ferric oxyhydroxide gels (HFGs) at different supersaturation to adsorb arsenate at high alkalinity, achieving zeroconsume of the alkali resources. As is shown in **Figure 9(b)**, using HFG(I) that synthesized under the lowest super-saturation condition as the sorbent to treat the

oxidation alkali leaching solution of the copper slag from real industry, the residual concentration of arsenic (As (V)) could decrease from 2084 to 71.8 mg/L, which fully meet the requirements for high-concentrated arsenic stabilization at high alkalinity and alkali resource recycling. To further improve the efficiency of filtration and separation, magnetic seed sowing and separation technology can also be introduced to make this process more complete. Related research is

Phosphorus and starch reportedly are the main wastewater contaminants that are difficult to remove efficiently [63–64]. When the phosphorus concentration in water exceeds 0.02 mg/L, phosphorus becomes a polluting element and causes eutrophication of water bodies [65–67]. Starch is a commonly used and cheap material, widely used in many chemical and material industries, but it produces high concentration of organic wastewater, which will affect the environment [68–70]. Therefore, phosphate and starch removal from wastewater has become the focus of many studies. The main phosphate and starch removal methods are similar, such as chemical precipitation [71–74], biological methods [75–79] and adsorption techniques [80–83]. Among them, Chemical precipitation and adsorption technology is commonly used in wastewater treatment due to the simple operation with low cost and large processing capacity compared to other methods [84–86]. However, chemical precipitation inevitably produces a large amount of fine precipitation and suspended solids, which seriously affect the sedimentation and filtration efficiency [84, 87]. And the adsorbents currently used in adsorption technology, such as activated carbon [70, 88], silica gel [89–90], membranes [91–93], etc., have

**4.3 Removal of phosphate and starch in wastewater**

processing and smelting, posing a huge threat to the environment [50–52].

**42**

*(a) Flow diagram of the comprehensive treatment of the arsenic alkali residue and (b) arsenic removal from arsenic alkali solution with different HFG samples synthesized at pH 3(I), 7(II), and 11(III) [58].*

**Figure 10.**

*The chemical precipitation and magnetic flocculation of removed hydroxyapatite contaminants [103].*

Du et al. [102–103] combined the magnetic flocculation technology with ironcontaining materials to prepare porous magnetic seeds with core-shell structure, which achieved simultaneous removal of starch and phosphate in wastewater. As shown in **Figure 10**, the core-shell magnetic seeds prepared by sulfation roasting of fine magnetite particles have a porous α-Fe2O3 structure on the surface, and the specific surface area is three times larger [103–106]. As shown in **Figure 10**, the phosphate and starch in the wastewater can be adsorbed on magnetic seeds surface, and then separated from the wastewater by magnetic separation. The phosphorus and starch content in the wastewater are reduced to 1.51 and 9.51 mg/L, respectively, and the removal rate reaches more than 75% [102].
