Hydrolyzed precipitation of iron

Iron is an impurity element hydrometallurgical the most commonly encountered. Its abundance in nature and its chemistry with many elements of the periodic table (such as Ca and Mg in the second main group element and the first transition element Ti, V, Cr, Mn, Co, Ni, Cu) The similarity makes it often subject to elemental substitution, so that if not all of the minerals of these elements, at least most of them contain iron. As the iron content in solid solution in mineral binding from a trace amount (<0.5wt%) to a large number (> 10wt%) range, the amount of iron in the sphalerite substituted up to 17.4% of zinc, pentlandite (Fe, Ni ₉ S ₈ contains up to 43% iron. Therefore, iron is contained to varying degrees in various leach liquors and process solutions in hydrometallurgy. The table below lists the estimated quantities of soluble iron produced by acid leaching or pickling in several major metal production processes. Therefore, the hydrolysis of the iron-containing solution naturally becomes the most important and most common reaction for the precipitation of iron in hydrometallurgy, and most of it is to remove iron impurities from the leachate and various process solutions, mainly from the sulfate medium. An additional benefit of removing iron by precipitation is that it can remove other harmful elements such as arsenic by co-precipitation with iron.

Table Estimated amount of soluble iron produced in some metallurgical industries

Under the oxidation potential and pH conditions encountered in hydrometallurgy, the iron in the solution has only two valence states, divalent and trivalent. As seen from Fig. 1, Fe ^{3 +} is far from the precipitation line of Zn ^{2 +} , Cu ^{2 +} , Co ^{2 +} , Ni ^{2 + ,} etc., indicating that the iron compound can be selectively precipitated by hydrolysis at a low pH of 3.5 to 5. Iron is removed from the solution of these metals. Fe ^{2 +} does not precipitate even under neutral conditions. Therefore, the problem of iron removal by precipitation in hydrometallurgy is based on the hydrolysis of Fe ^{3 +} , and Fe ^{2 +} needs to be oxidized to Fe ^{3 +} before being effectively removed.

The hydrolysis of iron is a very complicated process. The nature of the solution and the conditions of hydrolysis have an important effect on the hydrolysis results, resulting in different hydrolyzed products and different crystal structures. Because of this, there are many kinds of iron oxides in nature. There are 13 kinds of iron oxides, oxyhydroxides and hydroxides known to date, including ferrihydrite (Fe ₅ HO ₈ · 4H ₂ O), hematite (α-Fe ₂ O ₃ ), erythro ore. (γ-Fe ₂ O ₃ ), magnetite (Fe ₃ O ₄ ), goethite (α-FeOOH), tetragonal fibrite (β-FeOOH), fibrite (γ-FeOOH) and hexagonal fibril Mine (δ'-FeOOH). Except for goethite and hexagonal fibrite, the remaining iron oxide minerals may be good crystals. Figure 2 depicts the formation conditions of common iron oxides and the routes and transition conditions between them.

Figure 1 Metal hydroxide precipitation diagram 25 ° C

Figure 2 Common iron oxide formation and conversion routes and conditions

In addition to oxides, oxyhydroxides and hydroxides, it is possible to form complex salts by combining certain anions in the solution during the hydrolysis of iron. The most typical example is pyrite. Some of these hydrolysates may develop into compounds that remove iron from solution in hydrometallurgy. The hydrolysate selected as iron removal should have the following properties:

(1) It should have a small solubility, so that the residual iron in the solution can be minimized;

(2) It should be able to precipitate at a lower pH value to avoid loss of main metal precipitation during iron removal;

(3) It should be easy to crystallize, and the crystal grains are larger, which is convenient for filtration and washing;

(4) There should be a large hydrolysis rate, so that the iron removal process can be completed in a short time;

(5) It is better to coprecipitate with other harmful impurities in the solution to simplify the solution purification process;

(6) The hydrolysis and precipitation process should be as economical and simple as possible.

There are four types of iron-plating methods that have been developed and industrially applied, all of which are precipitated by a neutralization hydrolysis method. Three of them are used for iron removal, which are developed from the zinc hydrometallurgical industry and first industrialized. The iron compounds precipitated by them are called the yellow iron sputum method, the goethite method and the hematite method. The fourth type is mainly used for magnet synthesis. The following describes three methods for removing iron from water.

Redox potential and pH are two important factors controlling the behavior of iron in aqueous solution. The oxidizing environment is favorable for iron precipitation, and the reducing environment promotes iron dissolution; the acidic condition is favorable for iron dissolution, and the alkaline condition is favorable for iron precipitation. The equilibrium concentration of high-iron ions is strongly affected by the change of pH value of the solution. When the pH is <3, the equilibrium concentration of high-iron ions decreases by 2 to 3 orders of magnitude for every unit of pH increase. Therefore, simply increasing the pH of the high-iron solution for hydrolysis produces a large degree of supersaturation, causing a large nucleation rate and causing colloid precipitation. When the iron in the solution is more than 5 kg ∕m ³ , the precipitation of the colloidal Fe(OH) ₃ produced by the neutralization hydrolysis is difficult or even impossible to filter or settle. Such a precipitate entrains a large amount of solution, causing a serious loss of valuable components and cannot be used for iron removal in industrial production.

Temperature also has an important effect on the behavior of iron. High temperatures promote iron precipitation and allow precipitation to occur at lower pH. Therefore, the most important factors controlling the degree of precipitation of Fe ^{3 + in the} solution and the stability of the precipitate are temperature and pH. There are two main methods for inducing a hydrolysis reaction: heating the solution or neutralizing with a base. Babukan's hydrolysis of 0.5% ∕L Fe ₂ (SO ₄ ) ₃ -KOH to synthesize jarosite in the range of 20-200 ° C clarifies the temperature-pH relationship of its formation, as shown in Figure 3. The shaded portion in the figure is the stable region of jarosite, and as the temperature increases, the stable region tilts toward the pH value. The pH of jarosite formation ranges from 2 to 3 at 20 ° C, while the pH range extends from 1 to 2.3 at 100 ° C and pH from 0 to 1.2 at 200 ° C. When the pH is lower than the pH of this stable zone, no precipitation is formed. When the pH is higher than this zone, various other iron compounds are formed due to the difference in temperature. It is particularly noteworthy that hematite is formed above 100 °C and goethite is formed at lower temperatures. It appears that a pH between 1.5 and 1.6 is the ideal acidity for the formation of jarosite at 100 °C. The degree of precipitation of jarosite increases with the initial pH of the solution, and the initial pH is higher to form another iron compound.

Figure 3: The stability zone formed by jarosite and the relationship between temperature and pH

(precipitation from 0.5 mol of ∕LFe ₂ (SO ₄ ) ₃ solution at 20 to 200 ° C)

High-iron concentration liquids also have an important effect on the precipitation of iron. Determination of the isotherm of the Fe ₂ O ₃ -H ₂ SO ₄ -H ₂ O three-component system shows that at 110 ° C, the ferric sulfate acidic solution, precipitated at the lowest iron and acid concentration is goethite α-FeO (OH), yellow iron sputum H ₃ OFe ₃ (SO ₄ ) ₂ (OH) ₆ occurs at medium iron concentration, and another compound Fe ₄ (SO ₄ ) (OH) between yellow iron sputum and goethite ₁₀ ) It forms at a lower iron concentration and may be formed when the iron concentration is only a few g∕L in the late formation of the yellow iron sulphide. Only at a very high concentration of ferric sulphate, there is Fe ₃ (SO ₄ )(OH). generate.

A more in-depth discussion of the physical chemistry of iron hydrolysis precipitation can be found in the literature.

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