Hydroxide precipitation

First, the principle of hydroxide precipitation

Apart from a few alkali metal, hydroxides of most metals are of insoluble compounds. Therefore, in hydrometallurgical practice, the most common metal precipitation method is neutralization and hydrolysis to form a poorly soluble hydroxide precipitate. The typical precipitation reaction is:

(1)

The solubility product of the corresponding metal hydroxide is:

(2)

Also know from the balance of water dissociation:

(3)

Thus, the following relationship of the metal hydroxide can be obtained:

(4)

a solubility product of K _sp -metal hydroxide in the formula;

K _w - the ionic product of water.

It can be seen from the above formula that the pH of the precipitation of the metal hydroxide at a certain temperature is determined by the valence state of the metal ion and the solubility product of the hydroxide. If prescribed When =1mol ∕L, the precipitation starts. When the precipitation is complete at =10 ^-5 mol∕L, the pH corresponding to the start of precipitation and precipitation of the metal hydroxide can be determined from the above formula.

The solubility products of some common metal hydroxides and the pH of the precipitates are listed in the table below.

Table The solubility product of common metal hydroxide at 25 ° C and the pH of the precipitate

For a specific metal ion, there is a hydrolysis precipitation balance:

(5)

The hydrolysis equilibrium thus provides the following relationship between the activity of the remaining metal ions in the solution and the pH of the solution:

(6)

The above formula shows that the dissolution characteristics of the metal hydroxide are a function of pH. K in the formula is an equilibrium constant of the hydrolysis reaction formula (5). Comparing formula (6) with formula (4), it is known that lgK = lgK _{sp -} nlgK _w .

The functional relationship (6) can be plotted as a precipitation map. Mornez takes the pH of the solution as the abscissa and the logarithm of the metal ion activity in the solution as the ordinate, resulting in the curve of Figure 1. Each line in the figure corresponds to a hydrolyzed precipitation equilibrium, and the negative of the slope of the line is the valence of the precipitated metal ion. The dissolution behavior of the metal can be judged intuitively by the graph. The left side of the line is the condition in which the metal ions remain in the solution, and the right side of the line is the condition in which the metal ions are precipitated as hydroxide. The relative hydrolyzation performance of various metal ions is clearly shown in the figure, that is, the tendency of metal hydrolysis precipitation from left to right is weakened. In general, trivalent and tetravalent metal ions can be hydrolyzed under strong acid conditions, and divalent transition metal ions are hydrolyzed under weak to weak conditions. It can also be seen from the figure that the dissolution behavior of different valence ions of the same metal is also different, and the most typical cases are the difference of the conditions of hydrolysis and precipitation of Fe ^{2 +} and Fe ^{3 +} and Co ^{2 +} and Co ^{3 +} .

Figure 1 Metal hydroxide precipitation diagram 25 ° C

A strong base such as sodium hydroxide is generally not suitable as a precipitant for metal hydroxides. It is difficult to control the pH value even with very dilute alkali solution, and the precipitate of hydroxide formed is often colloidal and bulky, which is difficult to filter and wash. It is easy to adsorb other metal ions, which not only causes loss of metal, but also precipitates are not pure. Therefore, strong bases, including lime, are primarily used to recover small amounts of metal from very dilute solutions or to "sweep" metals from waste streams.

A suitable buffer can be used to control the pH of the solution, but this is usually only suitable for separation in chemical analysis, and cost for hydrometallurgy. In hydrometallurgy, the oxide, hydroxide or carbonate of the main metal of the solution is often used to control the pH of the solution to precipitate the hydroxide of the impurity metal.

It is also possible to form a basic salt of a metal in the precipitation of a metal hydroxide, and the magnitude of this tendency depends largely on the anion in the solution. Among the common anions in the hydrometallurgical process, sulfate is most likely to cause the formation of basic salts, and the pH of the metal basic sulfate is slightly lower than the pH of the corresponding metal hydroxide, in zinc hydrometallurgy. examples of iron jarosite is a representation of the other.

Second, the precipitation of aluminum hydroxide

Sodium aluminate solution was concentrated to give concentrated high temperature alkali leaching for producing alumina from bauxite Bayer process, aluminum hydroxide is precipitated from it a very important step. The Al in the Bayer solution is in the form of [Al(OH) ₄ ] ^- complex ions, which is unstable, and the precipitate of aluminum hydroxide is resolved by water. The reaction is as follows:

(7)

The precipitated aluminum hydroxide may be crystalline or colloidal, depending on the conditions of the precipitation, including the composition of the mother liquor, the temperature and the presence or absence of seed crystals. A typical Bayer solution contains about 80 kg ∕m ³ of Al ₂ O ₃ , and the Na ₂ O ∕ Al ₂ O ₃ ratio (referred to as the molar ratio, the same below) is between 1.5 and 2.5. The simple dilution or cooling can only obtain colloidal hydrogen. Alumina, difficult to separate and wash. In practice, seeding is added to help crystallization separation, which is customarily called “species”. The practice is to use the newly formed 5-150 μm aluminum hydroxide crystal in the previous cycle as a seed crystal, and greatly introduce it into a new crystallization cycle and cool down. The mixture was stirred slowly for about 4 d to obtain coarse-grained aluminum hydroxide crystals. In the initial stage of precipitation, the rate of crystallization is proportional to the surface area of ​​the seed crystal. Effective agitation is necessary, otherwise fine seed crystals tend to coalesce and reduce the rate of crystallization. About 70% of aluminum was crystallized by stirring at 25 to 35 ° C for 36 hours. Certain components such as dissolved iron, vanadium and calcium salts have a negative effect on crystallization and are therefore often referred to as inhibitors or poisoning agents. These inhibitors should be limited to the specified low levels to ensure the necessary crystallization rate. The precipitated aluminum hydroxide is settled to the bottom of the tank, filtered, washed and calcined to form an alumina product. The mother liquor was concentrated by evaporation to a density of 1.45 kg·m ^-3 and returned to leaching.

Another method of crystallizing aluminum hydroxide from a concentrated solution of sodium aluminate is to pass carbon dioxide to neutralize excess alkali, which is conventionally referred to as "carbon", generally at 70 ° C. The relevant neutralization reaction is as follows:

(8)

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