CN113816393A

CN113816393A - Iron removal method for preparing high-purity boric acid

Info

Publication number: CN113816393A
Application number: CN202111214180.XA
Authority: CN
Inventors: 李洪岭; 张宁红
Original assignee: Qinghai Liyada Chemical Co ltd
Current assignee: Qinghai Liyada Chemical Co ltd
Priority date: 2021-10-19
Filing date: 2021-10-19
Publication date: 2021-12-21
Anticipated expiration: 2041-10-19
Also published as: CN113816393B

Abstract

The invention relates to the technical field of high-purity boric acid production, and provides an iron removal method for preparing high-purity boric acid. According to different iron contents in the crude boric acid (raw material), different methods are adopted for removing iron; when the iron content in the crude boric acid is more than or equal to 0.001 percent, mixing the crude boric acid, water and hydrogen peroxide for oxidation reaction to obtain an oxidation reaction solution with the boric acid concentration of less than 180g/L, carrying out heat preservation flocculation treatment on the oxidation reaction solution at the temperature of more than 65 ℃ for more than 30min, and then carrying out solid-liquid separation and cooling crystallization. When the iron content in the crude boric acid is less than or equal to 0.0015 percent, the boric acid is crystallized at high concentration, the produced mother liquor is mixed with hydrogen peroxide for oxidation reaction, and the mother liquor is recycled after flocculation and filtration, so that the iron content in the system is maintained at a low balance level, and the product quality is ensured. The method provided by the invention breaks through the bottleneck that the recrystallization method has strict requirements on the quality of the raw materials, has simple steps and low cost, and is convenient for realizing the large-scale production of high-purity boric acid products.

Description

Iron removal method for preparing high-purity boric acid

Technical Field

The invention relates to the technical field of high-purity boric acid, in particular to an iron removal method for preparing high-purity boric acid.

Background

The high-purity boric acid is an important inorganic salt, is widely applied to high-end fields such as capacitor manufacturing, electronic industry, high-purity analytical reagent, photoelectric industry, metallurgy, medicine, glass, ceramics, textile, tanning, pigment, paint and national defense, and occupies an important position in national economy. The national standard GB538-2018 requires that the impurity content of the second-class boric acid (high-purity boric acid) is between 1 and 3ppm, and the iron content is required to be below 2 ppm.

At present, high-purity boric acid is generally obtained by purifying industrial boric acid. The purification method of boric acid mainly comprises a recrystallization method, an esterification-distillation method, an ion exchange method, an electrochemical method, a permeation membrane method and the like. Among them, other methods than the recrystallization method can effectively remove impurities, but mass production has not been achieved due to the disadvantages of complicated operation, expensive auxiliary materials and equipment, small processing capacity, and the like. The recrystallization method is widely adopted because of simple process, convenient operation, low cost and capability of effectively removing impurities.

The main impurities in the boric acid raw material comprise iron, chlorine, sulfate radicals, water insoluble substances and the like, wherein the iron removal difficulty is the greatest, the influence of the impurity content of the raw material is avoided, when the iron content in the boric acid raw material exceeds 0.0010%, a satisfactory purification effect cannot be obtained only through simple recrystallization and washing, the simple recrystallization method is only suitable for the boric acid raw material which reaches or is superior to the industrial standard, and the application range of the raw material is very narrow.

At present, when the iron content in the boric acid raw material is too high, the boric acid raw material is generally purified by the following three methods: 1. during the recrystallization, the following additives are added: ammonia water, hypochlorous acid, calcium carbonate, sodium hydroxide, nitric acid, sulfuric acid, hydrochloric acid, polyacrylamide and other iron-removing agents are used for oxidation, precipitation and adsorption iron removal, and although the method can achieve the purpose of iron removal, more other impurity ions (such as Ca) are added²⁺、Na⁺、Cl^-、SO₄ ^2-Etc.), the difficulty and cost of product purification are increased, and the factors influencing the product quality are increased; 2. continuous repeated recrystallization, the method has the defects of overlong technological process, limited iron removal content range, high production cost and the like, and is only suitable for boric acid raw materials with the iron content of less than or equal to 0.0010 percent; 3. and multiple purification means are used together, and the method has complex and tedious process and small processing capacity, and can not realize large-scale production.

Most of boron ores in China are mixed ores of boron-magnesium stone and boron-magnesium-iron, and the other part of boron ores are salt-brine type and salt lake solid boron ores and are mainly distributed in modern salt lakes in Qinghai and Tibet. The salt lake brine contains rich compound salt liquid of potassium, lithium, boron, magnesium, sodium and other elements. In recent years, due to social development and scientific and technological progress, the demand of human beings for compounds such as potassium, lithium, boron, magnesium, sodium and the like is increased rapidly, which also promotes the rapid development of the lithium extraction technology in salt lake brine, and a large amount of high-impurity crude boric acid is produced as a byproduct in the lithium extraction of salt lake brine and the like, and the crude boric acid is discarded or is prepared into industrial boric acid by recrystallization.

In recent years, although the boric acid production process in China is greatly developed and improved, the production technology of high-purity boron products cannot be broken through all the time, the quality of the boric acid produced in large scale always stays in the industrial standard range, and the content of iron ions in the boric acid cannot reach the standard of the high-purity boric acid mainly because the iron ions are not removed properly.

Disclosure of Invention

In view of the above, the present invention provides an iron removal method for preparing high purity boric acid. The iron removal method specifically comprises a first method and a second method, wherein the first method is suitable for the boric acid raw material with the iron content of more than 0.001 wt%, and the second method is suitable for the boric acid raw material with the iron content of less than 0.0015 wt%.

In order to achieve the above object, the present invention provides the following technical solutions:

an iron removal method for preparing high-purity boric acid comprises the following steps:

mixing crude boric acid, water and hydrogen peroxide for oxidation reaction to obtain oxidation reaction liquid; the concentration of boric acid in the oxidation reaction liquid is less than or equal to 180g/L, and the iron content of the crude boric acid is more than 0.001 wt%;

carrying out heat preservation flocculation treatment on the oxidation reaction liquid, and then carrying out solid-liquid separation to obtain a filtrate; the temperature of the heat preservation flocculation treatment is more than 65 ℃, and the treatment time is more than 30 min;

and directly cooling and crystallizing the filtrate.

Preferably, the crude boric acid has an iron content of 0.001 to 0.025%.

Preferably, the concentration of boric acid in the oxidation reaction liquid is 80-180 g/L.

Preferably, the mixing for the oxidation reaction comprises: mixing the crude boric acid with water, heating the obtained mixed feed liquid to 25-40 ℃, then mixing the mixed feed liquid with hydrogen peroxide, continuously heating until the crude boric acid is completely dissolved, and carrying out oxidation reaction by keeping the temperature at which the crude boric acid is completely dissolved.

Preferably, the temperature of the oxidation reaction is 40-75 ℃; and the heat preservation time of the oxidation reaction is 0.5-1 h.

Preferably, the crude boric acid, water and hydrogen peroxide further comprise, before mixing: and (2) carrying out water washing pretreatment on the crude boric acid, wherein the water washing pretreatment is based on that the free acid of the obtained washing water is less than or equal to 0.10g/L, and the free acid is calculated by HCl.

Preferably, the hydrogen peroxide is used in the form of hydrogen peroxide, and the concentration of the hydrogen peroxide is 27-30%; the mass of the hydrogen peroxide is 0.05-0.1% of the weight of the crude boric acid.

Preferably, the solid-liquid separation is carried out under a heat preservation condition, and the temperature of the heat preservation is above 65 ℃.

Preferably, the cooling crystallization further comprises: carrying out solid-liquid separation on the obtained crystallization system to obtain wet material boric acid and mother liquor; the mother liquor is returned to replace water for oxidation reaction after iron is removed; the method for removing iron from the mother liquor comprises the following steps: and heating the mother liquor to be above 40 ℃, stirring for 2-8 h, and then carrying out solid-liquid separation.

Preferably, the mother liquor is heated directly or by industrial preheating.

Preferably, the wet material boric acid also comprises the following steps: and washing the wet material boric acid by pure water, and returning the generated washing water to replace water for oxidation reaction.

Preferably, when the iron content of the crude boric acid is 0.0015 wt% or less, the iron removal method is replaced with a method comprising the steps of:

mixing crude boric acid with iron content of less than 0.0015 wt% with water, and heating to dissolve to obtain a hot-melt reaction solution;

directly carrying out solid-liquid separation on the hot-melt reaction solution to obtain a filtrate;

cooling and crystallizing the filtrate to obtain wet boric acid and mother liquor;

mixing the mother liquor and hydrogen peroxide for oxidation reaction, carrying out solid-liquid separation on the obtained oxidation liquid to obtain iron-removing mother liquor, and returning the iron-removing mother liquor to be used for dissolving crude boric acid;

and washing the wet material boric acid with water to obtain high-purity boric acid and washing water, wherein the washing water is returned to be used for dissolving the crude boric acid.

The invention provides an iron removal method for preparing high-purity boric acid, which comprises the following steps: mixing the crude boric acid, water and hydrogen peroxide for oxidation reaction to obtain an oxidation reaction solution; the concentration of boric acid in the oxidation reaction liquid is less than or equal to 180g/L, and the iron content of the crude boric acid is more than 0.001 wt%; carrying out heat preservation flocculation treatment on the oxidation reaction liquid, and then carrying out solid-liquid separation to obtain a filtrate; the temperature of the heat preservation flocculation treatment is more than 65 ℃, and the treatment time is more than 30 min; and cooling and crystallizing the filtrate. The invention utilizes the corresponding relation between boric acid concentration and pH value to control the concentration of an oxidation reaction solution to be less than 180g/L, the pH value of the oxidation reaction solution within the boric acid concentration range is more than 2.67, and the oxidation reaction solution is provided with Fe³⁺Conditions of complete precipitation; the invention firstly utilizes hydrogen peroxide to lead Fe in the crude boric acid²⁺Is completely oxidized into Fe³⁺Fe produced by oxidation³⁺With OH in solution^-The method comprises the steps of reacting to form ferric hydroxide colloid, wherein the formed colloid particles are very small and are not easy to see, and the removal is very difficult through solid-liquid separation, wherein the oxidation reaction liquid is subjected to heat preservation flocculation treatment at the temperature of over 65 ℃, the viscosity of the solution is low under the temperature condition, the motion resistance of the colloid particles is low, the collision flocculation of the ferric hydroxide colloid is facilitated, the thermal motion of the ferric hydroxide colloid particles is accelerated under the high-temperature environment, the chances of mutual collision and combination of the colloid particles are greatly improved, so that the ferric hydroxide colloid particles are coagulated and enlarged and settled, and the ferric hydroxide is coagulated into bulk precipitate; the invention controls the time of the heat preservation flocculation treatment to be more than 30min, can ensure the full collision combination of the ferric hydroxide colloid, and can easily remove the ferric hydroxide colloidal particles through solid-liquid separation; fixing deviceAnd the residual iron ions in the filtrate after liquid separation form dissolution balance, and only a small amount of iron ions can be separated out in the cooling and crystallization process, so that the high-purity boric acid is obtained through crystallization.

The invention utilizes the corresponding relation between the boric acid concentration and the pH value to control the concentration of the oxidation reaction liquid to be less than 180g/L so as to enable the oxidation reaction liquid to have Fe³⁺The condition of completely precipitating iron hydroxide colloid is combined with the control of flocculation treatment temperature and time, so that the Fe in the crude boric acid is realized³⁺The method has the advantages that the method does not introduce any impurity ions in the whole process, does not need to add any alkaline substance, realizes the aim of removing iron, does not add any new impurity, and creates superior conditions for preparing high-purity boric acid.

Furthermore, the method provided by the invention firstly carries out water washing pretreatment on the crude boric acid, removes impurities and free acid in the raw materials through the water washing pretreatment, avoids the influence of the impurities and the free acid on the pH value of the oxidation reaction liquid, is convenient for realizing accurate correspondence of the boric acid concentration and the pH value when the pH value of the oxidation reaction liquid is controlled, and realizes accurate control of the pH value.

Further, after cooling and crystallizing, the method heats and stirs the generated mother liquor to promote the residual Fe in the mother liquor³⁺Further precipitating, and returning the mother liquor subjected to iron removal to replace water for oxidation reaction, thereby realizing the recycling of the mother liquor, reducing the wastewater discharge and the iron removal cost, and simultaneously recycling the boric acid remained in the mother liquor to avoid the waste of the boric acid; and when the production is enlarged, the mother liquor and the filtrate obtained by solid-liquid separation after the heat preservation flocculation treatment of the last batch can be subjected to circular heat exchange, the heating of the mother liquor and the cooling crystallization of the filtrate are realized, the heat energy in the cooling crystallization is fully utilized, the waste of the heat energy is avoided, and the iron removal cost is saved.

Furthermore, after cooling and crystallization, the wet material boric acid is washed by water, and the generated washing water is returned to replace water for oxidation reaction, so that the cyclic utilization of the washing water can be realized, the iron removal cost is further reduced, and the wastewater discharge is reduced.

Further, when the iron content of the crude boric acid is relatively low (the iron content is 0.0015 wt% or less), the iron removal method may be replaced with a method including the steps of: mixing and dissolving crude boric acid and water, filtering a dissolving solution, cooling and crystallizing, washing wet boric acid generated by crystallization with water, returning generated washing water to be used for dissolving the crude boric acid, and returning mother liquor generated by cooling and crystallizing to be used for dissolving the crude boric acid after removing iron. The total iron content of the industrial boric acid consists of ferrous iron and ferric iron, the pH value of ferric iron which starts to form ferric hydroxide is 2.67, and the pH value of ferrous iron which starts to form ferrous hydroxide is 7.6; when the concentration of the boric acid dissolving solution is 200g/L, the corresponding pH value is 2.5, and experiments show that under the condition that the total iron in the boric acid dissolving solution is less than or equal to 0.0025g/L, pH value is less than or equal to 2.67, most of the iron in the dissolving solution exists in an ion form, so that when the iron content in the crude boric acid is less than 0.0015 wt% and the concentration of the boric acid dissolving solution is more than 180g/L, less iron ions enter boric acid crystals, most of the iron ions finally enter mother liquor, iron accumulated in a circulating way can be continuously increased in a system, the product quality is further influenced, and an iron outlet is necessary. Therefore, aiming at the boric acid raw material with the iron content of less than 0.0015 wt%, the invention adopts a method of heating, flocculating and removing iron after the circulating mother liquor is oxidized by adding hydrogen peroxide, thereby realizing the controllability of the quality of the product of the circulating operation of the process; in addition, the method has no requirement on the concentration of the crude boric acid in the dissolving solution, and only needs to ensure that the crude boric acid is completely dissolved, so that the method has stronger operability and controllability, and can effectively increase the yield and reduce the product cost on the premise of ensuring the quality of the high-purity boric acid.

The method provided by the invention adopts different methods to remove iron aiming at the iron content in the raw materials, the raw boric acid raw materials comprise industrial boric acid, by-product boric acid obtained by lithium extraction from salt lake brine and the like, the raw materials have wide application range, and a high-purity boric acid product with the iron content reaching the standard can be obtained through one-time purification, so that the bottleneck problem that the recrystallization method has strict requirements on the quality of the raw materials is broken through; the method provided by the invention has the advantages of simple process, low investment, low cost, no introduction of other impurity ions, low pollution, high yield, stable product quality and strong controllability, is convenient for realizing the large-scale production of the high-purity boric acid product, has good economic and social benefits, can provide a cheaper high-purity boric acid product for the high-tech field, solves the embarrassment situation that the existing high-purity boric acid product only can depend on import, and promotes the development of the high-tech field.

The results of the examples show that the average iron removal rates of the two methods provided by the invention are both more than 96%, and the crude boric acid with the iron content of 0.022% and the industrial boric acid are used as raw materials, and after iron is removed by the corresponding method, the content of iron in the obtained boric acid product is as low as 0.0001%, which is superior to the requirement of the second-class boric acid (high-purity boric acid) in GB 538-2018.

Drawings

FIG. 1 is a process flow diagram of iron removal from high purity boric acid when the iron content of the raw material boric acid is above 0.001 wt.% (method one);

FIG. 2 is a process flow diagram of iron removal from high purity boric acid (method two) in another way when the iron content in the raw boric acid is below 0.0015 wt%;

FIG. 3 is a diagram showing the relationship between the pH value and the boric acid solution prepared from crude boric acid in salt lake without water washing pretreatment;

FIG. 4 is a diagram showing the relationship between the pH value and the boric acid solution prepared from crude boric acid in salt lake after water washing pretreatment.

Detailed Description

The invention principle of the invention is as follows:

the reaction equation of the reaction of the present invention is expressed as:

2Fe²⁺+H₂O₂+H⁺＝2Fe³⁺+2H₂O

H₂O＝H⁺+OH^-

Fe³⁺+3OH^-＝Fe(OH)₃↓

(1) according to Fe (OH)₃＝Fe³⁺+3OH^-Ion balance of (3), Fe (OH)₃Ksp＝1.0×10^-39When in solution CFe³⁺≤1.0×10^-5At mol/L, Fe³⁺The ion precipitation is complete, at which point the pH of the solution can be calculated by the following formula:

Ksp[Fe(OH)₃]＝C(Fe³⁺)·C³(OH^-)＝1.0×10^-39

C[H⁺]·C[OH^-]＝1×10^-14

that is, when the solution is an ideal solution (no impurities and interfering ions are present), Fe is present at a solution pH of 2.67³⁺Conditions of complete precipitation.

(2) According to H₂O＝H⁺+OH^-The ionization of water is an endothermic process. Table 1 shows the ionic product constants K (w) of water at different temperatures.

TABLE 1 Ionic product constant of water at different temperatures

Temperature (. degree.C.)	0	10	20	25	40	50	60	70	80	90	100
												Kw/10^-14	0.134	0.292	0.681	1.01	2.92	5.47	9.55	15.8	25.1	38.0	55.0

As can be seen from Table 1, the higher the temperature, the larger the ionic product constant of water, and the OH produced by ionization^-The more, the more favorable Fe³⁺Precipitation of (4).

From the above principle, it can be seen that the process goes through H₂O₂Oxidized Fe³⁺When the solution has a pH of 2.67 or more, Fe (OH) is formed₃A colloid; in addition, the higher the temperature, the more favorable the Fe (OH)₃Flocculation and sedimentation of colloid.

Based on the principle, the invention provides two iron removal methods for preparing high-purity boric acid, which are respectively suitable for a boric acid raw material with the iron content of more than 0.001 wt% and a boric acid raw material with the iron content of less than 0.0015 wt%, and are marked as a method I and a method II, and the methods are respectively described below.

In the present invention, the first method comprises the following steps:

and directly cooling and crystallizing the filtrate.

The method mixes the crude boric acid, water and hydrogen peroxide for oxidation reaction to obtain oxidation reaction liquid. In the present invention, the iron content of the crude boric acid is 0.001 wt% or more, more preferably more than 0.0015 wt%, further preferably 0.002 to 0.025 wt%, further preferably 0.005 to 0.02 wt%, and in the present invention, the iron content of the crude boric acid is calculated as the soluble iron content; the invention has no special requirement on the type of the crude boric acid, and the crude boric acid known by the technicians in the field can be adopted, specifically, the crude boric acid is a byproduct of lithium extraction from industrial boric acid or salt lake brine (the crude boric acid is referred to as salt lake crude boric acid for short); in the invention, the impurities in the industrial boric acid comprise chloride ions, sulfate ions and iron, and the impurities in the salt lake crude boric acid comprise calcium oxide and magnesium oxide besides the chloride ions, the sulfate ions and the iron.

The invention analyzes the impurities of 6 crude boric acid samples in salt lake and 2 industrial boric acid samples, and the analysis data is shown in the table 2:

TABLE 2 analysis data (wt%) of main components of industrial boric acid and crude boric acid from salt lake

Sample source	H₃BO₃	MgO	CaO	Cl^-	SO₄ ^2-	Fe
							Crude boric acid in salt lake	97.67	0.11	0.04	0.77	0.089	0.0150
Crude boric acid in salt lake	94.64	0.19	0.027	0.80	0.20	0.0097
							Crude boric acid in salt lake	94.95	0.28	0.16	1.15	0.053	0.0110
Crude boric acid in salt lake	93.99	2.68	0.095	0.95	0.071	0.0087
							Crude boric acid in salt lake	94.08	0.18	0.082	0.73	0.061	0.0220
Crude boric acid in salt lake	92.94	——	——	0.055	0.18	0.0015
							Industrial boric acid	99.67	——	——	0.005	0.1	0.0010
Industrial boric acid	99.89	——	——	0.004	0.3	0.0015

The invention has no special requirement on the water, and the water which is well known to the technical personnel in the field can be adopted, such as tap water; in the embodiment of the present invention, water is preferably used for removing iron from the first batch of crude boric acid, and water generated from the last batch of crude boric acid or washing water generated from washing wet boric acid is preferably used for removing iron from the subsequent batch of crude boric acid instead of water in the oxidation reaction, which will be described in detail later.

Before mixing the crude boric acid, water and hydrogen peroxide, the crude boric acid is preferably subjected to water washing pretreatment, the water washing pretreatment is based on water washing until the free acid of the obtained washing water is less than or equal to 0.10g/L, and the free acid is marked by HCl. In the present invention, the water washing pretreatment is preferably: stirring the crude boric acid and water at room temperature for 10-30 min, and then carrying out solid-liquid separation to obtain crude boric acid subjected to water washing pretreatment; the dosage ratio of the crude boric acid to the water is preferably 1 kg: 0.7-1.51L, preferably performing centrifugal separation in a solid-liquid separation mode, and preferably performing centrifugal separation by using a flat plate type closed centrifuge; the invention removes soluble impurity ions and free acid in the crude boric acid by water washing pretreatment. In the specific embodiment of the invention, the salt lake crude boric acid contains more impurities, and preferably is subjected to washing pretreatment and then subsequent iron removal, and the industrial boric acid containing less impurities can be directly subjected to iron removal without washing pretreatment.

In the invention, the hydrogen peroxide is preferably used in a form of hydrogen peroxide, and the concentration of the hydrogen peroxide is preferably 27-30%; the dosage of the hydrogen peroxide is preferably 0.05-0.1% of the weight of the crude boric acid. In a specific embodiment of the present invention, when the crude boric acid requires water washing pretreatment, the weight of the crude boric acid is based on the weight of the crude boric acid after water washing pretreatment.

In the present invention, the mixing for the oxidation reaction in the first method preferably comprises: mixing crude boric acid and water, heating the obtained mixed feed liquid to 25-40 ℃, then mixing the mixed feed liquid with hydrogen peroxide, continuously heating until the crude boric acid is completely dissolved, and carrying out oxidation reaction at the temperature of completely dissolving the crude boric acid; the temperature of the oxidation reaction is preferably 40-75 ℃, more preferably 45-65 ℃, and the time of the oxidation reaction is preferably 0.5-1 h, more preferably 0.8-1 h; in the invention, the larger the dosage of the crude boric acid is, the higher the temperature required for heating to completely dissolve the boric acid is, and the temperature is specifically in the range of 40-75 ℃. The invention controls the temperature of the oxidation reaction within the range, and can promote the oxydol to react Fe²⁺Is completely oxidized into Fe³⁺(ii) a In the present invention, the concentration of boric acid in the oxidation reaction solution is not more than 180g/L, preferably 80-180 g/L, and more preferably 110-170 g/L, and the concentration of boric acid in the oxidation reaction solution is controlled according to the corresponding relationship between the concentration of boric acid and the pH value of the solution to ensure that the oxidation reaction solution contains Fe³⁺Under the condition of complete precipitation, when the concentration in the oxidation reaction liquid is less than or equal to 180g/L, the pH value of the oxidation reaction liquid is greater than or equal to 2.67 theoretically, and the invention controls the concentration of boric acid in the oxidation reaction liquid to be Fe³⁺Providing conditions for complete precipitation of Fe without the need for additional alkaline material³⁺Is completely precipitated.

The inventor finds that the lower the boric acid concentration in the oxidation reaction liquid of the iron-removing process (namely the method I) of the high-iron raw material, the lower the Fe³⁺The more complete the precipitation, the higher the boric acid concentration in the oxidation reaction solution, the more acidic the reaction solution is, and the condition of ferric hydroxide decomposition can occur, when the boric acid concentration in the oxidation reaction solution is higher than 180g/L, the pH value of the solution is reduced to below 2.67, and at the moment, the more iron ions in the solution are, the higher the iron content in the crystallized boric acid can be caused; the invention can ensure Fe in the solution by controlling the boric acid concentration in the oxidation reaction solution to be less than 180g/L³⁺Forming dissolution balance, wherein the content of iron in the boric acid obtained by subsequent crystallization can meet the standard of high-purity boric acid; the present invention is based on the aboveThe discovery that the high-efficiency removal of iron ions in the boric acid is finally realized by controlling the concentration of the boric acid; and when the total iron in the boric acid solution is less than or equal to 0.0025g/L, PH and less than or equal to 2.67, most of the iron in the solution exists in the form of ions, so that when the iron content in the crude boric acid is less than 0.0015 wt%, the concentration of the boric acid solution is more than 180g/l, preferably 180-250 g/l, more preferably 180-220 g/l, less iron ions enter the boric acid crystal, most of the iron ions finally enter the mother liquor, and the removal of iron in the mother liquor link can ensure that the Fe in the whole process system is removed³⁺The balance that the total iron is less than or equal to 0.0025g/L is maintained, and the content of iron in the boric acid obtained by subsequent crystallization can meet the standard of high-purity boric acid; based on the discovery, the invention finally realizes the high-efficiency removal of iron ions in the boric acid by using the high-iron raw material and controlling the concentration of the boric acid; the iron is removed in a link of using a low-iron raw material and adopting mother liquor, so that the yield is effectively increased and the product cost is reduced on the premise of ensuring the quality of high-purity boric acid; the low-iron raw material is subjected to iron removal by the first method, and can also achieve the purpose of iron removal, only the yield is low, in the specific embodiment of the invention, the low-iron raw material is preferably subjected to iron removal by the second method, and the detailed description is provided later.

Table 3 shows the correspondence between the concentration and the pH of the boric acid solution, wherein the boric acid solution is prepared from boric acid and water, the boric acid used is a premium grade boric acid meeting the standards of GB538-2018, and the water is laboratory pure water:

TABLE 3 boric acid concentration vs. pH

H₃BO₃(g/L)	pH value	H₃BO₃(g/L)	pH value
				44.38	5.13	116.95	3.26
51.58	4.98	143.9	3.10
				56.38	4.75	159.53	2.92
63.57	4.59	161.33	2.91
				74.97	4.33	164.9	2.73
81.57	4.13	170.9	2.73
				94.76	3.91	181.17	2.67
97.56	3.71	212.8	2.46
				101.96	3.5	217.7	2.44
113.95	3.31	220	2.44

As can be seen from Table 3, the lower the boric acid concentration, the higher the pH of the solution, and the more favorable the Fe³⁺The precipitation of (2) using crude boric acid with high iron as a raw material (i.e., method one) according to the inventive principle described above can theoretically realize Fe when the pH value of the reaction solution is 2.67 or more³⁺Complete precipitation of (2); however, the production efficiency is influenced by too low boric acid concentration, so that the concentration of boric acid in the reaction liquid is preferably controlled to be 80-180 g/L, and Fe can be realized on the basis of ensuring the production efficiency³⁺Is completely precipitated. In the embodiment of the present invention, it is preferable to determine the amount of the crude boric acid by performing calculation based on the target boric acid concentration of the reaction liquid, the boric acid content of the crude boric acid, and the amount of water, and when the iron-removing mother liquid is used in place of water in the oxidation reaction, the boric acid contained in the iron-removing mother liquid should be counted; in addition, when the consumption of the hydrogen peroxide is calculated, the iron removal of the oxidation reaction liquid is calculated on the basis of the feeding amount of the crude boric acid, and the boric acid amount contained in the iron removal mother liquor is not counted; the mother liquor de-ironing is calculated on the basis of the volume of the mother liquor.

After the oxidation reaction is finished, the obtained oxidation reaction liquid is subjected to heat preservation flocculation treatment and then is subjected to solid-liquid separation to obtain a filtrate. In the present invention, the temperature of the thermal flocculation treatment is 65 ℃ or higher, preferably 70 ℃ or higher, more preferably 90 ℃ or higher, and still more preferably 90 ℃ to the boiling temperature of the reaction solution. In the specific embodiment of the invention, the temperature of the heat-preservation flocculation treatment is preferably higher than the temperature of the oxidation reaction; the time of the heat preservation flocculation treatment is more than 30min, and preferably 0.5-1 h; the heat-preservation flocculation treatment is preferably carried out under the condition of standing or slow stirring. In the specific embodiment of the invention, the oxidation reaction liquid is preferably directly heated to the temperature of the heat preservation flocculation treatment under stirring, and then the heat preservation flocculation treatment is carried out after stopping or slowly stirring; the rotation speed of the slow stirring is preferably 1-30 r/min, and the optimal rotation speed is preferably determined according to the diameter of the tank body in actual production. The invention controls the temperature and time of the heat preservation flocculation treatment, so that the ferric hydroxide colloid is collided and flocculated under the high temperature condition to form a bulk precipitate, thereby realizing the filtering removal of the ferric hydroxide without the adsorption of the active carbon.

In the present invention, the solid-liquid separation of the treatment liquid obtained after the flocculation treatment is preferably performed under a heat-retaining condition, the temperature of the heat-retaining is preferably 65 ℃ or higher, and in a specific embodiment of the present invention, the temperature of the heat-retaining is preferably the same as the temperature of the flocculation treatment; the solid-liquid separation mode is preferably filter pressing; in the specific embodiment of the invention, the titanium steel pressure filter with a heat preservation device is preferably adopted for pressure filtration, and the effective filtration area of the pressure filter is preferably 100cm². The invention carries out solid-liquid separation under the condition of heat preservation, and can avoid the boric acid loss caused by crystallization of boric acid due to temperature reduction; in the invention, the treatment liquid after flocculation treatment is preferably subjected to pressure filtration, and the filtrate is directly cooled and crystallized. In the invention, the Fe content in the filtrate obtained after iron removal is less than or equal to 0.0025 g/L.

After the filtrate is obtained, the filtrate is directly cooled and crystallized. In the invention, the temperature of the cooling crystallization is preferably below 25 ℃, and more preferably 20-25 ℃; preferably, the filtrate is cooled to below 25 ℃ under the stirring condition to separate out boric acid crystals, and then the obtained crystal system is subjected to solid-liquid separation to obtain wet material boric acid and mother liquor; the solid-liquid separation is preferably performed by centrifugal separation. In the field, after iron is removed by filtration, hydrochloric acid or sulfuric acid is added into a filtrate to adjust the pH value of the solution, and cooling crystallization is carried out under an acidic condition in the traditional recrystallization method.

In the invention, the mother liquor is preferably returned to replace water after iron is removed to carry out oxidation thermal-melting reaction; the method for removing iron from the mother liquor is preferably as follows: and heating the mother liquor to above 40 ℃, preferably to 40-60 ℃, stirring for 2-8 h, preferably stirring for 4-8 h, and then carrying out solid-liquid separation. In the invention, the heating rate of heating the mother liquor is preferably 4-8 ℃/h; the mother liquor is preferably heated directly or subjected to heat exchange with industrial waste heat, and the method for performing heat exchange with the industrial waste heat is preferably as follows: the mother liquor and the waste heat generated by the production system are subjected to heat exchange, so that the waste of heat energy is avoided, and the iron removal cost is saved. In a specific embodiment of the invention, the heat exchange is specifically that the mother liquor and a filtrate obtained by solid-liquid separation after heat preservation and flocculation treatment of the last batch are subjected to circulating heat exchange in a cooling and crystallization process, specifically, a crystallizer used for cooling and crystallization is preferably a cylindrical crystallizer with a jacket, the material of the inner wall of the crystallizer is preferably titanium steel, the material of the outer wall of the crystallizer is preferably carbon steel, the inner diameter of the crystallizer is preferably 30cm, the height is preferably 150cm, and the crystallizer is preferably provided with a titanium steel stirrer and a speed reducer in a matching manner; when the mother liquor is heated by adopting a circulating heat exchange method, the mother liquor is preferably introduced into a jacket of the crystallizer, filtrate to be crystallized is added into the crystallizer, and the mother liquor is circulated by a pump to carry out heat exchange with the filtrate.

In the invention, the mother liquor after heating and stirring is preferably subjected to solid-liquid separation under the condition of heat preservation, the method for solid-liquid separation is preferably filter-pressing separation, and the mother liquor after iron removal is transferred into a container for storage and standby.

The inventor finds that iron ions are kept in a certain concentration during the crystallization process and are not precipitated, but exist in the mother liquor in the form of ions, and the iron ions are oxidized into Fe³⁺The iron in the mother liquor is further removed based on the discovery, so that the iron ions introduced in the mother liquor recycling process are reduced as much as possible; in addition, partial boric acid is remained in the mother liquor, and the method can recycle the mother liquor after removing iron and avoid the waste of the boric acid. In the specific embodiment of the invention, when the Fe content in the iron-removing mother liquor is lower than 0.0008g/L, the qualified iron-removing mother liquor can be obtained.

After wet material boric acid is obtained, the invention preferably washes the wet material boric acid with water, and returns the generated washing water to replace water for carrying out hot melt oxidation reaction. According to the invention, wet boric acid and water are preferably mixed for pulp washing, washing water is separated for recycling, the washed boric acid is dried, packaged and warehoused, and the content of the dried ferric borate can meet the requirement of class II boric acid (high-purity boric acid) in GB 538-2018. In the specific embodiment of the present invention, if the content of other impurity ions in the boric acid product is high and cannot meet the requirements in GB538-2018, the boric acid product may be further processed by methods well known to those skilled in the art, and the present invention is not particularly limited.

In the specific embodiment of the invention, the iron removal mother liquor and the washing water are recycled, pure water is adopted only during the first feeding, and the iron removal mother liquor and the washing water are adopted to replace the pure water subsequently; in order to avoid the enrichment of impurity ions in the system, the invention preferably discharges 1m per ton of boric acid produced³Mother liquor, and make up for 1m³Pure water; the supplemented pure water is specifically introduced by washing the wet material of the end product.

In the invention, the second method is suitable for the boric acid raw material with the iron content of less than 0.0015 wt%, and is specifically described as follows:

the second method comprises the following steps:

cooling and crystallizing the filtrate to obtain wet material boric acid and mother liquor;

and washing the wet material boric acid with water to obtain high-purity boric acid and washing water, and returning the washing water to be used for dissolving the crude boric acid.

In the invention, the raw material used in the second method is crude boric acid with an iron content of less than 0.0015 wt%, and preferably crude boric acid with an iron content of 0.001-0.0013 wt%. In the invention, the iron content of the crude boric acid is calculated by the content of soluble iron; the invention has no special requirement on the type of the crude boric acid, and the crude boric acid known by the technicians in the field can be adopted, specifically, the crude boric acid is a byproduct of lithium extraction from industrial boric acid or salt lake brine (the crude boric acid is referred to as salt lake crude boric acid for short); in the invention, the impurities in the industrial boric acid comprise chloride ions, sulfate ions and iron, and the impurities in the salt lake crude boric acid comprise calcium oxide and magnesium oxide besides the chloride ions, the sulfate ions and the iron. In the specific embodiment of the invention, when the salt lake crude boric acid is used as a raw material, the crude boric acid is preferably subjected to water washing pretreatment before iron removal, and the iron content in the boric acid subjected to the water washing pretreatment is below 0.0015 wt%, namely the iron removal can be carried out by using the second method of the invention; the method for water washing pretreatment is consistent with the method I, and is not described herein again; when the industrial crude boric acid with the iron content of less than 0.0015 wt% is used as the raw material, the iron can be removed by directly adopting the second method without water washing pretreatment.

In the present invention, the index of the crude boric acid raw material (after water washing pretreatment) used in the second method preferably meets the index of GB 538-90 first-class products and above, specifically, the high-class products or first-class products specified in GB 538-90, and the indexes of the high-class products, first-class products and qualified products of boric acid in GB 538-90 are shown in Table 4:

indexes of superior boric acid, first grade boric acid and acceptable boric acid in GB 538-90 of Table 4

The invention mixes crude boric acid with iron content less than 0.0015 wt% with water and heats the mixture until the crude boric acid is dissolved, thus obtaining hot-melt reaction liquid. When the second method is used for removing iron, the concentration of the crude boric acid in the hot-melt reaction solution has no special requirement, and the crude boric acid can be completely dissolved; in the specific embodiment of the invention, the concentration of the crude boric acid in the dissolving solution is preferably more than 180g/L, more preferably more than 200g/L, further preferably 200-250 g/L, and further preferably 200-220 g/L; the heating temperature is preferably 90 ℃ or higher.

After obtaining the hot-melt reaction solution, the invention directly filters the hot-melt reaction solution to obtain the filtrate. In the present invention, the filtration mode is preferably the same as the filtration mode after the heat preservation flocculation treatment in the above scheme, and is not described herein again.

After the filtrate is obtained, the filtrate is cooled and crystallized to obtain wet material boric acid and mother liquor. In the invention, the temperature of the cooling crystallization is preferably below 25 ℃, more preferably below 20 ℃, and the invention preferably directly cools and crystallizes the obtained filtrate without adding any substance to adjust the pH value of the filtrate; the device used for cooling crystallization is preferably the same as that in the first method, and is not described in detail herein; preferably, the filtrate is cooled to below 25 ℃ under the stirring condition to separate out boric acid crystals, and then the obtained crystal system is subjected to solid-liquid separation to obtain wet material boric acid and mother liquor; the solid-liquid separation mode is preferably centrifugal separation, and is specifically consistent with the centrifugal separation method in the first method, and is not described herein again.

After the mother liquor is obtained, the mother liquor and hydrogen peroxide are mixed for oxidation reaction, the obtained oxidation liquid is subjected to solid-liquid separation to obtain iron-removing mother liquor, and the iron-removing mother liquor is returned to replace the iron-removing mother liquor for dissolving crude boric acid. In the invention, the hydrogen peroxide is preferably used in the form of hydrogen peroxide, the concentration of the hydrogen peroxide is preferably 27-30%, and the addition amount of the hydrogen peroxide is preferably 2 per thousand of the volume of the mother liquor.

In the present invention, the step of mixing the mother liquor and hydrogen peroxide to perform the oxidation reaction preferably comprises: and mixing the mother liquor and hydrogen peroxide, carrying out oxidation reaction for 20-40 min, preferably for 30min, then heating to above 40 ℃, preferably to 40-60 ℃, stirring for 2-8 h, preferably for 4-8 h, and then carrying out solid-liquid separation. In the invention, the heating rate of heating the mother liquor is preferably 4-8 ℃/h; the mother liquor is preferably heated directly or subjected to heat exchange with industrial waste heat, and the method for performing heat exchange with the industrial waste heat is preferably as follows: the mother liquor is subjected to heat exchange with the waste heat generated by the production system, so that the waste of heat energy is avoided, and the iron removal cost is saved; the specific heat exchange method is the same as the first method, and is not described in detail herein. The invention adds hydrogen peroxide into the mother liquor to oxidize iron ions in the mother liquor, and forms ferric hydroxide flocculation precipitation from oxidized ferric iron by heating and stirring for a period of time, so that the iron is filtered and removed, thereby achieving the purpose of recycling the mother liquor.

After wet boric acid is obtained, the wet boric acid is washed by water to obtain high-purity boric acid and washing water, and the washing water is returned to be used for dissolving crude boric acid. In the present invention, the washing process of the wet material boric acid is preferably consistent with the above scheme, and is not described herein again. In the embodiment of the invention, pure water is used only during the first dissolution, and then the iron-removing mother liquor and the washing water are used to replace the pure water, so as to avoid the enrichment of impurity ions in the system, the invention preferably discharges 1m of boric acid per ton of boric acid produced³Mother liquor, and make up for 1m³Pure water; the supplemented pure water is specifically introduced by washing the wet material of the end product.

The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention.

Fig. 1 is a flow chart of a process (i.e., a first method) for removing iron from high-purity boric acid in the embodiment of the present invention when the content of iron in crude boric acid is more than 0.001 wt%, wherein crude boric acid, water (or washing water and iron-removed mother liquor) and hydrogen peroxide after pretreatment and washing are mixed for oxidation reaction, then temperature is raised for thermal insulation flocculation treatment, treatment liquid is finely filtered for removing iron, filtrate is cooled and crystallized, the mother liquor generated after solid-liquid separation of crystallization liquid is recycled after iron removal, wet material boric acid obtained by solid-liquid separation of crystallization liquid is washed with water to obtain a high-purity boric acid product, and washing water is recycled.

Fig. 2 is a flow chart of a process (namely a method two) for removing iron from high-purity boric acid by adopting another mode when the iron content in the crude boric acid is below 0.0015 wt%, wherein solution boric acid is firstly subjected to hot melting, a dissolved solution is subjected to heat preservation and fine filtration, a filtrate is subjected to cooling crystallization, a crystallization system is subjected to filtration and separation and then is washed (washing water is recycled), so that high-purity boric acid is obtained, and mother liquor generated by crystallization is added with hydrogen peroxide for removing iron and then is recycled.

Comparative example 1

In the comparative example, crude boric acid (namely crude boric acid in salt lake) which is a byproduct in lithium extraction from salt lake brine is used as a raw material, the test is carried out in two groups, one group adopts crude boric acid which is not subjected to water washing pretreatment, the other group adopts crude boric acid which is subjected to water washing pretreatment, and the oxidation reaction is carried out by adopting an iron-removing mother solution; mainly investigating the influence of free acid in solutions with different boric acid concentrations on the corresponding PH value and iron removal effect when (1) crude boric acid which is not subjected to pretreatment washing is fed; (2) when the pretreated and washed crude boric acid is fed, corresponding relations between solutions with different boric acid concentrations and PH values and corresponding iron removal effects are achieved;

wherein the salt lake crude boric acid is gray white sticky smooth fine particles and needle crystals, and comprises the following components in percentage by mass: h₃BO₃94.95％，MgO 0.28％，CaO 0.16％，Cl 1.15％，SO₄ ^2-0.053％，Fe0.011％；

The appearance of the salt lake crude boric acid after water washing pretreatment is white fine particles and needle-shaped crystals, and the salt lake crude boric acid comprises the following components in percentage by mass: h₃BO₃87.23％，MgO 0.0035％，CaO 0.15％，Cl 0.067％，SO₄ ^2-0.050％，Fe0.0089％。

The iron removal mother liquor comprises the following components: h₃BO₃44.38g/L，MgO 0.011g/L，CaO 0.15g/L，Cl 0.20g/L，SO₄ ^2-0.13g/L，Fe 0.0008g/L。

The method comprises the following specific steps:

firstly, salt lake crude boric acid which is not pretreated by water washing is used as a raw material, and different boric acid concentrations and pH value corresponding relations and iron removal comparative tests are carried out.

1. The method comprises the steps of preparing a boric acid solution by using raw boric acid in a salt lake without being washed with water as a raw material and pure water, testing the pH value of the boric acid solution, and drawing the corresponding relation between the concentration and the pH value of the boric acid solution in the graph 3 and comparing the corresponding relation with the corresponding relation between the concentration and the pH value of the boric acid solution in the table 3. As can be seen from fig. 3, compared with the boric acid solution prepared by using a high-grade boric acid, when the boric acid solution is prepared by using unwashed crude boric acid, the corresponding relationship between the boric acid concentration and the pH value fluctuates due to a higher content of free acid (Cl), so that the precise corresponding mode is destroyed, and the precise control of iron removal cannot be realized.

2. Iron removal experiment

(1) Respectively putting salt lake crude boric acid which is not pretreated by water washing into 11 containers containing 50L of deironing mother liquor, wherein the dosage of the crude boric acid is calculated according to the concentration of boric acid in reaction liquid of 100g/L, 110g/L, 120g/L, 130g/L, 140g/L, 150g/L, 160g/L, 170g/L, 180g/L, 190g/L and 200g/L (the specific dosage is shown in Table 5); heating the mixture to 40 ℃ under stirring with the total volume of each tank of the stock solution being about 52-60L, and then adding hydrogen peroxide (H) according to 0.1 percent of the weight of the crude boric acid₂O₂The concentration is 30 percent, the specific addition amount is shown in table 5), the heating is continued until the crude boric acid in the container is completely dissolved, the reaction is carried out for 30min under the temperature (the specific temperature is shown in table 5) of complete dissolution, the heating is continued to 90 ℃, and the stirring flocculation is carried out for 1 h; the container is a 100L stainless steel cylinder, and a stainless steel stirring and speed reducer is arranged in a matching manner.

(2) Carrying out heat preservation and pressure filtration on the feed liquid obtained in the step (1) in a pressure filter under the same pressure condition, wherein the pressure filter is a titanium steel material pressure filter with a heat preservation device, and the effective filtration area is 100cm²。

(3) Respectively adding the filtrates obtained in the step (2) into 11 containers, and cooling, crystallizing, separating and drying under the same crystallization condition to obtain boric acid products; the container is a 100L stainless steel cylinder, and a stainless steel stirring and speed reducer is arranged in a matching manner.

And detecting the pH value, the iron content and the chlorine content of the reaction solution with different boric acid concentrations, and detecting the boric acid content, the iron content and the whiteness of the corresponding iron-removed filtrate and boric acid products, wherein the detection data are shown in Table 5.

TABLE 5 comparison table of iron removal experimental data of crude boric acid in unwashed salt lake at different concentrations

As can be seen from table 5: the crude boric acid in salt lake without pre-treatment and washing is directly fed, because free acid exists in the raw material, the whole acidity of the prepared boric acid solution is enhanced, the corresponding iron removal rate is also linearly reduced, and the boric acid product cannot achieve the purpose of removing iron.

And secondly, adopting the salt lake crude boric acid subjected to water washing pretreatment as a raw material, and carrying out corresponding relation of pH values of different boric acid concentrations and iron removal comparative tests.

1. Adopting salt lake crude boric acid pretreated by water washing as raw material (adding 0.16m of salt lake crude boric acid into a container)³Stirring tap water at normal temperature for 30min, performing solid-liquid separation by a centrifugal machine for multiple times, wherein the free acid (calculated by HCl) of produced washing water is 0.08g/L), preparing a boric acid solution by using pure water, testing the pH value of the boric acid solution, and drawing the corresponding relation between the concentration and the pH value of the boric acid solution in the graph of FIG. 4 and comparing the corresponding relation with the corresponding relation between the concentration and the pH value of the boric acid solution in the table 3. As can be seen from fig. 4, the crude boric acid washed by pretreatment realizes accurate correspondence of boric acid concentration to pH value due to washing away of soluble impurities and free acid (Cl) as compared with the boric acid solution prepared using the top grade boric acid.

2. Iron removal experiment

(1) 160kg of crude boric acid in salt lake is weighed in a container, and 0.16m is added³Stirring tap water at normal temperature for 30min, and centrifuging for several times to obtain solid-liquid mixtureSeparating, wherein the free acid (calculated by HCl) of the produced washing water is 0.08g/l, and the wet material boric acid is fully and uniformly mixed for later use; the container is a 150L stainless steel cylinder, a stainless steel stirring and speed reducer is arranged in a matching way, and the centrifuge is a flat plate type closed centrifuge.

(2) According to the boric acid concentration of reaction liquid of 100g/L, 110g/L, 120g/L, 130g/L, 140g/L, 150g/L, 160g/L, 170g/L, 180g/L, 190g/L and 200g/L, the wet crude boric acid material obtained in the step (1) is sequentially and respectively put into 11 containers containing 50L of deironing mother liquor (the specific adding amount of the crude boric acid is shown in table 5), about 60L of each feed liquid is heated to 40 ℃ under stirring, hydrogen peroxide is added according to the amount of 1kg/T boric acid (the specific adding amount is shown in table 6), the heating is continued to completely dissolve the boric acid raw material, the heat preservation reaction is carried out for 30min at the temperature when the boric acid raw material is completely dissolved (the specific temperature is shown in table 6), and the heating is continued to 90 ℃ for flocculation for 1 h; the container is a 100L stainless steel cylinder, and a stainless steel stirring and speed reducer is arranged in a matching manner.

(3) Carrying out heat preservation and pressure filtration on the feed liquid obtained in the step (2) in the same pressure filter under the same pressure condition; the filter press is made of titanium steel materials, and the effective filtering area is 100cm²。

(4) Respectively putting the filtrate obtained in the step (3) into 11 containers, and cooling, crystallizing, separating and drying under the same crystallization condition to obtain a boric acid product; the container is a stainless steel cylinder, and a stainless steel stirring and speed reducer is arranged in a matching manner.

And (3) detecting the pH value, the iron content and the chlorine content of the reaction solution with different boric acid concentrations, and detecting the boric acid content, the iron content and the whiteness of the filtrate and boric acid products after iron removal, wherein the detection data are shown in a table 6.

TABLE 6 comparison table of iron removal experimental data of crude boric acid in salt lake after washing under different concentrations

As can be seen from table 6: the pretreated and washed crude boric acid in the salt lake is adopted for feeding, and free acid in the raw materials is removed, so that the prepared boric acid solution has no interference of the free acid, the pH value of the real concentration of the boric acid is reflected, and the corresponding relation between the concentration of the boric acid and the pH value in the table 3 is met; the comparative example clearly shows that when the concentration of the boric acid is below 180g/L, the iron removal effect is excellent, and the iron content of the obtained boric acid product reaches and is superior to the standard of GB538-2018 boric acid.

In addition, compared with crude boric acid which is not pretreated by water washing, when the boric acid which is pretreated by water washing is used as a raw material, the whiteness of the obtained boric acid product is obviously better.

Comparative example 2

The optimal condition for deironing the mother liquor is examined by taking time and temperature as two variables, and the method comprises the following specific steps:

the test adopts the mother liquor without iron removal, and the components of the mother liquor without iron removal are as follows: Fe0.0025g/L, H₃BO₃44.1g/L and the pH value is 5.2; taking 50L of the mother liquor, placing into a container, respectively, performing iron removal comparative tests at four temperature stages of 30 deg.C, 40 deg.C, 50 deg.C and 60 deg.C for different time, because all Fe is in the oxidation reaction process²⁺Has been oxidized to Fe³⁺Therefore, hydrogen peroxide does not need to be added again for oxidation when the iron in the mother liquor is removed; stirring at constant temperature for 8h in each temperature section, sampling at each integral point, filtering under the same condition, detecting the iron content in the filtrate, calculating the iron removal rate under the condition, and observing the state of the mother liquor. The iron content and iron removal rate of the mother liquor at different temperatures and time points are shown in Table 7.

TABLE 7 comparison table of iron removal effect data of mother liquor at different temperatures and times

As can be seen from Table 7, the temperature increase helps to promote the formation of ferric hydroxide, the mother liquor temperature is only 30 ℃, the reaction is stirred at a constant temperature for 8h, and the mother liquor is in a clear state and Fe³⁺The content is only slightly reduced, and the iron removal rate is only 12 percent; the mother liquor can achieve the iron removal effect required (Fe is less than or equal to 0.0008g/l) when stirred for 6 hours at the constant temperature of 40 ℃; the iron removal can reach the standard after 2 hours at 50 ℃; the above results show that the temperature rise is favorablePromoting the formation of ferric hydroxide, and the longer the constant temperature reaction time is, the better the flocculation effect of the formed ferric hydroxide is, the better the effect of filtering and removing iron is, and the Fe in the mother liquor³⁺The more complete the removal.

Example 1

The method adopts crude boric acid in salt lake as raw material, and removes iron after pretreatment and washing.

Putting the salt lake crude boric acid into a container, adding tap water, stirring at normal temperature for 30min, and performing solid-liquid separation by using a centrifuge to obtain wet material crude boric acid, wherein the dosage ratio of the salt lake crude boric acid to the tap water is 1 kg: 1L of the compound.

The salt lake crude boric acid is an off-white sticky smooth fine particle and a needle crystal in appearance, and comprises the following components in percentage by mass: h₃BO₃94.08％，MgO 0.18％，CaO 0.082％，Cl 0.73％，SO₄ ^2-0.061％，Fe0.0220％；

The appearance of the crude boric acid after water washing pretreatment is white fine particles and needle-shaped crystals, and the crude boric acid comprises the following components in percentage by mass: h₃BO₃89.47％，MgO 0.0027％，CaO 0.012％,Cl 0.055％，SO₄ ^2-0.032％，Fe0.0120％。

The iron removal method comprises the following specific steps:

(1) 10kg of the crude boric acid after the water washing pretreatment is put into a 56L container for deironing mother liquor, and stirring and heating are started; the container is a cylinder with an inner wall of H150cm and a diameter of phi 30cm, made of titanium steel and made of carbon steel, and a titanium steel stirring and speed reducer is arranged in a matching manner; the iron removal mother liquor comprises the following components: h₃BO₃48.57g/L，MgO 0.011g/L，CaO 0.10g/L,Cl 0.36g/L，SO₄ ^2-0.11g/L，Fe0.0008g/L。

(2) Heating the feed liquid in the step (1) to 40 ℃, adding 10g of hydrogen peroxide according to 0.1 percent of the mass of the boric acid, heating until the raw materials are completely dissolved, wherein the temperature of the reaction liquid is 75 ℃, the pH value is 2.73, and H is₃BO₃The concentration is 181.27g/L, Fe, the content is 0.020g/L, the reaction is carried out for 30min under the condition of heat preservation at the temperature, then the heating is continued to 90 ℃, the heating is stopped, and the flocculation reaction is carried out for 1 h.

(3) Flocculating the step (2)Pouring the prepared boric acid liquid into a heat-preservation filter press for many times for filter pressing separation, and transferring the filtrate into a crystallizing tank; h of the filtrate₃BO₃The concentration of 180.20g/L, Fe is 0.0018g/L, pH, the value is 2.77, the filter press is a titanium steel material filter press with a heat preservation device, and the effective filtering area is 100cm²。

(4) Transferring the pressure filtrate obtained in the step (3) into a crystallizing tank for cooling and crystallizing, and introducing the mother liquor obtained in the last batch into a jacket of the crystallizing tank for heat exchange iron removal; the mother liquor obtained in the last batch comprises the following components: h₃BO₃47.36g/L，MgO 0.055g/L，CaO 0.25g/L，Cl 0.52g/L，SO₄ ^2-0.10g/L, Fe0.0025 g/L; the crystallizer is a carbon steel cylinder with jacket H150cm and phi 30cm inner wall titanium steel material outer wall, and is matched with a titanium steel stirring and speed reducer.

(5) Cooling the crystallization liquid in the step (4) to 25 ℃ under stirring, and then carrying out centrifugal separation to obtain wet material boric acid and mother liquor, wherein the mother liquor comprises the following components: h₃BO₃48.09g/L，MgO 0.069g/L，CaO0.21g/L，Cl 0.35g/L，SO₄ ^2-0.0088g/L and Fe0.0018g/L, wherein the mother liquor is deironing when the filtrate of the next batch is crystallized, and the deironing is recycled for blanking; mixing 10.3kg of wet material boric acid and 10L of pure water, washing, circularly discharging and using separated washing water, and drying to obtain 9.2kg of boric acid product with whiteness of 87, wherein the mass analysis data of the boric acid product is as follows: h₃BO₃100.56％，Cl^-0.0001％，SO₄ ^2-0.0003 percent of Fe and 0.0002 percent of Fe all reach the Chinese standard of GB538-2018 products (high-purity boric acid).

(6) Keeping the temperature of the mother liquor which is subjected to heat exchange to 45 ℃ in the step (4) and stirring for 8 hours, pouring the mother liquor into a heat-preservation filter press for multiple times for filter pressing separation, and transferring the filtrate into a container for storage for later use; the content composition of the iron removal mother liquor is as follows: h₃BO₃45.88g/L，MgO 0.055g/L，CaO 0.26g/L，Cl 0.52g/L，SO₄ ^2-0.10g/L，Fe0.0008g/L。

Example 2

The appearance of the salt lake crude boric acid is off-white sticky smooth fine particles and needle crystals, and the component content analysis is as follows by mass fraction: h₃BO₃89.78％，MgO 0.26％，CaO 0.19％,Cl 0.56％，SO₄ ^2-0.022％，Fe0.021％；

The appearance of the crude boric acid after water washing pretreatment is white fine particles and needle-shaped crystals, and the analysis content of the components is as follows: h₃BO₃90.78％，MgO 0.0031％，CaO 0.022％,Cl 0.010％，SO₄ ^2-0.0087％，Fe0.0098％。

(1) 6kg of crude boric acid after the water washing pretreatment and washing is put into a container containing 56L of iron-removing mother liquor, and stirring and heating are started; the container is a cylinder with an inner wall of H150cm and a diameter of 30cm, made of titanium steel, and an outer wall of carbon steel, and is matched with a titanium steel stirring and speed reducer; the components of the iron removal mother liquor content are as follows: h₃BO₃48.57g/L，MgO 0.011g/L，CaO 0.10g/L,Cl 0.36g/L，SO₄ ^2-0.11g/L，Fe0.0008g/L。

(2) Heating the feed liquid in the step (1) to 40 ℃, adding 3g of hydrogen peroxide according to 0.05 percent of the mass of the boric acid, heating until the raw materials are completely dissolved, wherein the temperature of the feed liquid is 65 ℃, the pH value is 3.27, and the pH value is H₃BO₃The concentration is 135.55g/L, Fe, the content is 0.011g/L, the reaction is carried out for 30min under the condition of heat preservation at the temperature, then the heating is continued to 90 ℃, the heating is stopped, and the flocculation reaction is carried out for 1h by standing.

(3) Pouring the boric acid solution flocculated in the step (2) into a heat-preservation pressure filter for multiple times for pressure filtration separation, and transferring the filtrate into a crystallization tank; h in the filtrate₃BO₃The concentration of 135.50g/L, Fe is 0.0010g/L, pH, the value is 3.3, the filter press is a titanium steel material filter press with a heat preservation device, and the effective filtering area is 100cm²。

(4) Transferring the press filtrate obtained in the step (3) into a crystallization tank, stirring, cooling and crystallizing, and introducing the last batch into a jacket of the crystallization tank to obtain the filtrateCarrying out heat exchange on the mother liquor to remove iron; the mother liquor obtained in the previous batch comprises the following components in percentage by weight: h₃BO₃45.29g/L，MgO 0.032g/L，CaO 0.22g/L，Cl 0.61g/L，SO₄ ^2-0.0069g/L, Fe0.0022 g/L; the crystallizer is a carbon steel cylinder with jacket H150cm and phi 30cm inner wall titanium steel material outer wall, and is matched with a titanium steel stirring and speed reducer.

(5) And (4) cooling the crystallization liquid in the step (4) to 25 ℃ under stirring, and performing centrifugal separation to obtain wet boric acid and mother liquor, wherein the mother liquor comprises the following components in percentage by weight: h₃BO₃47.22g/L，MgO 0.056g/L，CaO0.19g/L,Cl 0.32g/L，SO₄ ^2-0.0096g/L and Fe0.0006g/L, wherein the mother liquor is subjected to iron removal in the crystallization process of the filtrate of the next batch, and the iron is removed and then recycled for blanking; 6.5kg of wet material boric acid and 6L of pure water are mixed and washed, the separated washing water is recycled, and 6.1kg of boric acid layer products with whiteness of 95 are obtained after drying, and the quality analysis is as follows: h₃BO₃100.87％，Cl^-0.0001％，SO₄ ^2-0.0001 percent and Fe0.0001 percent are all superior to the national standard of GB538-2018 products (high-purity boric acid).

(6) Keeping the mother liquor which is subjected to heat exchange to 47 ℃ in the step (4) at the temperature, stirring for 8 hours, pouring the mother liquor into a heat-preservation pressure filter for multiple times for pressure filtration and separation, and transferring the filtrate into a container for storage and standby; the iron-removing mother liquor comprises the following components in percentage by weight: h₃BO₃45.5g/L，MgO 0.033g/L，CaO 0.20g/L，Cl 0.63g/L，SO₄ ^2-0.0057g/L，Fe0.0007g/L。

Example 3

The method is characterized in that outsourcing industrial boric acid is adopted as a raw material for iron removal, the appearance of the industrial boric acid is white fine particles, and the components are analyzed in percentage by mass as follows: h₃BO₃99.54％，MgO—，CaO—，Cl0.01％，SO₄ ^2-0.30％，Fe 0.0015％；

(1) Putting 8kg of the industrial boric acid into a 56L container of iron-removing mother liquor, starting stirring and heating; the container is a 100L stainless steel cylinder, and a stainless steel stirring and speed reducer is arranged in a matching way; the iron removal mother liquor comprises the following components: h₃BO₃48.57g/L，MgO 0.011g/L，CaO 0.10g/L，Cl0.36g/L，SO₄ ^2-0.11g/L，Fe0.0008g/L。

(2) Heating the feed liquid in the step (1) to 40 ℃, adding 5g of hydrogen peroxide according to 0.05 percent of the mass of the boric acid, heating until the raw materials are completely dissolved, wherein the temperature of the feed liquid is 73 ℃, the pH value is 2.98, and the pH value is H₃BO₃The concentration is 179.6g/L, Fe, the content is 0.0030g/L, the reaction is kept at the temperature for 30min, the heating is continued to 90 ℃, the heating is stopped, and the flocculation reaction is kept for 30 min.

(3) Pouring the boric acid solution flocculated in the step (2) into a heat-preservation pressure filter for multiple times for pressure filtration separation, and transferring the filtrate into a crystallization tank; h in the filtrate₃BO₃The concentration of 180.5g/L, Fe is 0.0011g/L, pH value is 3.0, the filter press is a titanium steel material filter press with a heat preservation device, and the effective filtering area is 100cm²。

(4) Transferring the press filtrate obtained in the step (3) into a container, stirring, cooling and crystallizing; the crystallizer is a cylinder for processing 100L stainless steel, and is matched with a stainless steel stirring and speed reducer.

(5) Cooling the crystallization liquid obtained in the step (4) to 22 ℃ under stirring, and performing centrifugal separation to obtain wet material boric acid and mother liquor, wherein the mother liquor comprises the following components: h₃BO₃46.22g/L，Mg O 0.013g/L，CaO0.097g/L,Cl 0.35g/L，SO₄ ^2-0.013g/L and Fe0.0017g/L, the mother liquor is recycled after iron removal, 9kg of wet material boric acid and 9L of pure water are mixed and washed, the separated washing water is recycled for use, 8.1kg of boric acid product is obtained after drying, the whiteness is 91, and the mass analysis is as follows: h₃BO₃100.33％，Cl^-0.0001％,SO₄ ^2-0.0001 percent and 0.0001 percent of Fe are all superior to the national standard of GB538-2018 products (high-purity boric acid).

Example 4

The appearance of the salt lake crude boric acid is off-white sticky smooth fine particles and needle crystals, and the component content analysis is as follows by mass fraction: h₃BO₃94.64％，MgO 0.19％，CaO 0.027％,Cl 0.80％，SO₄ ^2-0.20 percent and Fe0.0097 percent; the appearance of the crude boric acid after water washing pretreatment is white fine particles and needle-shaped crystals, and the analysis content of the components is as follows: h₃BO₃89.15％，MgO 0.027％，CaO 0.020％,Cl0.017％，SO₄ ^2-0.0042％，Fe0.0069％。

(1) 2.5kg of the crude boric acid after the water washing pretreatment and washing is put into a container containing 56L of iron-removing mother liquor, and stirring and heating are started; the container is a cylinder with an inner wall of H150cm and a diameter of 30cm, made of titanium steel, and an outer wall of carbon steel, and is matched with a titanium steel stirring and speed reducer; the components of the iron removal mother liquor content are as follows: h₃BO₃43.59g/L，MgO 0.010g/L，CaO 0.0097g/L,Cl 0.21g/L，SO₄ ^2-0.0045g/L，Fe0.0007g/L。

(2) Heating the feed liquid in the step (1) to 25 ℃, adding 1.3g of hydrogen peroxide according to 0.05 percent of the mass of the boric acid, heating until the raw materials are completely dissolved, wherein the temperature of the feed liquid is 40 ℃, the pH value is 4.15, and the pH value is H₃BO₃The concentration is 80.32g/L, Fe, the content is 0.0039g/L, the reaction is kept for 30min at the temperature, then the heating is continued to 65 ℃, the heating is stopped, and the flocculation reaction is stirred at a slow speed for 30 min.

(3) Pouring the boric acid solution flocculated in the step (2) into a heat-preservation pressure filter for multiple times for pressure filtration separation, and transferring the filtrate into a crystallization tank; h in the filtrate₃BO₃The concentration of 80.5g/L, Fe is 0.0009g/L, pH value is 4.15, the filter press is a titanium steel material filter press with a heat preservation device, and the effective filtering area is 100cm²。

(4) And (4) transferring the press filtrate obtained in the step (3) into a crystallizing tank, stirring, cooling and crystallizing, wherein the crystallizer is a carbon steel cylinder with H150cm and phi 30cm inner wall and titanium steel outer wall, and a titanium steel stirring and speed reducer is arranged in a matching manner.

(5) Cooling the crystallization liquid obtained in the step (4) to 25 ℃ under stirring, and performing centrifugal separation to obtain wet boric acid and mother liquor, wherein the content of the mother liquor isThe method comprises the following steps: h₃BO₃46.97g/L，MgO 0.027g/L，CaO0.018g/L,Cl 0.29g/L，SO₄ ^2-0.0058g/L and Fe0.0012g/L, wherein the mother liquor is subjected to iron removal in the crystallization process of the filtrate in the next batch, and the iron is removed and recycled for blanking; mixing the wet material boric acid 2.55kg and pure water 2.5L, washing, circularly discharging and using the separated washing water, and drying to obtain boric acid layer product 2.2kg, wherein the whiteness is 93, and the quality analysis is as follows: h₃BO₃99.98％，Cl^-0.0001％，SO₄ ^2-0.0001 percent and 0.0002 percent of Fe, which meet the national standard of GB538-2018 products (high-purity boric acid).

Example 5

(1) 5kg of the crude boric acid after the water washing pretreatment and washing is put into a container containing 56L of iron-removing mother liquor, and stirring and heating are started; the container is a cylinder with an inner wall of H150cm and a diameter of 30cm, made of titanium steel, and an outer wall of carbon steel, and is matched with a titanium steel stirring and speed reducer; the components of the iron removal mother liquor content are as follows: h₃BO₃43.59g/L，MgO 0.010g/L，CaO 0.0097g/L，Cl 0.21g/L，SO₄ ^2-0.0045g/L，Fe0.0007g/L。

(2) Heating the feed liquid in the step (1) to 30 ℃, adding 2.5g of hydrogen peroxide according to 0.05 percent of the mass of the boric acid, and heating until the raw materials are completely dissolved, wherein the feed is prepared at the momentThe temperature of the solution was 60 ℃, pH 3.35, H₃BO₃The concentration is 115.2g/L, Fe, the content is 0.0065g/L, the reaction is carried out for 30min under the condition of heat preservation at the temperature, then the heating is continued to 80 ℃, the heating is stopped, and the flocculation reaction is carried out for 60min by standing.

(3) Pouring the boric acid solution flocculated in the step (2) into a heat-preservation pressure filter for multiple times for pressure filtration separation, and transferring the filtrate into a crystallization tank; h in the filtrate₃BO₃The concentration of the titanium dioxide filter is 115.6g/L, Fe, the content of the titanium dioxide filter is 0.0012g/L, pH, the value of the titanium dioxide filter is 3.35, the filter press is a titanium steel filter press with a heat preservation device, and the effective filtering area is 100cm²。

(5) Cooling the crystallization liquid obtained in the step (4) to 23 ℃ under stirring, and performing centrifugal separation to obtain wet boric acid and mother liquor, wherein the mother liquor comprises the following components in percentage by weight: h₃BO₃41.89g/L，MgO 0.033g/L，CaO0.021g/L,Cl 0.35g/L，SO₄ ^2-0.0063g/L, Fe0.0015g/L, and the mother liquor is deironing in the crystallization process of the filtrate of the next batch, and is recycled for blanking after deironing; 5.1kg of wet material boric acid and 5L of pure water are mixed and washed, the separated washing water is recycled, 4.7kg of boric acid layer products are obtained after drying, the whiteness is 90.3, and the quality analysis is as follows: h₃BO₃100.63％，Cl^-0.0001％，SO₄ ^2-0.0001 percent and Fe0.0001 percent, which are superior to the national standard of GB538-2018 products (high-purity boric acid).

Comparative example 3

The comparative example adopts crude boric acid as a byproduct in lithium extraction from salt lake brine (namely, crude boric acid in salt lake) as a raw material, and adopts the salt lake crude boric acid subjected to water washing pretreatment and an iron removal mother liquor to prepare an oxidation reaction solution; mainly comprising the following steps: the oxidation reaction liquid is flocculated at different temperatures for the same time to remove iron.

Wherein the salt lake crude boric acid is gray white sticky smooth fine particles and needle crystals, and comprises the following components in percentage by mass: h₃BO₃93.99％，MgO 2.68％，CaO 0.095％，Cl 0.95％，SO₄ ^2-0.071％，Fe0.0087％；

The appearance of the crude boric acid after water washing pretreatment is white fine particles and needle-shaped crystals, and the crude boric acid comprises the following components in percentage by mass: h₃BO₃90.21％，MgO 0.13％，CaO 0.033％，Cl 0.027％，SO₄ ^2-0.049％，Fe0.0052％。

The iron removal mother liquor comprises the following components: h₃BO₃46.5g/L，MgO 0.016g/L，CaO 0.10g/L，Cl 0.25g/L，SO₄ ^2-0.089g/L，Fe 0.0006g/L。

The method comprises the following specific steps:

(1) 5.0kg of crude boric acid pretreated by water washing is put into a container containing 56L of iron-removing mother liquor, stirred and heated to 25 ℃, and 30 percent of H is added according to 0.05 percent of the mass of the boric acid₂O₂2.5g, heating until the raw materials are completely dissolved, wherein the temperature of the oxidation feed liquid is 61 ℃, the pH value is 3.28, and the pH value is H₃BO₃CaO with a concentration of 120.7g/L, MgO:0.12 g/l; 0.11g/l, Cl 0.27g/l, SO₄ ^2-0.13g/l, Fe: 0.0049g/L, and keeping the temperature at the temperature for reaction for 30 min; the container is a cylinder made of carbon steel and provided with an inner wall made of titanium steel and an outer wall made of carbon steel and having an inner diameter of H150cm and a diameter of phi 30cm, and a titanium steel stirring and speed reducer is arranged in a matching manner.

Averagely dividing the oxidation reaction liquid (the total volume is 60L) in the step (1) into 6 parts, placing the 6 parts in 6 containers, respectively heating to the temperature of 61 ℃, 65 ℃, 75 ℃, 85 ℃, 90 ℃ and 98 ℃ (boiling), standing, preserving heat, flocculating for 30min, preserving heat and filtering in a filter press under the same condition, and sampling the filtrate to detect the values of boric acid, iron and PH; the filter press is a titanium steel material filter press with a heat preservation device, and the effective filtering area is 100cm². The iron removal effect of flocculation at different temperatures for 30min is shown in Table 8.

TABLE 8 data table of effect of flocculation deferrization at different temperatures of oxidation reaction solution

As can be seen from the comparative data of the flocculation temperature in Table 8, the higher the flocculation temperature is at the same time, the better the effect of the flocculation iron removal is; the flocculation iron removal rate at 61 ℃ is only 18.4 percent, and the iron removal rate at 65 ℃ is as high as 53 percent; the amplification test and the statistics of test production data show that when Fe in the iron-removing filtrate is less than or equal to 0.0025g/L, the Fe content of the product is less than or equal to 0.0002 percent, and the product reaches the national standard of GB538-2018 products (high-purity boric acid).

Example 6 (Low iron material iron removal process)

The appearance of the salt lake crude boric acid is off-white sticky smooth fine particles and needle crystals, and the component content analysis is as follows by mass fraction: h₃BO₃92.94％，MgO—，CaO—,Cl 0.055％，SO₄ ^2-0.18 percent and Fe0.0015 percent; the appearance of the crude boric acid after water washing pretreatment is white fine particles and needle-shaped crystals, and the analysis content of the components is as follows: h₃BO₃88.97％，Cl 0.019％，SO₄ ^2-0.0037％，Fe0.0013％。

(1) Taking 11kg of crude boric acid after the water washing pretreatment and washing, putting the crude boric acid into a container containing 56L of iron removal mother liquor, starting stirring and heating; the container is a cylinder with an inner wall of H150cm and a diameter of 30cm, made of titanium steel, and an outer wall of carbon steel, and is matched with a titanium steel stirring and speed reducer; the components of the iron removal mother liquor content are as follows: h₃BO₃52.12g/L，Cl 0.022g/L，SO₄ ^2-0.16g/L，Fe0.0003g/L。

(2) Heating the feed liquid in the step (1) to be completely dissolved, wherein the temperature of the feed liquid is more than or equal to 90 ℃, and H₃BO₃The concentration of 218.52g/L, Fe content is 0.0025 g/L.

(3) Pouring the boric acid solution completely dissolved in the step (2) into a heat-preservation filter press for multiple times for filter-pressing separation, and transferring the filtrate into a crystallizing tank; h in the filtrate₃BO₃The concentration of 220.21g/L, Fe is 0.0025g/L, and the filter press is a titanium steel material filter press with a heat preservation deviceEffective filtration area 100cm²。

(4) Transferring the press filtrate obtained in the step (3) into a crystallizing tank, stirring, cooling and crystallizing, cooling the crystallized liquid to 25 ℃, performing centrifugal separation to obtain wet material boric acid and mother liquid, mixing 11.5kg of the wet material boric acid and 11.5L of pure water, performing slurry washing, discharging and hot-dissolving use of separated washing water circularly, and drying to obtain 10.5kg of a product, wherein the mass analysis is as follows: h₃BO₃100.11％，Cl^-0.0001％，SO₄ ^2-0.0001 percent and 0.0001 percent of Fe are all superior to the national standard of GB538-2018 products (high-purity boric acid).

(5) The mother liquor 55m separated in the step (4) is³The content of the mother liquor is as follows: h₃BO₃55.31g/L,Cl 0.032g/L，SO₄ ^2-0.26g/L, Fe0.0023g/L, adding 11g of hydrogen peroxide according to 0.2 per mill of the volume of mother liquor, stirring and oxidizing at the temperature for 30min at a low speed, stirring and heating to 60 ℃, preserving heat and oxidizing for 2h, pouring into a heat preservation filter press for many times to carry out filter pressing and separation, and transferring the filtrate into a container for storage and standby; the iron-removing mother liquor comprises the following components in percentage by weight: h₃BO₃57.20g/L，Cl 0.037g/L，SO₄ ^2-0.22g/L，Fe0.0003g/L。

Example 7 (Low iron material iron removal Process)

(1) 10.5kg of the purchased industrial boric acid is put into a container containing 56L of iron-removing mother liquor, and stirring and heating are started; the container is a cylinder with an inner wall of H150cm and a diameter of 30cm, made of titanium steel, and an outer wall of carbon steel, and is matched with a titanium steel stirring and speed reducer; the components of the iron removal mother liquor content are as follows: h₃BO₃57.20g/L，Cl 0.037g/L，SO₄ ^2-0.22g/L，Fe0.0003g/L。

(2) Heating the feed liquid in the step (1) to be completely dissolved, wherein the temperature of the feed liquid is more than or equal to 90 ℃, and H₃BO₃The concentration is 221.05g/L, FeThe content is 0.0024 g/L.

(3) Pouring the boric acid solution completely dissolved in the step (2) into a heat-preservation filter press for multiple times for filter-pressing separation, and transferring the filtrate into a crystallizing tank; h in the filtrate₃BO₃The concentration of 221.15g/L, Fe is 0.0024g/L, the filter press is a titanium steel material filter press with a heat preservation device, and the effective filtering area is 100cm²。

(4) Transferring the press filtrate obtained in the step (3) into a crystallizing tank, stirring, cooling and crystallizing, cooling the crystallized liquid to 25 ℃ under stirring, performing centrifugal separation to obtain wet material boric acid and mother liquid, mixing and washing 12kg of the wet material boric acid and 12L of pure water, performing circular blanking and use on separated washing water, and drying to obtain 11kg of product, wherein the mass analysis is as follows: h₃BO₃99.97％，Cl^-0.0001％，SO₄ ^2-0.0001 percent and 0.0001 percent of Fe are all superior to the national standard of GB538-2018 products (high-purity boric acid).

(5) 54.8m of mother liquor separated in the step (4)³The content of the mother liquor is as follows: h₃BO₃49.2g/L,Cl 0.038g/L，SO₄ ^2-0.25g/L, Fe0.0022g/L, adding 11g of hydrogen peroxide according to 0.2 per mill of the volume of the mother liquor, slowly stirring and oxidizing for 30min at the temperature, stirring and heating to 60 ℃, preserving heat and oxidizing for 2h, pouring into a heat preservation filter press for many times for filter pressing and separation, and transferring the filtrate into a container for storage and standby; the iron-removing mother liquor comprises the following components in percentage by weight: h₃BO₃57.20g/L，MgO 0.037g/L，CaO 0.22g/L，Cl 0.71g/L，SO₄ ^2-0.0060g/L，Fe0.0002g/L。

As can be seen from the above examples, different iron removal modes are adopted according to different iron contents in the raw materials; when the iron content in the raw material is more than 0.001 wt%, the method accurately controls the concentration of the boric acid in the oxidation reaction liquid according to the corresponding relation between the boric acid solution and the pH value, wherein the concentration is Fe³⁺The complete precipitation of the water can be provided with conditions, and simultaneously, the water can be promoted to be separated from enough OH by controlling the temperature of the heat-preservation flocculation treatment^-So that Fe is present³⁺The ferric hydroxide is completely precipitated, and the invention controls the time of heat preservation flocculation treatment to ensure that ferric hydroxide colloid is collided and flocculated to form bulk precipitateThereby realizing the filtration and removal of the filtrate; furthermore, when the iron content in the raw materials is less than or equal to 0.0015 wt%, the invention can also adopt high-concentration hot-melt crystallization, and iron is removed by adding hydrogen peroxide into the produced mother liquor, so that the obvious effects of yield increase and energy saving are realized on the premise of ensuring the product quality; the method does not introduce any impurity ions while realizing iron removal, has high iron removal rate, and reduces the iron content in the boric acid product to be less than 0.0002%.

The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.

Claims

1. The iron removal method for preparing the high-purity boric acid is characterized by comprising the following steps of:

and directly cooling and crystallizing the filtrate.

2. The method for removing iron according to claim 1, wherein the crude boric acid has an iron content of 0.001 to 0.025%.

3. The method for removing iron according to claim 1, wherein the concentration of boric acid in the oxidation reaction solution is 80 to 180 g/L.

4. The iron removal method according to any one of claims 1 to 3, wherein the mixing for oxidation comprises: mixing the crude boric acid with water, heating the obtained mixed feed liquid to 25-40 ℃, then mixing the mixed feed liquid with hydrogen peroxide, continuously heating until the crude boric acid is completely dissolved, and carrying out oxidation reaction by keeping the temperature at which the crude boric acid is completely dissolved.

5. The iron removal method according to claim 4, wherein the temperature of the oxidation reaction is 40-75 ℃; and the heat preservation time of the oxidation reaction is 0.5-1 h.

6. The iron removal method according to any one of claims 1 to 3, wherein the crude boric acid, water and hydrogen peroxide before mixing further comprises: and (2) carrying out water washing pretreatment on the crude boric acid, wherein the water washing pretreatment is based on that the free acid of the obtained washing water is less than or equal to 0.10g/L, and the free acid is calculated by HCl.

7. The iron removal method of claim 1, wherein the hydrogen peroxide is used in the form of hydrogen peroxide, and the concentration of the hydrogen peroxide is 27-30%; the mass of the hydrogen peroxide is 0.05-0.1% of the weight of the crude boric acid.

8. The iron removal method according to claim 1, wherein the solid-liquid separation is performed under a temperature maintaining condition at a temperature of 65 ℃ or higher.

9. The iron removal method of claim 1, further comprising, after said cooling crystallization: carrying out solid-liquid separation on the obtained crystallization system to obtain wet material boric acid and mother liquor; the mother liquor is returned to replace water for oxidation reaction after iron is removed; the method for removing iron from the mother liquor comprises the following steps: and heating the mother liquor to be above 40 ℃, stirring for 2-8 h, and then carrying out solid-liquid separation.

10. The iron removal method of claim 9, wherein the mother liquor is heated directly or by heat exchange with industrial waste heat.

11. The iron removal method of claim 9, wherein obtaining the wet boric acid further comprises: and washing the wet material boric acid by pure water, and returning the generated washing water to replace water for oxidation reaction.

12. The iron removal method according to claim 1, wherein when the iron content of the crude boric acid is 0.0015 wt% or less, the iron removal method is replaced with a method comprising the steps of: