Patent Description:
A mainstream production process of titanium dioxide in China is the sulfuric acid method. <NUM> t of TiO<NUM> produced can generate <NUM> t to <NUM> t of waste acid, which has an acid concentration of <NUM>% to <NUM>% and includes ferrous sulfate with a concentration of <NUM>% to <NUM>% as well as a certain amount of the TiO<NUM> and metal sulfates in addition to H<NUM>SO<NUM>. Direct neutralization of the waste acid not only wastes a large amount of resources, but also produces a large amount of red gypsum to be stored in landfills and high-salt wastewater to be discharged, causing secondary pollution of the water environment. Therefore, the direct neutralization cannot truly solve environmental problems. As a result, it has always been a technical problem facing titanium dioxide production workers and environmental protection workers to effectively recycle this type of waste acid.

The waste acid in titanium dioxide production is mainly treated by single evaporation and concentration in China. Specifically, the waste acid is directly evaporated and concentrated to <NUM>% to <NUM>%, compounded with concentrated sulfuric acid, and then returned to the ilmenite leaching section to realize the recovery of acid components. However, this process may produce ferrous sulfate monohydrate with high impurity content and viscosity, and disposal of this type of low-quality ferrous sulfate monohydrate is also a headache faced by enterprises. The single evaporation and concentration only recovers sulfuric acid, causing the ferrous sulfate to be lost as a waste, and requires extremely high energy consumption. In recent years, with the rapid rise of domestic new energy vehicles, lithium iron phosphate has become one of the main materials for battery cathodes. One of the raw materials for preparing a battery precursor ferric phosphate is ferrous sulfate, such that there is an increasing demand for ferrous sulfate in the market. In the past, the ferrous sulfate was only an impurity that needed to be removed, and effective recovery was not taken seriously. Accordingly, it is bound to bring great economic and environmental benefits to enterprises and reduce huge burdens by seeking a technical means that can simultaneously recycle ferrous sulfate and sulfuric acid while saving a large amount of steam efficiency.

Patent <CIT> disclosed a method of recycling waste acid in titanium dioxide production using a solid-liquid membrane separation + freezing crystallization + nanofiltration membrane separation process, to recover metatitanic acid, ferrous sulfate, and <NUM>% of finished sulfuric acid. Although the process recycles various products, only partial recovery is completed; while a nanofiltration concentrate, which accounts for <NUM>% to <NUM>% of an original waste acid volume, does not undergo resource disposal. Based on the engineering application experience and experimental data of freezing crystallization and the proficient application of nanofiltration membrane technology, the process has a recovery rate on the ferrous sulfate of less than <NUM>%.

Patent <CIT> disclosed a three-stage negative-pressure evaporation and crystallization process. The process adopts a staged treatment of waste acid, including: conducting freezing crystallization to obtain ferrous sulfate heptahydrate, conducting double-effect evaporation and concentration + freezing crystallization to obtain metal sulfate crystals, and then subjecting secondary freezing crystallization mother liquor to single-effect evaporation and concentration + freezing crystallization to obtain metal sulfate crystals and <NUM>% of finished acid. This process is intended to overcome the heater clogging during waste acid concentration. Based on the experiences from freezing crystallization of titanium liquid and concentration of waste acid in titanium dioxide production for ages, this process shows the following three shortcomings. On one hand, the three-stage repeated process from high temperature to low temperature is bound to consume extremely high energy. On the other hand, the segmented partial removal of calcium, magnesium, and iron may inevitably lead to the heater clogging. Furthermore, there is a recovery rate on the ferrous sulfate of less than <NUM>%, and the second and third stages of concentration and crystallization mainly produce miscellaneous metal sulfates.

<CIT> discloses a method for recycling iron (II) sulfate and sulfuric acid from waste acid TiO2 production by using heat from the TiO2 roaster. The recycling method comprises the following steps: applying water absorbing agent under room temperature and normal pressure under stirring to the waste acid including ferrous sulfate/magnesium sulfate/manganese sulfate and controlling the temperature at <NUM>-<NUM>; cooling the suspension to room temperature, separating the water hydrate crystals by solid-liquid separation, which is a heat exchanger and obtaining a filtrate having a higher sulphuric acid concentration; the filtrate is subjected to freezing to -<NUM> to <NUM>, so that the ferrous sulfate heptahydrate is precipitated in the solution and the solid solution of the ferrous sulfate heptahydrate is separated by solid-liquid separation and the spent acid is further concentrated; these steps might be repeated until the concentration of the sulfuric acid reaches <NUM> % by mass; exhaust gas of the roasting furnace is used to heat and dehydrated the water absorbing agent hydrate to regenerate the water absorbing agent; the water vapour is cooled by the room temperature waste acid partition wall and returned to the pickling process to be used for the titanium acid washing water. The filter cake comprises Ti, Mn,Mg and is further recovered; the sulfuric acid is sent to the acid hydrolysis step for recycling. The waste heat of the calcining furnace is obtained after the hot gas of the rotary kiln head and kiln tail is removed from the cyclone separator.

An objective of the present disclosure is to provide a method for recycling ferrous sulfate heptahydrate and sulfuric acid from a waste acid in titanium dioxide production. The method saves energy consumption, has a high system recovery rate of ferrous sulfate and a high purity of ferrous sulfate heptahydrate, and can provide a finished acid with a concentration that meets the demand.

The objective of the present disclosure is achieved as follows: The present disclosure provides a method for recycling ferrous sulfate heptahydrate and sulfuric acid from a waste acid in titanium dioxide production, including the following steps:.

In the present disclosure, the waste acid in titanium dioxide production is at least one selected from the group consisting of a waste acid produced after high-pressure filtration of hydrolyzed metatitanic acid, a waste acid produced after adiabatic flash and the high-pressure filtration of the hydrolyzed metatitanic acid, and a waste acid produced after direct filtration of the hydrolyzed metatitanic acid on blades of a leaf filter; and the waste acid in titanium dioxide production has an acid concentration of <NUM>% to <NUM>% and a ferrous sulfate concentration of <NUM>% to <NUM>% in step <NUM>.

In the present disclosure, a rotary kiln flue gas is at <NUM> to <NUM>, and the waste acid in titanium dioxide production pre-concentrated by the rotary kiln flue gas spraying is at <NUM> to <NUM> in step <NUM>.

In the present disclosure, a device for the freezing crystallization includes the heat exchanger, a refrigeration unit, a crystallization tank, a centrifuge or a disc filter, a pump unit, and a tank body; and a cold source for the freezing crystallization includes a crystallization completion liquid of <NUM> to <NUM> and a water chilling unit in step <NUM>.

In the present disclosure, the freezing crystallization is conducted at <NUM> to <NUM> for <NUM> to <NUM>; the ferrous sulfate heptahydrate has a grade of <NUM>% to <NUM>%; and the crystallization mother liquor has an acid concentration increased to <NUM>% to <NUM>% in step <NUM>.

In the present disclosure, the waste acid evaporation system in step <NUM> has a concentration device provided with a preheater, a heater, a cooler, a condenser, an evaporator, a maturation cooling tank, a pump unit, and a tank body, and a heat source for evaporation is waste heat derived from a primary steam, a tail gas of titanium dioxide calcination, or a tail gas of acid hydrolysis; and the filter press dehydration system in step <NUM> is a filtration device being one selected from the group consisting of a membrane filter press, a centrifuge, and a belt filter press combined with supporting facilities thereof.

In the present disclosure, the waste acid evaporation system has an effective number of <NUM> to <NUM>; and the evaporation condensed water has an acid concentration of less than <NUM>%.

In the present disclosure, the precipitated miscellaneous salt-containing suspended concentrated acid has an acid concentration of <NUM>% to <NUM>%.

In the present disclosure, the filtered finished acid obtained in step <NUM> is pumped into a freezing crystallization system to allow the freezing crystallization at <NUM> for <NUM> to obtain a ferrous sulfate heptahydrate crystal; a resulting freezing crystallization mother liquor is collected into an evaporation feed tank and then pumped into a double-effect evaporation device to obtain an acid-salt suspension concentrate; and the acid-salt suspension concentrate overflows to a maturation cooling tank to allow standing for approximately <NUM>, and is then pumped into a membrane filter press through a screw to obtain <NUM>% of a finished acid and the metal miscellaneous salt filter cake.

The present disclosure combines the waste heat from a large amount of high-temperature rotary kiln exhaust gas produced by titanium dioxide enterprises, the high-quality ferrous sulfate heptahydrate obtained by freezing crystallization, and the solubility of ferrous sulfate in a sulfuric acid system. The present disclosure proposes a relatively-single multi-effect evaporation and concentration method for a waste acid in titanium dioxide production that contains high concentrations of ferrous sulfate and sulfuric acid. The method can save energy consumption by not less than <NUM>%, and simultaneously recover the ferrous sulfate with high system recovery rate, ferrous sulfate heptahydrate with high purity, and finished acid with a concentration that meets customer demands.

The present disclosure also has the following advantages:.

The present disclosure is described in more detail hereinafter with reference to the accompanying drawings and specific embodiments.

<FIG> shows a schematic diagram of a working flow in the present disclosure.

The examples of the present disclosure are described in detail below with reference to the drawings.

The present disclosure provides a method for recycling ferrous sulfate heptahydrate and sulfuric acid from a waste acid in titanium dioxide production, including the following steps:.

In the present disclosure, the pre-concentrating is conducted by rotary kiln flue gas spraying, the rotary kiln flue gas spraying in series with flash evaporation, or a combination of the flash evaporation, the rotary kiln flue gas spraying, and the flash evaporation in sequence; and the rotary kiln flue gas spraying concentrates the waste acid in titanium dioxide production to <NUM>% to <NUM>%.

In the present disclosure, the filtered finished acid obtained in step <NUM> is pumped into a freezing crystallization system to allow the freezing crystallization at <NUM> for <NUM> to obtain a ferrous sulfate heptahydrate crystal; a resulting freezing crystallization mother liquor is collected into an evaporation feed tank and pumped into a double-effect evaporation device to obtain an acid-salt suspension concentrate; and the acid-salt suspension concentrate overflows to a maturation cooling tank to allow standing for approximately <NUM>, and is then pumped into a membrane filter press through a screw to obtain <NUM>% of a finished acid and the metal miscellaneous salt filter cake.

A waste acid in titanium dioxide production had an acid concentration of <NUM>%, a ferrous sulfate content of <NUM>%, and an SS content of <NUM>/L; the waste acid in titanium dioxide production was pumped to a Venturi tower to allow direct-contact heat exchange, evaporation, and pre-concentration with a <NUM> rotary kiln tail gas, to obtain a pre-concentrated acid at <NUM> with an acid concentration increased to <NUM>% and a ferrous sulfate concentration of <NUM>%; the pre-concentrated acid was then pumped into a freezing crystallization system to allow freezing crystallization at <NUM> for <NUM> to obtain ferrous sulfate heptahydrate crystals with a grade of <NUM>% and a recovery rate on ferrous sulfate of <NUM>% after sampling detection; while a resulting freezing crystallization mother liquor had an acid concentration of <NUM>% and a ferrous sulfate concentration of <NUM>%; the freezing crystallization mother liquor was collected into an evaporation feed tank and then pumped into a double-effect evaporation device to obtain an acid-salt suspension concentrate; the acid-salt suspension concentrate overflowed to a maturation cooling tank to allow standing for approximately <NUM>, and was then pumped by a screw into a membrane filter press to obtain <NUM>% of a finished acid and a metal miscellaneous salt filter cake.

Example <NUM>: A waste acid in titanium dioxide production had an acid concentration of <NUM>%, a ferrous sulfate content of <NUM>%, and an SS content of <NUM>/L; the waste acid in titanium dioxide production was pumped to a Venturi tower to allow direct-contact heat exchange, evaporation, and pre-concentration with a <NUM> rotary kiln tail gas, to obtain a pre-concentrated acid at <NUM> with an acid concentration increased to <NUM>% and a ferrous sulfate concentration of <NUM>%; the pre-concentrated acid was then pumped into a freezing crystallization system to allow freezing crystallization at <NUM> for <NUM> to obtain ferrous sulfate heptahydrate crystals with a grade of <NUM>% and a recovery rate on ferrous sulfate of <NUM>% after sampling detection; while a resulting freezing crystallization mother liquor had an acid concentration of <NUM>% and a ferrous sulfate concentration of <NUM>%;.

further, a filtrate with an acid concentration of <NUM>% was pumped into a freezing crystallization system to allow the freezing crystallization at <NUM> for <NUM> to obtain ferrous sulfate heptahydrate crystals with a grade of <NUM>% and a secondary recovery rate on ferrous sulfate of <NUM>% after sampling detection; while a resulting freezing crystallization mother liquor had an acid concentration of <NUM>% and a ferrous sulfate concentration of <NUM>%; the freezing crystallization mother liquor was collected into an evaporation feed tank and pumped into a double-effect evaporation device to obtain an acid-salt suspension concentrate; and the acid-salt suspension concentrate overflowed to a maturation cooling tank to allow standing for approximately <NUM>, and was then pumped into a membrane filter press through a screw to obtain <NUM>% of a finished acid and the metal miscellaneous salt filter cake. The ferrous sulfate had a total recovery rate of <NUM>%.

The specific effect data of the examples were shown in Table <NUM>.

Samples of the waste acid in titanium dioxide production were collected from two different manufacturers. In order to find out the of low recovery rate of ferrous sulfate in the methods of direct freezing crystallization of waste acid in titanium dioxide production to obtain ferrous sulfate heptahydrate of Patents <CIT> and <CIT>, the direct freezing crystallization of waste acid in titanium dioxide production to obtain ferrous sulfate heptahydrate was simulated. Two sets of parallel experimental data were shown in Table <NUM>, and recovery rates of ferrous sulfate were both less than <NUM>%. In order to demonstrate the advancement of the present disclosure, the "pre-concentration + freezing crystallization" process of waste acid in titanium dioxide production was simulated, and two sets of parallel experimental data were shown in Table <NUM>. The minimum recovery rates of ferrous sulfate in Example <NUM> to Example <NUM> reached averagely not less than <NUM>%:.

The present disclosure may have the following advantages:.

Claim 1:
A method for recycling ferrous sulfate heptahydrate and sulfuric acid from a waste acid in titanium dioxide production, comprising the following steps:
step <NUM>: pre-concentrating the waste acid in titanium dioxide production to obtain a pre-concentrated acid;
step <NUM>: cooling the pre-concentrated acid by a heat exchanger, and conducting freezing crystallization for a period of time to obtain the ferrous sulfate heptahydrate and a crystallization mother liquor;
step <NUM>: concentrating the crystallization mother liquor in a waste acid evaporation system to obtain precipitated miscellaneous salt-containing suspended concentrated acid and evaporation condensed water; and
step <NUM>: subjecting the precipitated miscellaneous salt-containing suspended concentrated acid to maturing and cooling, and then transferring a resulting product into a filter press dehydration system to allow a treatment to obtain a metal miscellaneous salt filter cake and a filtered finished acid; wherein the metal miscellaneous salt filter cake serves as a raw material for extracting precious metals comprising titanium, manganese, magnesium, scandium, and vanadium as well as iron to allow resource recovery, and the filtered finished acid is returned to an ilmenite acid hydrolysis section;
wherein in step <NUM>, the pre-concentrating is conducted by rotary kiln flue gas spraying, the rotary kiln flue gas spraying in series with flash evaporation, or a combination of the flash evaporation, the rotary kiln flue gas spraying, and the flash evaporation in sequence the rotary kiln flue gas spraying concentrates the waste acid in titanium dioxide production to <NUM>% to <NUM>%.