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 a double salt 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.

I. Hydrolysis precipitation of yellow iron

Yellow iron scorpion is also commonly referred to as jarosite, and has a small solubility in acidic solutions.矾 refers to a double salt composed of two or more kinds of metal sulphates. It is easier to crystallize out of solution than its corresponding single salt, and can form larger crystal grains, which is favorable for solid-liquid separation. Yellow iron sputum is a group of double salts of Fe(III) basic sulphate, and its molecular formula can usually be written as M ₂ O·3Fe ₂ O ₃ ·4SO ₃ ·6H ₂ O or MFe ₃ (SO) ₂ (OH) ₆ Wherein M ⁺ is one of the following monovalent cations (or cesium ions): H ₃ O ⁺ , Na ⁺ , K ⁺ , NH ₄ ⁺ , Ag ⁺ , Rb ⁺ and Pb ^{2 +} and so on. In the chemical composition of the ferrotium, the ratio of high-iron ions to sulfate ions (Fe ^{3 +} : SO ₄ ^{2 -} = 1.5) is much larger than 1 ∕ 2, and thus belongs to the basic salt rather than the normal salt. Compared with the normal salt, it is formed under the conditions of low acidity of the solution and small content of SO ₃ , and can be regarded as an intermediate product of the transition of the hydroxide to the normal salt. In the normal salt, the bond of the high iron ion is an O ^{2 -} ion in the SO ₄ ^{2 -} ion, and in the hydroxide it is an OH ^- ion. When the acidity of the solution is increased, it will change to a positive salt, and when the acidity is lowered, a hydroxide will be precipitated.

There are six kinds of yellow iron scorpions in the natural world, namely: jarosite, sassafras, yellow ammonium iron, silver iron, yellow sodium iron and lead iron. They are all formed in an acidic environment, mostly the intermediate product of the oxidation of pyrite to limonite, which occurs mostly in the initial stage of the development of the sulfide ore oxidation zone. The type of monovalent cation M ^{+ has} an effect on the precipitation of scutellaria. In the range of 160-200 ° C, Na ₂ SO ₄ , Na ₂ CO ₃ , NH ₄ OH or K ₂ SO _{4 were added} as the monovalent cation source of precipitated pyrite, and it was found that the residual iron concentration in the solution after precipitation was very high. Unlike the same, the residual iron concentration decreases in this order, but the difference becomes smaller than 180 °C. Several jarosite oxalyl jarosite most unstable, can be seen grass jarosite _{H 3 OFe 3 (SO 4)} 2 (OH) 6 generated although there is no presence of alkali metal, but even so a small amount of an alkali metal It is converted into alkali metal yellow iron sputum, and the degree of substitution of the hydrated proton H ₃ O ⁺ by the alkali metal ion increases as the temperature rises. Potassium has the highest stability, and the NH ₄ ⁺ ion radius is larger than K ⁺ . Although the radius of Na ⁺ and Li ⁺ plasma is smaller than K ⁺ , they have more hydrated molecules and larger hydrated ions, so their The stability of the shovel is not as good as that of potassium. However, considering that the potassium salt is relatively expensive, industrial ammonium is usually the preferred source of monovalent cations for the precipitation of pyrite.

Once formed, the yellow iron sputum is very stable and insoluble in acid, so the precipitation reaction of the yellow iron sputum can be used to remove iron from the sulphate solution, thereby reducing the solubility of iron at a given acidity. The precipitation reaction can be expressed by the following formula:

(1)

As seen in the above formula, free acid is produced during the precipitation of the yellow iron sputum, and the pH of the solution required to neutralize to maintain the precipitation requirement is required as the reaction progresses. Therefore, the neutralizing agent used for precipitating the yellow ferrite is not only used to neutralize the initial acid, but also to neutralize the acid produced by the hydrolysis of high iron. However, as mentioned above, it is not appropriate to use a strong base such as sodium hydroxide for neutralization, and it is difficult to control the pH even with a very thin alkali. In the practice of electrolytic zinc plants, zinc calcine (mainly containing ZnO) is used as a neutralizing agent.

The literature collects the free energy data of various yellow iron scorpions. The equilibrium constant of the composition of the yellow iron scorpion dissociated into its constituents can calculate the solubility of iron under given conditions. The rate at which the jarosite precipitates is formed varies with temperature. The formation rate of pyrite is slow at 25 ° C, and precipitation from a solution having a pH range of 0.82 to 1.72 may take 6 months. Increasing the temperature improves the precipitation rate, and the precipitation speed becomes faster at temperatures above 80 °C, and it can be completely precipitated within several hours at 100 °C. The precipitation rate is significantly accelerated above 100 ° C, but there is an upper limit to the precipitation temperature in terms of the stability of the yellow iron. Although the upper limit of the temperature varies depending on the composition of the solution, 180 to 200 ° C seems to be the upper limit of the stability of the yellow shovel.

As mentioned above, in addition to pH and temperature, the formation and stability of pyrite is closely related to many factors such as the concentration of monovalent cations, the concentration of iron, and the presence or absence of seeds or impurities. If the yellow iron scorpion is regarded as a poorly soluble electrolyte, the dissociation reaction formula can be written as:

(2)

Correspondingly, the solubility product is written as

(3)

It can be seen that the addition of alkali metal sulphate promotes the formation of scutellaria. However, in the above formula, the concentration of monovalent cation M ^{+ is} the lowest, which has the least effect on the precipitation of iron in solution. The yellow iron strontium can be precipitated from a solution containing K ⁺ as low as 0.02 mol ∕L, but in general, iron precipitation The degree increases with increasing concentration ratio of monovalent cation M ⁺ to Fe ^{3 +} , and experiments have shown that the ideal M ⁺ concentration should satisfy the atomic ratio specified by the molecular formula MFe ₃ (SO ₄ ) ₂ (OH) ₆ . From the solution containing Fe ^{3 +} 0.025 to 3 mol ∕L, the yellow iron sputum can be completely precipitated, and the lower limit of the precipitation is 10 ^-3 mol ∕L. As long as there is an excess of M ⁺ ions present in the solution, the amount and composition of the precipitated pyrite are independent of the Fe ^{3 +} concentration in the initial solution. On the other hand, the concentration of OH ^- ion is the highest, and the acidity of the solution has the greatest influence on the precipitation of iron slag. Under the actual operating conditions of the plant (precipitation temperature ~ 100 ° C), the concentration of Fe ^{3 +} remaining in the solution during the precipitation of the yellow ammonium iron slag has the following relationship with the initial H ₂ SO ₄ concentration:

[Fe ^{3 +} ]/[H ₂ SO ₄ ]=0.01

The above formula shows that the higher the initial H ₂ SO ₄ concentration, the higher the Fe ^{3 +} concentration remaining in the yellow ferrite precipitate. And the longer it takes to reach equilibrium.

The precipitation of scutellaria is basically a process of nucleation and growth, and the amount and speed of precipitation are closely related to the use of seed crystals. The presence of a precipitation reaction in a homogeneous system to produce a solid surface may require an induction period, and the presence of seed crystals is expected to eliminate this induction period and accelerate the rate of iron sputum precipitation. Although many factors such as the wall effect and the purity of the reagents used may affect the nucleation process of the new phase due to the size of the reaction device, the literature on the role of the seed crystal is quite different, and some even think that the seed crystal has little effect, but The general view is to confirm the role of seed crystals in promoting the formation of yellow iron. The addition of seed crystals can greatly increase the precipitation rate of the yellow iron scorpion and inhibit the induction period, and the initial velocity of the precipitation increases linearly with the amount of seed crystal added. The addition of seed crystals also allows the precipitation of pyrite at lower pH and temperature.

The behavior of lead, silver and other divalent metals such as Cu, Ni, Co, etc. in the precipitation of pyrite can not be ignored. Lead can form lead bismuth under the condition that the acidity is not high:

(4)

The amount of lead iron is related to iron concentration and acidity. The higher the iron concentration, the higher the acidity of lead bismuth. These shovel also form solid solutions with other yellow iron scorpions such as scutellaria and alkali metal stellate. If the lead concentration in the solution originally has a recovery value, the formation of lead bismuth causes a loss of lead. In order to prevent the formation of lead iron sputum, three measures have been proposed. (1) The acidity is increased to prevent the formation of lead bismuth, and the lead bismuth is soluble in 1 mol ∕L sulfuric acid at 95 ° C; (2) Precipitated iron in the range of 180 to 190 ° C, lead iron in the temperature range is unstable; (3) effective precipitation of iron at a sufficiently high concentration of alkali metal ions, which will form an alkali metal more stable than lead iron Yellow iron shovel. For example, in a slurry in which Fe ^{3 +} is 0.1 mol ∕L, H ₂ SO ₄ is 0.1 mol ∕L, and PhS is 4.5 kg/m ³ , at 150 ° C, K ₂ SO ₄ or Na ₂ SO ₄ or (NH _{4 2} ) When SO ₄ is 0.3 mol ∕ L, the formation of lead bismuth can be effectively prevented. When the concentration of alkali metal ions is low, a mixture of alkali metal and lead is produced.

Precious metals such as silver are also easily precipitated into silver iron or silver-lead iron

(5)

When yellowing iron strontium is precipitated from a solution containing 100 × 10 ^-4 % or less of Ag, more than 95% of silver is incorporated into the iron sputum. The divalent metals such as Zn ^{2 +} , Cu ^{2 +} and Ni ^{2 +} are only incorporated to the alkali metal yellow iron samarium to a small extent, which makes the pyrite method easy to use for solutions from these metals ( In particular, the sulphate solution removes iron without causing metal loss. The order in which the metal is incorporated into the alkali metal yellow iron samarium is: Fe ^{3 +} >Cu ^{2 +} >Zn ^{2 +} >Co ^{2 +} >Ni ^{2 +} . But the amount of these metals incorporated into the lead iron is much larger. Trivalent metals such as Ga and In are relatively easily incorporated into the pyrite compound.

There is also a view that the divalent metal ion is substituted for Fe ^{3 +} in the structure of the yellow iron sputum rather than the alkali metal ion. The general tendency of the divalent metal to bind to the yellow iron sputum is enhanced with increasing ion concentration, pH and alkali metal ion concentration, and decreases as the Fe ^{3 +} concentration decreases.

2. Hydrolysis precipitation of goethite

Goethite is a type of iron oxyhydroxide called α-type iron oxyhydroxide α-FeO(OH). There are four kinds of hydroxy iron oxide homomorphisms in nature, and the other three are: tetragonal pyrite β-FeO(OH), fibrite γ-FeO(OH) and hexagonal fibrite δ-FeO(OH) . Goethite is the most common iron oxyhydroxide mineral in nature, reflecting its stability under weathering conditions. In fact, it is usually the product of the weathering of iron-bearing sulfide ore, oxidized ore, carbonate and silicate in nature. Studies have shown that goethite is the most likely product of high-iron hydrolysis in the range of pH 1.5-3.5 at the boiling point of atmospheric pressure and the total concentration of sulfate in the range of 3 mol ∕L. Most goethite contains other elements in the form of solid solutions.

Goethite may also be seen to type [alpha] - water iron oxide _{α-Fe 2 O 3 · H} 2 O, which is structurally the same as diaspore ore, is orthorhombic. In the crystal structure of goethite, there are only Fe ^{3 +} , O ^{2 -} and OH ^- 3 ions, and the mixing ratio of the three is 1:1:1. Where O ^{2 -} is at the apex of the octahedron, and Fe ^{3 + is} at the center of the octahedron and is surrounded by O ^{2 -} . O ^{2 -} ions and ions Fe ^{3 +} 4 phase coupling, i.e. common for between four octahedra, wherein each valence bond only 1/2 price. OH ^- ion is used in total between two octahedrons, and each valence bond is also 1/2. The high-iron ions located in the center of the octahedron have strong polarization, which causes the outer electron cloud of the surrounding coordination ions to shift, causing the electron clouds of the outer and negative ions to overlap and form a covalent bond. Since O ^{2 -} is more susceptible to deformation than OH ^- , the coordinating oxygen ion will have a stronger covalent bond than the coordinating hydroxide ion, that is, the polarity of the bond is weak.

Thermodynamic calculations indicate that goethite has a larger lattice energy than trihydrate iron oxide, indicating that goethite is more stable than the latter. Therefore, under normal conditions (less acidity and temperature not higher than 140 ° C), the thermodynamically stable structure of the high-iron hydrolysate should be goethite rather than colloidal ferric hydroxide. However, in practice, when the high-speed iron is precipitated from the aqueous solution by the neutralization method, the precipitate obtained is a colloidal iron oxide trihydrate rather than a crystalline goethite. The main reason for this is that pH has a great influence on the degree of supersaturation of high iron in the solution. Therefore, when the hydrolysis is moderate, the high super-iron supersaturation occurs with the increase of the pH of the solution, forming a large nucleation rate. The hydrolyzed product is precipitated as a limb. In view of the fact that it is difficult to control the supersaturation of the system due to the neutralization and hydrolysis of high-iron solution, it is necessary to avoid the precipitation of iron hydroxide in the rubber. The key is to control the high-iron concentration in the solution to a very low level, generally less than 1kg·m ^{- 3} . The goethite method is proposed for this problem. It adopts the hydrolysis conditions of air oxidation, low supersaturation and higher temperature, which is beneficial to the dehydration and condensation of hydrates, and also facilitates the orderly arrangement of the relevant particles, so that the hydrolyzed products are crystal rather than limbs. The goethite method has two modes to control high iron concentration. One is to first reduce the high-iron ions in the solution to a low price, and then neutralize to a pH of 4.5 to 5. At this time, because the high-iron concentration is very low, colloidal iron hydroxide is not precipitated, and ferrous ions are at this pH. No Fe(OH) ₂ precipitate formed under the value. Then, the ferrous iron is reoxidized into high iron at a temperature of about 90 ° C. A small amount of high iron ions are hydrolyzed to form a small amount of crystal nuclei, and slowly develop into goethite crystals and precipitate. The relevant reaction equation is :

(7)

High-iron reducing agents can have many options, but the reducing agents used in production should be inexpensive, easy to handle, and do not introduce any hazards after oxidation. From this practical point of view, zinc sulfide concentrate is the best reducing agent for the purification of the zinc sulfate electrolyte by goethite. As a result of reducing the high iron by zinc sulfide, the zinc in the ZnS enters the solution as Zn ^{2 +} ions, and the sulfur remains in the slag as a solid form of elemental sulfur, without any harm to the subsequent operation. The total reaction formula of zinc sulfide to reduce high iron is:

(8)

Thermodynamic calculations show that the standard electromotive force of this redox reaction is 0.506V, which has sufficient thermodynamic driving force. Practice has shown that the reaction speed is also relatively high, generally only 3 to 4 hours at 90 ° C temperature can reach a considerable reduction depth. For example, the equilibrium constant obtained from the standard electromotive force of the reaction formula (8) is Kc = [Fe ^{2 +} ] ² [Zn ^{2 +} ] ∕ [Fe ^{3 +} ] ² = 10 ^17.09 , and the activity of the zinc ion is 0.1. For mol∕L, [Fa ^{2 +} ]∕[Fe ^{3 +} ]≈10 ^{9 is obtained} , indicating that zinc sulfide makes the reduction of high iron more thorough.

The reoxidation of ferrous iron in the goethite method uses oxygen in the air as the oxidant. The oxidation reaction equation is:

(9)

The standard oxidation potential of air at a temperature of 25 ° C is E = 1.22 - 0.059 pH. At pH = 4, the standard potential of oxygen is 0.984 V, and only the standard potential of the Fe ^{3 +} ∕Fe ^{2 +} pair (0.771 V) is 0.213 V higher. However, since Fe ^{3 +} has been pre-reduced to Fe ^{2 +} at this time, the actual potential E of this pair Greatly reduced. For example, when Fe ^{3 +} /Fe ^{2 +} =10 ^-4 , E Dropped to 0.538 V, the potential of the oxidation reaction (9) was increased to 0.316V. At the same time, in the hydrolyzed iron system, the high-iron high-mass produced by oxidation is hydrolyzed and precipitated immediately, so that [Fe ^{3 +} ]/[Fe ^{2 +} ] in the system can always be kept at a lower value.

Ferrous oxide precipitation includes two successive steps of ferrous oxidation and high iron hydrolysis. The process of oxygen ferrous oxide includes the dissolution of oxygen, the diffusion of oxygen molecules from the phase interface to the interior of the solution, the adsorption of ferrous ions to oxygen molecules, the cleavage of oxygen molecules into oxygen atoms, and the electronic exchange between ferrous ions and oxygen atoms. Wait for multiple steps. The cracking of oxygen molecules into oxygen atoms is a key step in controlling the speed. Increasing the rate of oxygen molecular cleavage reaction can be carried out in three ways: by increasing the oxygen partial pressure, such as using an oxygen-enriched blast and using compressed air and maintaining the entire reaction process at a higher pressure to increase the temperature; using catalysis, generally with Cu ^{2 +} as a catalyst.

After the adsorbed oxygen molecules are converted into adsorbed oxygen atoms, electron transfer between the oxygen atoms and the ferrous ions occurs, and as a result, the ferrous ions are oxidized to high iron ions, and the oxygen atoms are reduced to O ^{2 -} ions. :

The other oxygen atom will also be reduced to the ion O ^{2 -} in the same manner, and the formed O ^{2 -} will strongly bind to the high iron to form a complex ion such as (Fe-O-Fe) ^{4 +} . It then with OH ^- ions incorporated, and further dewatered synthesis, goethite is generated:

Another model for the control of high-iron concentration by the goethite method was developed by the Australian Electrolytic Zinc Company. Instead of reducing it first, it directly adds the hot high-iron solution together with the neutralizing agent to the sedimentation tank at a controlled rate to make the high-speed rail. The concentration is maintained at 1 kg·m ^-3 or less. At a temperature of 70 to 90 ° C and maintaining the pH at about 2.8, goethite is continuously precipitated with the addition of high iron. The relevant reactions are:

(10)

3. Hydrolysis precipitation of hematite

Hematite Fe ₂ O ₃ trigonal system, the structure is corundum type, there are two crystal forms, namely α-Fe ₂ O ₃ (hematite) and α-Fe ₂ O ₃ (magnetic hematite). The transition temperatures of these two different crystal forms are approximately 400 ° C. γ-Fe ₂ O ₃ is thermodynamically unstable and is in a metastable state, and will change to α-Fe ₂ O ₃ at around 400 °C. Natural hematite α-Fe ₂ O _{3 is} mainly a weathered product of iron-containing silicates, sulfides and carbonates and is the most stable iron compound in the natural environment. The first product obtained by heating the iron hydroxide analyzed from the low-temperature solution water is iron oxide monohydrate, which is goethite, and then hemihydrate iron oxide, which is hemihydrate, and further heated to obtain α-type Fe ₂ O ₃ . The transition temperature of goethite and gamma-type Fe ₂ O ₃ is approximately 160 ° C. If a high-temperature hydrolysis method is employed, as the hydrolysis temperature is continuously increased, one water, half water, and anhydrous ferric oxide can be sequentially obtained. A pyrite process for high temperature hydrolysis in the industry for precipitating iron. The higher the temperature, the faster the hydrolysis rate, and the better the precipitation of iron at higher acidity. At a high temperature of 200 ° C, even if the sulfuric acid concentration is as high as 100 kg ∕ m ³ , the residual iron concentration in the solution can be reduced to 5-6 kg ∕m ³ .

Fourth, the application of iron hydrolysis precipitation in hydrometallurgy

The most typical use of hydrolyzed precipitation to remove iron is the practice of zinc roasting-leaching-electrowinning. Although the calcination is to convert zinc sulfide to zinc oxide, the iron in the raw material is almost completely combined with zinc to form zinc ferrite during the calcination process. Zinc oxide in the dissolution of calcine in dilute sulfuric acid can only reach a total leaching rate of 85% to 93%, while the zinc in the zinc ferrite leached with hot acid causes a large amount of iron to enter the solution, which has once become an electrolytic zinc production. The bottleneck problem. After painstaking and fruitful efforts, several iron removal methods capable of producing iron compounds that were easy to filter were developed in the mid and late 1960s, and the first industrial application in the electrolytic zinc industry, the roasting-leaching-electrowinning method has been long-established since then. Development has become the main method for producing electrolytic zinc. At present, 80% of the world's electrolytic zinc is produced by this method. These iron removal methods are also largely applicable to the iron removal practice of other solutions.

(a) Huang Tieyu method

The yellow iron shovel method is the most representative of the practice in the wet zinc smelting plant as an effective method for removing iron. The development of the yellow iron shovel method was successful in the mid-1960s, when the Australian Electric Zinc Company, the Norwegian Zinc Company and the Spanish Asturian Company independently developed the technology and applied for patents almost simultaneously. Since then, the yellow iron sputum method has been widely used and become the main iron removal technology in electrolytic zinc production. At present, at least 16 large electrolytic zinc plants in the world have adopted this technology. The yellow iron sputum method used to remove iron is to adjust the pH of the solution to 1.5 and maintain this pH value, and to add a monovalent cation to precipitate the yellow iron strontium from the acidic sulfate solution at around 95 °C. The most commonly used monovalent cations in the industry are NH ₄ ⁺ and Na ⁺ . After the precipitation of the yellow iron sputum, the concentration of iron in the solution generally falls to 1 to 5 kg ∕m ³ .

The typical operation of the yellow iron samarium method in wet zinc smelting is divided into three basic steps: neutral leaching, hot acid leaching and yellow iron sputum precipitation. In the neutral leaching stage, the acidic electrolytic lean liquid is neutralized by zinc calcine ZnO to obtain a zinc ferrite-containing slag and a neutral zinc sulfate solution for supplying zinc. The zinc ferrite slag is dissolved in the hot acid caused by the electrolytic lean solution of sulfuric acid in the hot acid leaching section, and the obtained Zn and Fe-containing leaching solution is treated in the precipitation section of the yellow iron slag, and the acidity is first adjusted by zinc baking. The alkali metal yellow iron samarium is precipitated by adding ammonium sulfate or sodium sulfate. The slag iron back to neutral leaching, and the yellow slag slag is discarded. It should be pointed out that the zinc ferrite contained in the zinc baking sand used as the neutralizing agent in the precipitation of the yellow iron slag will not dissolve and enter the iron slag residue, so the newly formed yellow iron slag residue should not be directly discarded, so as to avoid loss of baking. Undissolved zinc ferrite in the sand neutralizer. In view of the fact that the yellow iron sorghum is once stable, it is quite stable to acid. In practice, the yellow iron slag residue can be pickled under the conditions of similar hot acid leaching to dissolve the zinc ferrite remaining in the recovered slag, and the yellow shovel itself does not Dissolved.

The specific operating conditions and sequence of the three basic steps of the yellow iron sputum method are different in different manufacturers, but the purpose is the same; the maximum recovery of zinc without considering a small amount of associated elements such as Pb and Ag. For example, the hot acid leaching of zinc ferrite and the precipitation of yellow iron sputum can be combined into one, the so-called conversion method, and the total reaction is as follows:

(11)

The solution of this combined step can then be neutralized with fresh calcine to produce a solution supply solution and a slag return cycle. If the concentrate contains a relatively large amount of Pb and Ag, an additional procedure is employed to obtain a slag containing Pb ∕Ag, a precipitate of pyrite and a neutral Zn electrolyte. This type of process includes a pre-neutralization job. In the usual process of the yellow shovel, the calcination is used to reduce the acidity of the hot acid leaching solution, thereby rapidly and effectively precipitating the yellow shovel. The Zn ^{2 +} , Cd ^{2 +} , Cu ^{2 +} , Pb ^{2 +} and Ag present in the calcination are lost to the yellow iron slag. The introduction of a pre-neutralization operation between the hot acid leaching and the yellow iron slag precipitation operation can reduce the metal loss in the yellow iron shovel. In the pre-neutralization operation, a part of the acid in the solution is neutralized by calcination, and the resulting slag is returned to the hot acid leaching stage to dissolve Zn and Fe therein, while Pb and Ag remain in the lead-silver slag. The partially neutralized solution is then added to the desired neutralizing agent for precipitation of the yellow iron sputum.

Figure 4 is a schematic flow chart of the integrated yellow iron shovel method. Its design incorporates most of the improvements in the various yellow iron shovel schemes.

Figure 4 integrated yellow iron shovel method

In addition to being used in the wet zinc smelting industry, the yellow iron sputum method is also used as a de-ironing process in the extraction of metals such as copper, nickel and cobalt , especially in the sulphate system. For example, in the Chambishi roasting-leaching-electrowinning method for treating cobalt-copper concentrates, the iron removal before copper electrowinning is the use of jarosite iron. Since the sulphation roasting itself provides K ⁺ ions, there is no need to add high cost potassium sulphate when precipitating jarosite.

The advantage of the yellow iron sputum method is that the precipitation is easy to filter, and the loss of Zn, Cd and Cu in the precipitation is the least, and the sulfate and alkali metal ions can be controlled at the same time, and it is easy to combine with various hydrometallurgical processes. However, it also has its own defects, such as: 1) the cost of the reagents used is high; 2) the volume of the slag is larger, 1.4kg ∕ (m ³ · t), the heap occupies a large area; 3) needs sufficient washing Remove the harmful environment or the available metal; 4) Store under controlled conditions to avoid liberating harmful components from polluting the environment. It is expected to overcome these disadvantages by converting pyrite to hematite for thermal production by thermal decomposition or hydrothermal decomposition and recycling sodium sulfate/ammonium sulfate to the sedimentation operation of the yellow iron crucible.

(2) Goethite method

The technology for removing iron from precipitated goethite was first developed and industrialized by Vieille Montagne, a Belgian company, called the VM method. The key to successful precipitation of goethite is to maintain a low concentration of Fe ^{3 +} in the solution, such as <1kg ∕m ³ , otherwise a gelatinous Fe(OH) will be obtained in the pH range of the precipitated goethite (2 to 3.5). ₃ or basic ferric sulfate Fe ₄ SO ₄ (OH) ₁₀ . The VM method solves this problem by using a reduction-precipitation method. As shown in Fig. 5, 100 kg of ∕m ³ Zn, 25 to 30 kg of 3m ³ Fe ^{3 +} and 50 to 60 kg of ∕m ³ H are obtained from hot acid leaching. ₂ SO ₄ zinc sulphate dissolution is first subjected to reduction operation, that is, before the precipitation of goethite, the Fe ^{3 +} in the solution is first reduced to Fe ^{2 +} by zinc concentrate (ZnS) in a separate operation, after reduction The unreacted ZnS is separated from the elemental sulfur produced by the reaction and sent back to the baking furnace . The reduced liquid is pre-neutralized with calcined ZnO to 3 to 5 kg of 3m ³ H ₂ SO ₄ , and the obtained iron slag is returned to the hot acid leaching operation, and the solution is sent to the precipitation reactor. The Fe ^{2 + is} oxidized to Fe ^{3 +} by passing air to the precipitator to hydrolyze and precipitate the goethite crystals.

Figure 5 VM needle iron ore method

In the precipitation of goethite, it is necessary to continuously add calcination to neutralize the acid produced by the hydrolysis reaction, and to control the pH within an appropriate range, such as pH=2 to 3.5. The VM method requires special attention to control the oxidation rate of Fe ^{2 +} so that the concentration of Fe ^{3 +} in the solution is always within 1 kg ∕m ³ during the hydrolysis of precipitated goethite. Unlike the yellow iron sputum method, it is not necessary to provide a monovalent cation when the goethite is precipitated, and the obtained goethite slag cannot be pickled to recover the undissolved zinc which is neutralized by the calcine. To prevent this loss of zinc, one countermeasure is to use low-iron sphalerite calcine as a neutralizer.

The EZ method developed by Australian Electrolytic Zinc Company directly adds the solution to be hydrolyzed with Fe ^{3 +} to the hydrolysis precipitator, and controls the concentration of Fe ^{3 + in the} hydrolyzate to not exceed 1 kg ∕m ³ to control hydrolysis, so the EZ method is also called partial decomposition. law. The precipitated goethite is continuously hydrolyzed at 70 to 90 ° C while continuously adding zinc calcine to neutralize the acid produced by the hydrolysis, maintaining the pH of the solution at 2.8 to be suitable for hydrolysis.

Compared with the two goethite methods, the same amount of iron is precipitated, and the acid produced by the VM method has less EZ method, so the zinc to be consumed for neutralizing the hydrolyzed acid is less, and less zinc is lost with the zinc calcine. The effect of removing iron is better than the EZ method. However, the VM method involves two processes of reduction and oxidation, which is relatively cumbersome. In addition, the VM method using air oxidation of Fe ^{+ 2} slow speed, and the other with the high cost of the oxidizing agent.

Compared with the yellow iron sputum method, the goethite method does not require sulfate and alkali metals, and can be applied to any acid leaching system, including chloride systems and nitrate systems, and the effect of removing iron is also better (from 30kg ∕m ³ to Less than 1kg·kg∕m ³ ), but goethite has poor stability to acid, and undissolved zinc ferrite in the precipitate cannot be recovered by pickling like the yellow iron sputum method.

(3) Hematite method

Japan's Akita company's Iijima zinc smelter and Germany's Ruhr's zinc company Dartrun electric zinc plant use hematite treatment to treat zinc leaching neutral leaching residue to recover zinc and other valuable components in the presence of zinc ferrite. . The treatment of iron slag from wet zinc smelting by the hematite method stems from the pressure of environmental protection. The principle flow of the hematite method is shown in Figure 6. The high-iron slag from the main process of leaching is replenished with acid in the acid-resistant brick and lead autoclave in the village, and the reaction temperature is 95-100 °C. The leaching is carried out in an atmosphere of SO ₂ (partial pressure 0.15 to 0.25 MPa), so it is also referred to as SO ₂ leaching. Under this condition, the ferrite in the slag is easily dissolved, and the high iron is reduced to divalent with zinc and copper in the ferrite into the solution:

(12)

(13)

Figure 6 Flow chart of the hematite method

After removing excess SO ₂ from the solution and removing copper by precipitation with H ₂ S, the solution containing about Zn 90 kg ∕m ³ , Fe 60 kg ∕m ³ , H ₂ SO ₄ 20 kg ∕m ³ was neutralized with lime in two stages. . The first stage is neutralized to pH = 2 to produce a saleable high grade gypsum , which is then neutralized to pH = 4.5, which precipitates gypsum containing valence metals such as Ca and In, while elements which hinder hematite precipitation, such as Al Etc. is also removed with gypsum precipitation at this stage. The solid obtained by gravity sedimentation of the slurry produced in the second stage is returned to the first section of the neutralization tank. After the sedimentation, the liquid is filtered by high pressure to obtain a mixed precipitate of oxide-hydroxide, and sent to the smelting plant for recovery of gallium and indium . At the same time, some iron and other impurities are precipitated by air oxidation. The precipitated gypsum helps to remove the sulfate produced by the oxidation of SO ₂ to maintain the sulfate balance.两段中和后的溶液（含Fe 40～45kg∕m ³ ）用赤铁矿法沉淀除铁。沉铁在衬钛高压釜中进行，通入新鲜蒸汽和氧气，温度从95℃升高到200℃，压力提高到1.8MPa（氧分压0.15～0.25MPa），溶液中的硫酸亚铁被氧化成硫酸铁并发生水解：

（14）

高压釜中停留时间约3h，主要水解产物为赤铁矿，含有w（Fe）＝59%å’Œw（S）＝3%，固液分离后赤铁矿也主要销售给水泥厂。分离出赤铁矿的溶液含Fe5～7kg∕m ³å’ŒH ₂ SO ₄ 60～70kg∕m ³ ，返回焙砂的中性浸出段。

采用赤铁矿法的饭岛锌冶炼厂自1972年投产以来，至今已成功运行了26年，经1997年扩产，电锌产量巳达190000t∕a。由于锌精矿铁含量增加，生产效率提高和工厂扩产，赤铁矿法处理的铁量逐年增加，并在技术上作了若干改进。例如，锌焙砂弱酸浸出的渣与元素硫混合用电解贫液补加硫酸后在衬铅和耐酸砖的高压釜中再浸出。加入元素硫使溶液中大部分铜作为硫化铜沉淀。热酸浸出的排料除去过量的SO ₂后，在搅拌槽中通入H ₂ S沉淀其余的铜。沉铜槽的排料浓密、压滤，得到的滤渣含铜、铅和贵金属，送熔炼厂回收。沉铜浓密机溢流含30kg∕m ³æ¸¸ç¦»é…¸ï¼Œç”¨ç»†ç£¨çš„石灰石两段中和。第一段中和游离酸（至pH＝1）得到纯的石膏，离心过滤后销售给水泥厂。

近些年来，随着锌精矿中铁含量的增加，焙砂中进入铁酸盐中的铜增加，焙砂弱酸浸出的铜减少而进入浸渣的铜增加，因而浸渣赤铁矿法处理厂中需要沉淀的铜大为增加，从而使渣处理厂沉淀铜的成本提高。1992年以前，渣处理厂中溶液中的铜用元素硫和硫化氧沉淀：

（15）

（16）

饭岛锌冶炼厂1992年用于沉淀铜的硫化氧气体消耗成本占总的消耗性成本的25%。这无疑太高，需要开发一个不用硫化氢沉淀铜的新方法。后来发现硫化锌精矿可以代替硫化氢气体，它沉淀除铜的反应如下

（17）

（18）

当生产上用硫化锌精矿沉铜时，铜的沉淀并不完全。后来使用更细的精矿增加SO ₂分压解决了这一问题。现在这种方法有效地脱除了铜。

高铁水解成赤铁矿和铝水解沉淀铝矾都产生酸，因而降低赤铁矿沉淀釜的料液中游离硫酸的浓度和铝的浓度对促进高铁的水解很有效：

原来第二段中和的溶液有30%返回第一段，从1997å¹´3月以来，第二段溶液返回的量逐渐增加，赤铁矿水解高压釜的料液中游离硫酸浓度从7kg∕m ³é™åˆ°4kg∕m ³ ，铝的浓度降到2kg∕m ³ä»¥ä¸‹ï¼Œé™¤é“æ•ˆçŽ‡æé«˜åˆ°88%以上，使操作成本因素如氧气或蒸汽的成本降低。

虽然赤铁矿法在环保方面比黄铁矾法和针铁矿法更有利，它仍然受到环境方面的压力。为了使沉淀的赤铁矿能全部售出给水泥厂，必须解决赤铁矿中的含砷和含硫问题。由于火法冶金不仅成本高，而且很难满意脱除砷，于是饭岛炼锌厂研究在沉淀赤铁矿前从溶液中脱砷，提出了图7所示的改进赤铁矿法新流程。

图7 改进的赤铁矿法新流程

在改进的赤铁矿法中，弱酸提出的渣在105℃下SO ₂气氛中浸出而不加锌精矿或元素硫，产生的含银和铅的渣过滤分离。滤液用石灰第一段中和到pH＝1，产生纯石膏。然后在该中和段的溶液中加入锌灰，沉淀砷化铜，铜和砷的脱除率达到99%。脱砷后液第一段加石灰石中和到pH＝4，沉淀出含Ga，Inå’ŒAl的石膏。该段的溶液大部分送赤铁矿沉淀高压釜，其余溶液用于浸出砷化铜。浸除在单独的高压釜中氧气氛下进行，铜被浸出而砷沉淀为砷酸铁。浸液中的铜用锌灰置换，然后将溶液返回焙砂中性浸段。改进的赤铁矿法进行了中试和可行性研究，得到的赤铁矿质量及成本都令人满意。

德国鲁尔公司（Ruhr－Zink GmbH）的赤铁矿法主要包括以下步骤：

（1）中性浸出渣两段热酸浸出。第一段为热酸浸出，中性提出渣用第二段超热酸浸出的滤液在95℃下浸出，浸出的终酸浓度50kg∕m ³ 。渣中的大部分有价金属如锌、铜和镉随同铁一起溶解。浸出的排料浓密后溢流泵送至还原段，底流在过热酸浸段中沸点以上浸出，酸浓度140kg∕m ³ 。过热酸浸中铁酸盐都溶解，残留的低铁富铅的Pb－Ag渣经浓密和高压膜压滤机过滤，滤液返回热酸浸出。

（2）高铁还原。为了在沉淀赤铁矿前净化溶液并能在最尽可能低的温度下沉淀铁，需要将离解的高铁先还原成亚铁。硫化锌精矿可用作还原剂，它的成本低，但需大大过量，反应温度在90℃左右。未反应的含元素硫的渣过滤后返回焙烧。

（3）溶液的净化与中和。还原后液用焙砂在中和槽和浓密机中两段中和，使所有影响赤铁矿质量的元素大部分沉淀析出，特别是砷和锑 。铜则部分共沉淀。这些元素富集在中和渣中，再在终浸作业中完全溶解。终浸用废酸进行，终酸浓度为40kg∕m ³ 。在浓密机中固液分离后，底流送去热酸浸出作业，溢流送去用海绵铁置换沉铜，将铜的浓度降至500g∕m ³ä»¥ä¸‹ï¼Œå†è¿”至前面的中和作业。置换的铜用废酸洗涤后出售。

（4）赤铁矿沉淀。这是最重要的部分。中和净化的浸液（含Fe ^{2 ＋} 25～30kg∕m ³ ，Zn120～130kg∕m ³ ）用蒸汽加热到180℃以上，其中的亚铁在氧压1.8MPa下氧化并水解成含w（Fe）＝60%左右的细粒赤铁矿，铁的沉淀率达90%～95%。具体流程如图8所示。

赤铁矿法投资和操作费用远高于黄铁矾法和针铁矿法，但它可能回收锌精矿的全部成分，产生的全是可销售的产品，所有作为中间产品的渣帮可进一步加工而无需堆存。

图8 鲁尔公司电解锌厂赤铁矿法原则流程

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