Hydrolyzed precipitation of yellow iron

Yellow iron scorpion is also commonly referred to as jarosite, and has a small solubility in acidic solutions. Alum means a double salt of two or more metals consisting of sulfates, which are more than a single crystallization from a corresponding salt solution, but also larger grains are formed, facilitate 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 in a few 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 monovalent cation concentration, iron concentration 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 sputum 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 the 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.

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