IL46740A

IL46740A - Purification of phosphoric acid

Info

Publication number: IL46740A
Application number: IL46740A
Authority: IL
Original assignee: Azote & Prod Chim
Priority date: 1974-03-19
Filing date: 1975-03-03
Publication date: 1977-07-31
Also published as: JPS50131892A; ZA751297B; FI750803A; GB1438269A; IL46740A0; DE2511345A1; MX3087E; YU60675A; NL167391C; BE826810A; NL167391B; FI58767C; SE393361B; JPS5440075B2; FR2264774B1; NO138526B; DE2511345B2; NL7503004A; YU36663B; ES435760A1

Description

Purification of phosphoric acid AZOTE ET PRODUITS CHIMIQUES S.A.

C. 44695 PROCESS FOR PURI- PHOSPHORIC ACID The present invention concerns a process for purifying wet process phosphoric acid often called "green acid".

Wet process phosphoric acid is prepared by reacting phosphate rock with sulphuric acid and separating calcium sulphate. This acid contains anionic impurities such as S04 and F, metallic impurities such as Ca, Mg, Fe, Al, As, Pb and Cu, and also colloidal silica. Metallic impurities give troublesome deposits during storage. Furthermore in alkali. or ammonia . solutions metallic impurities precipitate as gelatinous phosphates which make the solution cloudy and clog up filters. Furthermore anionic and metallic impurities must be eliminated when manufacturing phosphates for human or animal food or detergents.

Many purification processes were proposed. According to one of these processes phosphoric acid is partially neutralized with sodium hydroxide, soda, potassium hydroxide, potassium carbonate or ammonia in order to precipitate metallic impurities as phosphates. According to this process sulphate ions are not eliminated. They have to be precipitated in a further treatment with a barium salt or, sometimes, with a calcium salt.

Phosphoric acid has also been purified by counter-curren extraction with an organic solvent. The large quantities of solvent necessary are expensive. Furthermore with this process sulphate ions are not either eliminated as sulphuric acid is extracted together with phosphoric acid. On the other hand colloidal . silica must in some cases be flocculated separately.

The present invention concerns a process allowing to reduce in a simple way the impurity concentration to about one tenth of the original concentration.

The present invention concerns a process for purifying wet process phosphoric acid characterized by the following steps : -in a first step wet .process phosphoric acid is reacted with urea arid urea phosphate is precipitated and separated from mother-liquors. -in the second step urea phosphate is reacted with nitric acid and urea nitrate is separated from purified phosphoric acid.

The preparation of urea phosphate was known for a long time. It is, for example, described in British Patent Specification No. 1,149,924. Israel Patent Specification No. 36262 also describes the preparation of urea phosphate and its subsequent conversion into urea ammonium polyphosphate.

For preparing urea phosphate wet process phosphoric acid is reacted with urea and urea phosphate is crystallized from the reaction mixture.

The reaction of formation of urea phosphate CO( H2)2 + H3P04 > CO(NH2)2, H3P04 (1). is exothermic. As urea phosphate solubility increases with temperature the urea phosphate crystallization rate is improved by cooling the reaction mixture. Crystallization is preferably performed at temperatures ranging between 0 and 25 °C, more particularly between 15 and 20°C. Even lower temperatures can be used but the cooling operation becomes expensive.

Crystallized urea phosphate is separated by filtration or draining. Part of mother-liquors is preferably recycled into the reaction mixture in order to maintain its fluidity at a value compatible with an efficient agitation. The recycled part of mother-liquors is particularly such that, in the reaction mix-ture/ the weight ratio between mother-liquors and phosphoric acid is comprised between 0.5 and 2.5, preferably between 1.5 and 2.0. Crystallized separated urea phosphate is washed, preferably with a solution saturated in urea phosphate issuing from a previous operation. The mother-liquors obtained after urea-phosphate separation contain the major part of impurities (SO^, F, Fe, Al) of the original phosphoric acid and a certain amount of P2^5 anc urea nitrogen. In order to avoid losses, these mother-liquors are advantageously used for the manufacture of liquid or solid fertilizers.

Urea phosphate obtained can be immediately treated with nitric acid to obtain purified phosphoric acid. But urea phosphate being easier to store and transport than phosphoric acid it may be advantageous in some cases to store and transport urea phos fertilizer- , animal food, extinguishing agent, metal phosphata-tion agent. Therefore according to needs urea phosphate can either be sold or used to prepare purified phosphoric acid.

In the second ste urea phosphate is reacted with nitric acid in order to obtain purified phosphoric acid and urea nitrate according to reaction: CO(NH2)2, H3P04 + HN03÷ ¾CO(NH2)2, HN03 + H3P04 (2) Urea nitrate is a white crystallized solid which is separated from phosphoric acid by filtration or draining. Recycling to the reaction mixture part of phosphoric acid improves filtration or draining. The recycled part is preferably such that the weight ratio between recycled phosphoric acid and urea phosphate is com-prised between 0.4 and 3, preferably about 1. Stoichiometric quantities of nitric acid are preferably used but nevertheless in some cases, an excess of nitric acid is favourable as it decreases the solubility of urea nitrate in the reaction mixture.

Drained urea nitrate is washed with water having preferably a temperature ranging between 0 et 5 °C to avoid dissolving too much urea nitrate. The quantity of water used amounts particularly to about 200 by weight of urea nitrate. Thus obtained urea nitrate can easily be used as nitrogen solution for the preparation of liquid fertilizers.

As may be seen hereabove the process is particularly advantageous as all the by-products can be used. An example is given hereunder to illustrate the invention.

In a mixer are introduced 1818g of phosphoric acid containing 54,5% of P205 and the following impurities (weight %) F 0,49 S04 3,38 Ca 0,54 Mg 0,33 Fe 0,09 Al 0,17 and 845g of urea with 46% of N as well as 2540g of recycled mother-liquors and 500g of recycled washing water1. The weight ratio between mother liquors and phosphoric acid is about 1.7.

The mixture is allowed to react during an hour and maintained at a temperature of about 18 "C. It is then centrifuged in order to separate 1980g of urea phosphate and 3732g of mother-liquors containing 3l6g of N and 815g of P?0 2540g of which are recy-cled to the mixer. Urea phosphate is washed on a drum with a saturated solution of urea phosphate obtained by dissolving 254g of purified urea phosphate in 229g of water. After washing I980g of purified urea phosphate are obtained, 254g of which are used to prepare the washing solution. Purified urea phosphate (1726g) and 1223g of 53% nitric acid are introduced together with 1726g of recycled phosphoric acid. The weight ratio between recycled phosphoric acid and urea phosphate is about 1. The mixture is let to react for an hour and then centrifuged.

There are obtained 2980g of phosphoric acid containing 43% of ?2°5 and !6 5g °f urea nitrate. Urea nitrate is washed on a drum with 420g of water at 0°C.

The 625g of washing water containing 460g of P2°5 obtained are added to purified phosphoric acid. After purification 1490g of urea nitrate and 1879g of phosphoric acid containing 39.5% P 05 and having the following composition are obtained : F 0.03 Mg 0.02 Fe 0.01 Al 0.01

Claims

C L A I M S

1. ) Process for purifying wet process phosphoric acid characterized by the following steps : - in a first step, wet process phosphoric acid is reacted with urea and cr stallized urea phosphate is precipitated and separated from mother-liquors - in a second step urea phosphate is reacted with nitric acid and urea nitrate is separated from purified phosphoric acid.

2. ) Process according to claim 1· in which urea phosphate is crystallized at a temperature ranging between 0 and 25 °C.

3. ) Process according to claim 2 in which urea phosphate is crystallized at a temperature ranging between 15 and 20°C.

4. ) Process according to claims' 1 to 3 in which in the first step such a fraction of mother-liquors is recycled into the reaction mixture that the weight ratio between mother-liquors and phosphoric acid ranges between 0.5 and 2.5.

5. ) Process according to claim 4 in which the weight ratio between mother-liquors and phosphoric acid ranges between 1.5 and 2.0.

6. ) Process according to claims 1 to 5 in which, in the second step, such a fraction of phosphoric acid is recycled to the reaction mixture that the weight ratio between recycled phosphoric acid and urea phosphate ranges between 0.4 and 3.

7. ) Process according to claim 6 in which the weight ratio between, phosphoric acid and urea phosphate is about 1.

8. ) Process according to claims 1 to 7 in which, in the second step, urea nitrate is washed with water having a temperature ranging between 0 and 5°C.