Crystallization and separation of ferrous sulfate

After settling to remove most of the insoluble residue titanium sulfate solution, contains large amounts of soluble iron salts - ferrous sulfate. The second step in the purification of titanium liquid is to crystallize the soluble iron salt in the titanium solution as FeSO ₄ ·7H ₂ O by crystallization.

From the principle of crystallography, the decisive factor for the precipitation of solid crystals from steam, solution or melt is its supersaturation and supercooling. The lower the temperature, the greater the supersaturation, the faster the rate of crystal growth. . In the production of titanium dioxide by sulfuric acid method, the crystallization of the titanium liquid is generally the crystallization of the solute (ferrous sulfate) after the solution is cooled or evaporated to make the solubility of the solute in the solution supersaturated. The main methods can be divided into two categories: one is to remove a part of the solvent, so that it is supersaturated to precipitate crystals, such as evaporation or vacuum crystallization; the other is to remove the solvent, using solute at different temperatures in the solvent. The difference in solubility is a method of lowering the solubility by lowering the temperature to precipitate a solute crystal which is supersaturated. Table 1 shows the solubility of ferrous sulfate in a titanium solution having a TiO ₂ concentration of 120 g/L and an effective acid of 240 g/L at different temperatures.

Table 1 Solubility of ferrous sulfate in titanium Huai

The production of sulfuric acid method is different from the chlorination method. It does not need to remove all the ferrous sulfate in the titanium liquid. According to different hydrolysis process requirements, it is necessary to retain a part of ferrous sulfate in the titanium liquid, and control the iron and titanium in industrial production. More than to control the amount of ferrous sulfate precipitated during crystallization. The ratio of iron to titanium has a certain influence on the particle size of the hydrolyzate metatitanic acid. According to different hydrolysis processes, the ratio of iron to titanium is generally controlled between 0.18 and 0.37.

(1) Crystallization method without removing solvent

This method mainly includes natural cooling crystallization and freezing crystallization. Natural crystallization is to let the titanium liquid cool naturally (or water-cooled) to room temperature (15~20 °C). At this time, ferrous sulfate crystallizes large crystals along the wall or the bottom of the tank. This method is simple but takes too long ( About 3~4d), the crystallization efficiency is low, and the crystallization of the titanium liquid still contains 40-60g/L of ferrous sulfate (calculated as Fe ²⁺ ), and the ferrous sulfate content in the titanium liquid in summer will be higher. The crystallization efficiency of this method depends on the crystal area. In the early days, in order to increase the crystal area, some lead strips were placed in the crystallization tank to precipitate the ferrous sulfate crystals. This ancient method has been used in industrial production.

In industrial production, the method of freezing crystallization is mainly used. The freezing crystallization uses a refrigerant (refrigerated water or calcium chloride brine) to lower the temperature, and removes heat to cause the ferrous sulfate to be supersaturated and precipitated. This method has high efficiency and short time, and the crystallization effect mainly depends on the heat exchange area, the temperature of the refrigerant, the agitation and the thermal conductivity of the material of the heat exchanger (freezing coil).

The frozen crystals are generally carried out in a freezing tank, and the refrigerant is circulated through a coil (lead, copper or titanium tube) provided in the freezing tank, and the titanium liquid is heat-exchanged with the coil under stirring to continuously lower the temperature. The freezing efficiency depends not only on the temperature of the refrigerant and the heat exchange area of ​​the coil, but also on the freezing rate. If the freezing speed is too fast, the ferrous sulfate will quickly accumulate on the surface of the coil to reduce the heat transfer effect. Usually, the titanium liquid from the precipitation tank (about 50 ° C) is cooled to room temperature with tap water, and then chilled or calcium chloride brine is used to continue freezing to the temperature required by the process, which can reduce the accumulation of ferrous sulfate crystal on the coil. Even if the crystal of the wall is also loose and easy to remove, it can save the refrigeration energy consumption. In industrial production, two sets of coils are generally arranged in the freezing tank, one set of water, one set of refrigerant, or a valve is switched. In foreign countries, several freeze tanks are connected in series as shown in Fig. 1, first The coil in the freezer is cooled with tap water, the second is cooled with the frozen titanium solution, and the third is cooled with chilled water or frozen calcium chloride brine. Water is used to rinse the crystalline ferrous sulfate on the surface of the coil before each crystallization operation to improve the heat transfer effect.

Stirring in the freezing tank is also very important. Stirring can promote the formation of crystals. The speed of stirring not only affects the heat transfer effect, but also affects the crystal size and degree of walling after crystallization. Stirring slow crystals tends to form walls, and even a large amount of precipitated crystals will sink to the bottom, making discharge difficult; stirring too fast ferrous sulfate crystal particles are too fine, which brings difficulties to the subsequent separation of ferrous sulfate. The stirring speed in industrial production is generally controlled at about 60 rpm, and the stirring paddle is usually anchored or framed. [next]

(2) Crystallization method for removing a part of solvent

The method mainly removes a part of the solvent by evaporation under normal pressure to increase the concentration and thereby reduce the solubility of the solute in the solvent, which is also called evaporation crystallization. Evaporation crystallization is carried out at isothermal temperature, and the solution is concentrated by evaporation to saturate the crystals. This method is suitable for the crystallization of certain salts (such as sodium chloride) which have little change in solubility at different temperatures. Another method is to carry out under vacuum, while removing a part of the solvent by vacuum evaporation, the temperature is also lowered, and the concentration of the liquid can be reduced by 0.2% for every 1 degree of temperature reduction of the aqueous solution, which can further promote the precipitation of crystals. The crystallization of the titanium liquid is mostly carried out by this method.

Vacuum crystallization, also known as adiabatic evaporative crystallization, is mainly carried out by lowering the boiling point of the solution under vacuum, because the vacuum pressure is lower than the vapor pressure of the liquid to boil the liquid, part of the solution is evaporated to concentrate, and the solute is lowered. Solubility, in addition, due to the latent heat of vaporization during evaporation, a large amount of heat energy is absorbed, and the solution is rapidly cooled until supersaturation to precipitate crystals. Therefore, it has more crystallization than indirect freezing crystallization. The method can be operated in a gap or continuously. This method is adopted by a large foreign sulfuric acid titanium dioxide factory.

The advantages of the method: a. The crystallizer has a simple structure, large production capacity, small equipment footprint, low overall average cost; b. simultaneous evaporation and cooling, high crystallization efficiency; c. solution adiabatic fin cooling, no heat required The heat transfer area required for the exchange, the ferrous sulfate wall wall pattern is also less; d. The titanium solution after crystallization has a high concentration and a high temperature, which can reduce the burden of the subsequent concentration process; e. compared with the frozen crystallization Low comprehensive energy consumption and low production costs.

Disadvantages: The operation control is more complicated. Sometimes the ferrous sulfate crystal particles are too fine to affect the filtration operation, and consume more steam and cooling water.

The vacuum crystallizer can be operated in a single device gap, or several units can be connected in series for continuous operation (Figure 2). It is desirable to divide into a plurality of chambers for continuous vacuum crystallization in a horizontal evaporative crystallizer.

The method in Figure 2 is based on steam, which is converted into kinetic energy by a steam jet pump to generate vacuum. Under the action of vacuum, the iron liquid passes through three different temperatures (37 ° C, 25 ° C, 15 ° C). The crystallizer causes continuous precipitation of ferrous sulfate. [next]

Figure 3 is the operation of a continuous vacuum crystallizer characterized by multi-stage crystallization in the same vacuum crystallizer to control crystal growth and increase crystal growth by increasing the number of stages and increasing the residence time of the solution in the crystallizer. Fine crystals are precipitated in a metastable state range to avoid critical supersaturation.

Another advantage of multi-stage vacuum crystallization is that the more stages of crystallization, the smaller the compression ratio of the steam ejector can be, which saves steam and some of the steam can be used to preheat the mother liquor. Table 2 is the reference operation data for a 6-stage vacuum crystallizer.

Table 2 Temperature and flow rate in each chamber of the vacuum crystallizer

(3) Fraction of ferrous sulfate

Separation from ferrous sulfate belongs to the category of solid-liquid separation. The solid-liquid separation operation runs through the entire process of producing sulfuric acid titanium dioxide, such as: dust recovery during ilmenite pulverization - gas-solid separation; sedimentation of titanium liquid - solid-liquid separation; filtration of titanium liquid - solid-liquid separation; Washing with metatitanic acid - particle fractionation during liquid-solid separation and surface treatment. Therefore, the selection of suitable filtration and separation equipment is very important for the production of sulfuric acid titanium dioxide.

After crystallization, the ferrous sulfate in the titanium solution exists in the form of FeSO ₄ ·7H ₂ O, and the coarser crystal particles can be realized by vacuum ultrafiltration or centrifugal separation.

a. Vacuum suction filtration This is an ancient and simple method. There are still many small and medium-sized sulfuric acid titanium dioxide plants in use. The vacuum suction filter is also called a ferrous suction filter or a ferrous separation tank. It is generally rectangular and divided into two upper and lower layers. The upper layer is open at normal pressure, the lower layer is a negative pressure chamber, and the middle is separated by a grid and coated with a filter cloth. The material to be separated is added from the upper portion, the lower layer is evacuated, and the filtrate flows through the filter cloth into the lower chamber, and the ferrous sulfate crystal remains in the upper portion of the separation tank. The equipment is simple in material selection, and can be made of steel-lined rubber, steel-lined FRP, hard PVC and acid-resistant concrete. It has no moving parts, is easy to manufacture, saves investment, has less maintenance, is intuitive to operate, has a large amount of disposable processing, and is manually unloaded by gap operation. Material, filter cloth can use polyester fiber, polypropylene fiber, wool felt, etc., without the environmental pollution of acid mist and ferrous dust when centrifuge is separated. Disadvantages: large equipment area, high labor intensity, high vacuum energy consumption, uneven distribution of materials and vacuum, consumption of washing water, a large amount of small water to return to use. [next]

b. Centrifugal separation, centrifugal separation In addition to the three-legged centrifuge, it is generally operated continuously, with high degree of automation, low labor intensity, small equipment footprint, and low energy consumption. Due to the high centrifugal force, the ferrous sulfate has low moisture content. The washing water is small, and there is no large amount of water produced by vacuum suction filtration. The disadvantages are: high equipment cost, complicated construction and high maintenance cost. Some materials have poor corrosion resistance and a small amount of fine ferrous sulfate. Centrifugal methods are often used in modern large-scale plants, and the commonly used centrifugal separators are as follows.

The cone blue centrifuge (Fig. 4) is also called the WI vertical discharge centrifuge or the centrifugal discharge centrifuge. The machine is suitable for separating the suspension of solid phase particles ≥ 0.25mm, the structure is simple and there is no complicated automatic operation mechanism, the maintenance is convenient, the continuous operation, the processing volume is large, and the price is cheap. The disadvantage is that the cone angle and speed of the centrifuge are not adjustable, so it can only be applied to materials of a specific type. The whole structure is weakest mesh, casting of ferrous sulfate crystals sieve material removed during centrifugation, while the metal mesh sieve friction will not only wear and destructive to produce large crystal titanium fine particles into the solution after separation In the middle, it becomes very difficult to control the next filtration, and sometimes it is necessary to perform secondary separation to remove fine particles.

Horizontal piston centrifuge (Fig. 5) The machine can perform relatively sufficient washing while separating solid phase materials. The output can be continuously operated, the thickness of the filter residue layer and the discharge speed can be adjusted, and the wear degree of the crystal is compared. The cone blue centrifuge is small and suitable for separating suspensions with solid phase particles ≥ 0.25 mm. The disadvantage of this machine is that it is sensitive to the change of the concentration of the suspension. The solid content is required to be ≥30%, preferably 40%~60%. Therefore, the machine must be passed through the thickener before the machine is directly overflowed and filtered. The filtrate is mixed and sent away, and the suspension with a relatively high concentration at the bottom and a constant concentration enters the machine for separation. In addition, the structure of the machine is complicated, the maintenance is troublesome, and the screen of the piston pushing part is more likely to wear. Although the machine feeds continuously, the unloading is carried out by the piston one by one. Therefore, it is also called a pulsating centrifuge, and the number of pushing materials is generally 20~120 times/min. If the filter residue is required to have good washing effect and low moisture content, it must be lengthened according to the structural principle of the machine to prolong the residence time of the material, but this will bring difficulties to the discharge, and then a multi-stage piston discharge centrifuge is designed. . [next]

Horizontal disc filter (Fig. 6) This machine is actually a continuous vacuum filtration equipment . It is suitable for suspensions with high solid concentration (20% solid content), large difference in solid-liquid specific gravity and fast sedimentation speed ( In the suspension characteristics, it belongs to the suspension of type A, that is, the solid phase material can form the cake of filter cake in a short time. Under the circular filter disc, it is divided into several vacuum filter chambers, so it can be washed while filtering. While draining, the washing water and the filtrate can be applied to each other for multiple successive countercurrent washings, and a better washing effect can be obtained with less washing liquid, and the filter cloth is more convenient to regenerate, and there is no problem of the wear of the crystallizer by the centrifuge. Unloading is smooth. The disadvantages are large equipment area, high horizontal installation accuracy, complicated connection of the distribution heads in each vacuum chamber, high material requirements, and slightly higher moisture content in the ferrous iron than the horizontal piston discharge centrifuge. However, due to its relatively stable operation at low speeds, there are few problems with ferrous wear and filtration. The latter two of the above three types of centrifuges are used more in the titanium dioxide industry.

The surface and voids of ferrous sulfate after filtration or centrifugation contain a small amount of titanium liquid, which must be washed and recovered with water, otherwise it will not only affect the yield of titanium dioxide, but also reduce the quality of ferrous sulfate. When separating the ferrous sulfate by the ferrous separation tank, since the ferrous layer in the separation tank is very thick, it must be washed with a large amount of water, so that a low concentration of drought (lower TiO ₂ content) is generated. water. In order to save water, avoid reducing the concentration of titanium liquid, increase the concentration burden, reduce the amount of water, and prevent local hydrolysis, the second rinse of ferrous recovered water (small water) is usually used for the first wash, due to ferrous sulfate. The solubility in water is higher than in titanium and sulfuric acid, which also reduces the degree of re-dissolution of ferrous iron during water washing.

In order to prevent the excessive dissolution of ferrous sulfate during washing, the iron-to-titanium ratio is increased, and the temperature of the washing water is not too high, and cold water is preferably used in summer. When using a centrifuge to separate ferrous iron, since the ferrous layer on the ancient wall is very thin, only a small amount of water is washed to achieve better results, and the centrifugation and washing are basically carried out simultaneously, which is also separated by a centrifuge. One of the main advantages over using a vacuum suction filter.

The titanium liquid after separation of ferrous sulfate not only has a decrease in viscosity and density, but also because the seven crystal water in the ferrous sulfate removes part of the water in the titanium liquid, so that the concentration of the titanium liquid is increased by about 30 g/L, which is effective. The acid is increased by about 80g/L, and the volume is also reduced by 17%~20%.