The chemical precipitation method is also one of the methods of separation and purification. It is a process in which the target component (ion) in the solution is selectively precipitated as a poorly soluble compound by the action of a precipitating agent. When the precipitated component is an impurity ion and the valuable component remains in the solution, it is generally referred to as purification. On the contrary, it is called the preparation process of poorly soluble compounds. If the hard-to-turn compound is the final product, it is called a chemical concentrate or a separate product. The precipitates are usually various salts, hardly soluble sulfides and poorly soluble hydroxides.

According to the mechanism of chemical precipitation, they can be divided into two categories.

(1) Stepwise hydrolysis

Hydrolysis step is one of various methods commonly used in the leaching solution separated metal ions. When the acid immersion liquid is neutralized with a base or diluted with water, the metal cation therein is precipitated as a hydroxide or a base or a salt. If the process of precipitation of a metal hydroxide is also called hydrolysis, the reaction formula is:

Mn+ + nOH- == M(OH)n

The formation of pure hydroxide precipitates is only an ideal situation and can only be achieved when the metal and acid radical concentrations are low. The actual leachate cannot satisfy this condition, so the metal salt precipitates at the same time as the hydroxide precipitate is formed, that is, the so-called basic salt precipitate.

Since some metal ions in the leachate are often in a low-cost form, they cannot be separated from the host metal by a simple hydrolysis method. Leachate from copper, for example, iron ore, must first be oxidized to Fe2 + to Fe3 + the iron is removed by hydrolysis. Therefore, the oxidative hydrolysis purification method is commonly used in practice. The low-priced impurities are first oxidized to a high-priced form, and then neutralized by a neutralizing agent.

(2) Insoluble salt precipitation method

In the chemical treatment of mineral raw materials, various precipitants are often used to form certain insoluble compounds with certain metal ions in the solution to separate impurities and extract useful components. Commonly used precipitating agents include sulfides, chloride, carbonate, phosphate, xanthates, oxalate and the like. They all use the difference in the solubility product of each compound to separate the target components.

Since most metal sulfides are insoluble salts, that is, the solubility is very small, the solubility of different metal sulfides is different; the sulfide precipitation method is to use sodium sulfide or hydrogen sulfide as a metal precipitant, which can be realized by controlling the sedimentation conditions. Separation of metal.

In addition to sulfide precipitation method, properties of certain metals may also be utilized insoluble phosphate, arsenate, phosphate, oxalate, fluoride, chloride, uranyl formate, tungstate, molybdate and the like is The components are separated. For example, the ion-adsorbed rare earth ore is usually leached with 5~10 brine or 1.5~3.5% (NH4)2S04 solution, and the rare earth is transferred into the solution. Under the condition of pH=1~2.0, the mixed rare earth can be obtained by precipitation with oxalic acid. The oxalate salt is filtered and washed to obtain a mixed rare earth oxide.

In addition to the addition of a precipitant to precipitate some of the poorly soluble compounds, it is also possible to precipitate some of the components in the form of insoluble compounds by means of concentrated crystallization or salting out, such as crystallization in an alkali leaching solution of tungsten mineral raw materials. Crystallization of sodium tungstate and the like are precipitated.

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