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When the ratio is greater than 0.08, P 2 O 5 is degraded, and the effective phosphorus is lowered. In China's phosphate rock quality requirements, P 2 O 5 ≥ 31%. According to this ratio, if P 2 O 5 is 31%, the content of Fe 2 O 3 ≥ 2.48%, which will degrade P 2 O 5 . The use of waste acid increases impurities such as iron and aluminum, which causes a decrease in soluble phosphorus. Therefore, it is necessary to strictly control the content of iron and aluminum in the production of calcium. The reaction formula of FeSO 4 in waste acid and Fe 2 O 3 in phosphate rock to degrade effective P 2 O 5 is as follows:
Treatment and recycling of waste acid---Supply calcium (phosphorus fertilizer)
Titanium white powder in the production of sulfuric acid, resulting in a large number of difficult to deal with waste acid, and calcium superphosphate production and require large amounts of sulfuric acid, the possibility of using waste acid to produce calcium superphosphate it? Some experts believe that it is not advisable to make calcium directly from waste acid.
(I. Introduction
In the production of titanium dioxide in the sulfuric acid process, it takes about four tons of titanium dioxide per ton of sulfuric acid to decompose the iron titanium ore. During the decomposition process, most of the sulfuric acid reacts with ilmenite to form soluble titanium and iron sulfates, and some of the sulfuric acid does not react and becomes a free acid present in the product, which is leached into an acidic sulfate solution. When the titanium liquid is hydrolyzed, titanyl acid and new sulfuric acid are formed in the titanium oxysulfate to increase the sulfuric acid concentration of the solution. Finally, after filtration, the solid meta-titanic acid is removed, and the filtrate is concentrated acid. About 1 ton of titanium dioxide is produced, and about 8 tons of concentrated acid with sulfuric acid of 210-310 g/L can be obtained. These waste acids are very large and difficult to handle. If they are discharged into rivers, they will seriously pollute the environment.
SSP is manufactured (Normal over calcium phosphate) just requires a lot of concentration of approximately 65% sulfuric acid. If the above-mentioned waste acid is added to a certain concentration of concentrated sulfuric acid with a concentration of 93% to 98%, it is formulated into a sulfuric acid with a concentration of 65% for making calcium, which can utilize waste acid, reduce environmental pollution, and reduce sulfuric acid. The use of dosage, lower production costs, and improved economic efficiency are good things for both, and the best of both worlds. Therefore, some manufacturers have used titanium white waste acid to directly produce calcium, and have achieved good economic benefits, and published related articles. Some people think that only seeing the available side of sulfuric acid in the waste acid, and not seeing the side effects of impurities in the waste acid, the following views are raised on this issue.
(2) It is not advisable to use calcium white waste acid to make calcium directly.
It is well known that the growth of plants requires the absorption of phosphorus. The required phosphorus is soluble phosphorus. Only soluble phosphorus can be absorbed by the roots of plants. If it is insoluble phosphorus, plants are difficult to absorb. For this reason, it is necessary to convert the phosphate rock containing insoluble phosphorus (the main component is tricalcium phosphate) with sulfuric acid to convert it into soluble phosphorus dicalcium phosphate (also called monocalcium phosphate), and then apply it to plants. Where the growth takes place, the plant can absorb phosphorus, and the reaction formula of insoluble phosphorus into soluble phosphorus is as follows:
Ca 3 (PO 4 ) 2 +2H 2 SO 4 ===2CaSO 4 +Ca(H 2 PO 4 ) 2
However, the spent acid contains many impurity elements that convert soluble phosphorus into insoluble phosphorus.
1, titanium
When the concentration of the titanium ion concentration and the phosphate ion concentration is small, a poorly soluble titanium phosphate precipitate can be formed. When sulfuric acid added with spent acid reacts with phosphate rock, titanium reacts with phosphoric acid to form insoluble titanium phosphate, which reduces soluble phosphorus. Its reaction formula is as follows:
TiOSO 4 +2H 3 PO 4 ===TiH 2 (PO 4 ) 2 ·H 2 O↓+H 2 SO 4
2, iron, aluminum
In the salt treatment of titanium dioxide production, the purpose of adding phosphoric acid is to react with ferric or ferrous iron in metatitanic acid to form insoluble ferric or ferrous phosphate, which is light yellow by calcination. Iron phosphate, which is calcined directly by trivalent iron or ferrous iron to produce reddish brown iron oxide, is much lighter, thereby increasing the whiteness of titanium dioxide. From this process, it is known that phosphoric acid reacts with ferric iron or iron to form insoluble trivalent iron or ferrous phosphate. In fact, ferric iron can form insoluble ferric phosphate at a lower pH and higher acidity, and ferrous iron can produce insoluble ferrous phosphate at a pH greater than 6. In the waste acid, it is mainly ferrous iron, and there is little or no trivalent iron. Therefore, in the preparation of calcium, since the initial acidity is large, there is no action of ferric iron and phosphoric acid, so the soluble phosphorus does not decrease; The pH value will not be greater than 6, and the ferrous iron will not form insoluble ferrous phosphate, and the soluble phosphorus will not decrease. However, when the calcium continues to mature, the sulfuric acid continues to interact with the phosphate rock, the acidity will gradually decrease, and the pH will gradually increase. When it rises to more than 6, the ferrous ion combines with the phosphate ion to form an insoluble phosphate. Iron, especially when calcium is applied to farmland with a pH of 6.5-7.5, ferrous ions must react with phosphate ions to form insoluble ferrous phosphate, which reduces soluble phosphorus. The more ferrous iron is contained in the calcium, the more ferrous iron is formed, so that the decrease of soluble phosphorus is greater. Therefore, in the whole system, the more iron, the greater the loss of phosphorus.
When the acidity of the whole system is lowered and the pH is raised to more than 6, aluminum ions, like ferrous ions, can react with phosphate to form insoluble aluminum phosphate, which also reduces soluble phosphorus.
The so-called phosphorus, refers to water-soluble P 2 O 5 and soluble citrates sum of P 2 O 5. Phosphorus in iron phosphate and aluminum phosphate, although hardly soluble in water, is soluble in citrate and becomes soluble phosphorus and is absorbed by plants. For this reason, people often use the combination of calcium and organic fertilizers, and the organic acid such as citric acid and malic acid produced during the decomposition of organic fertilizer and the coordination of iron, aluminum and calcium in the soil to reduce the calcium. The chemical precipitation and immobilization prolong its effectiveness. It may compensate for the loss of soluble phosphorus caused by iron and aluminum phosphate. However, there are not many organic fertilizers to be mixed, and less citric acid is produced by decomposition. This compensation is far from satisfying the loss of phosphorus caused by the increase of iron and aluminum to form insoluble phosphate. Moreover, what is referred to herein as being soluble in citrate is a phosphate formed by sesquioxide (R 2 O 3 ), that is, a phosphate of trivalent iron and trivalent aluminum, and the phosphate of ferrous iron may not be capable of Dissolved in citrate, the divalent iron salt can only become trivalent iron salt after being fully oxidized by air. [next]
In order to prevent the iron and aluminum content from being too high and the formation of insoluble phosphate to reduce the effective phosphorus, China's quality requirements for phosphate rock, the iron and aluminum oxides are specified to be less than 4%; the former Soviet Union data also indicates , when
FeSO 4 +Ca(H 2 PO 4 ) 2 ===Fe(H 2 PO 4 ) 2 +CaSO 4
Fe 2 O 3 +3H 2 SO 4 ===Fe 2 (SO 4 ) 3 +3H 2 O
Fe 2 (SO 4 ) 3 +3Ca(H 2 PO 4 ) 2 ===2Fe(H 2 PO 4 ) 3 +3CaSO 4
During the calcium aging process, as the free acid is gradually reduced, the following conversion reactions occur:
3Fe(H 2 PO 4 ) 2 +2H 2 O===Fe 3 (PO 4 ) 2 ·2H 2 O+4H 3 PO 4
Fe(H 2 PO 4 ) 3 +2H 2 O===FePO 4 ·2H 2 O+2H 3 PO 4
Some people may say that the calcium used in the use of titanium white waste acid is qualified for its factory-tested effective phosphorus. This is tested under conditions of high acidity, pH <6, ferrous ions and phosphate ions have not reached the refractory ferrous phosphate, or citrate is dissolved, but once calcium is added Sell ​​it and apply it to the farmland with a pH of 6.5~7.5. The ferrous iron and phosphate ferrous phosphate have a pH greater than 6, and the ferrous iron will quickly react with the phosphate to form insoluble. Ferrous phosphate, the farmland has no citrate to dissolve the phosphorus, so it will definitely reduce the available phosphorus.
For the above reasons, it has been considered that it is not preferable to directly produce calcium by using titanium white waste acid because it greatly reduces the effective phosphorus which can be absorbed by plants.
(3) Exploring the method of making calcium by using titanium white waste acid
According to the above analysis, the reason for the influence of the titanium white waste acid to produce calcium is that the waste acid contains impurities such as titanium, iron, aluminum, etc., and if it is desired to use calcium white waste acid to make calcium, it is necessary to remove these impurities first.
1, in addition to titanium
As mentioned above, titanium ions and phosphate ions easily form insoluble titanium phosphate. To remove titanium, it is necessary to add phosphoric acid to the spent acid to form a titanium phosphate precipitate before the calcium is produced.
Titanium phosphate is a white pigment with wide application and promising prospect. It is a good anti-rust pigment and is an additive for catalysts, surfactants, rubber, plastics and paper industry. It has been developed and applied in countries such as the former Soviet Union and Japan.
It is known that the hydrolysis rate of titanium is generally about 96%, and 3% of unhydrolyzed soluble titanium is present in the spent acid after hydrolysis. If these waste acids are applied to the production of calcium, the soluble titanium in the spent acid is bound to interact with the soluble phosphate in the calcium to form insoluble titanium phosphate, resulting in loss of soluble phosphorus. If this part of unhydrolyzed titanium can be recovered, it can not only improve the recovery rate of titanium, improve the economic benefits of the enterprise, but also reduce the loss of soluble phosphorus caused by the use of such titanium-containing waste acid.
In fact, it is not difficult to recycle titanium. The following two methods of recycling in China and the former Soviet Union are introduced.
(1) Domestic method, after heating the waste acid to 90~120 °C, slowly add phosphoric acid (slow temperature and H 3 PO 4 reaction is slow), time 1h, the amount added is TiO 2 :P 2 O 5 =(1.5~2 ):1. Its main reaction formula is as follows:
TiOSO 4 +2H 3 PO 4 ===TiH 2 (PO 4 ) 2 ·H 2 O↓+H 2 SO 4
Titanium phosphate precipitation is allowed to stand for 12~24h, then vacuum filtered, pickled, washed, dried, and calcined at 400 °C for 2~4h, then cooled and pulverized to 320 mesh.
Using this method, the waste acid produced by producing 1 ton of titanium dioxide can produce 64-85 kg of titanium phosphate, and the recovery rate of TiO 2 can reach about 85%. If it is 8,000 tons/year of titanium white factory, it can produce 512~680 tons of titanium phosphate per year, which can create an output value of about 5 million yuan. [next]
(2) Former Soviet law, in the production of anatase titanium dioxide, the suspension of metatitanic acid after hydrolysis is cooled to 75 ° C, and under stirring, the ratio of 0.5 kg of H 3 PO 4 is added to 1 kg of TiO 2 , slowly The phosphoric acid is added to the suspension of metatitanic acid, and the phosphoric acid acts on the unhydrolyzed titanyl sulfate to form a titanium phosphate precipitate mixed in the metatitanic acid, and then the titanium phosphate is suction-filtered, washed with water and salted with the titanic acid (only Adding K 2 CO 3 , without adding H 3 PO 4 ), calcining, and pulverizing to obtain a titanium white powder mixed with titanium phosphate. This method can increase the recovery rate of TiO 2 by 1.1%, and the quality of the finished product can still reach the standard of anatase titanium dioxide. However, the addition of H 3 PO 4 increases the amount, which leads to a decrease in whiteness of titanium dioxide.
Direct addition of H3PO4 to the suspension of metatitanic acid after hydrolysis, the utilization rate of H 3 PO 4 is low, indicating that some H 3 PO 4 acts with soluble titanium, while another part of H 3 PO 4 acts with metatitanic acid. .
In order to increase the utilization rate of H 3 PO 4 , it was later changed to filter the hydrolyzed metatitanic acid, H 3 PO 4 was added to the filtrate, H 3 PO 4 and soluble titanium were allowed to interact to form titanium phosphate, and then titanium phosphate was added. The slurry is then added to the hydrolyzed metatitanic acid, and the subsequent steps are carried out according to the previous process. This will increase the utilization of H 3 PO 4 to 96%, the recovery of TiO 2 to 2.1%, and the product still meets high quality standards. As the recovery of TiO 2 is increased, the economic benefits are also increased, which is an additional gain.
2, in addition to iron, aluminum
From the relationship between sulfuric acid, ferrous sulfate and water (see above), it can be known that in higher concentrations of sulfuric acid, the solubility of ferrous sulfate is relatively low. When the sulfuric acid concentration is 20% at 55 ° C, the solubility of ferrous sulfate is 17.1%; when the sulfuric acid concentration is adjusted to 50%, the solubility of ferrous sulfate is only 1.3%; when the concentration of sulfuric acid is adjusted to 64%, sulfuric acid The solubility of iron is reduced to 0.6%. According to this principle, a method of concentrating the spent acid can be employed to crystallize the ferrous sulfate from the spent acid, thereby greatly reducing the iron content in the spent acid, making it suitable for the preparation of calcium. If the waste acid is concentrated to 64%, it contains only 0.6% of ferrous sulfate, so it is not necessary to add concentrated sulfuric acid to make calcium, which reduces the consumption of concentrated sulfuric acid and reduces the cost of raw materials. At the same time, such low sulfuric acid When the ferrous content is calcium, the insoluble ferrous phosphate is rare, and the loss of soluble phosphorus is rare. Further, the concentrated high-concentration sulfuric acid can be returned to the ilmenite. Acid hydrolysis can also save a part of acidification acid to reduce production costs and improve economic efficiency. The method of concentrating waste acid for returning and re-acidification has already been learned from the mature experience of titanium white manufacturers in China. It is completely possible to use this concentrated method to remove a large amount of ferrous sulfate from waste acid. The purpose of calcium is to use acid.
The waste acid contains less aluminum, and even if it is concentrated, it is not easy to cause the aluminum sulfate to be supersaturated and crystallized. Therefore, it is difficult to remove aluminum from the spent acid, and it is only possible to compensate for the loss of effective phosphorus caused by the difficulty in removing aluminum by removing more iron.
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