DE1567619C3 - Process for the preparation of low-vanadium alkali phosphates - Google Patents

Description

30th

The present invention relates to a process for the preparation of low-vanadium alkali metal phosphates, in particular of pyro- and polyphosphates from raw phosphoric acids, which by digestion of phosphate ores with mineral acids and which are called wet process phosphoric acids or wet phosphoric acids are designated.

It is known that polyphosphates produced from wet phosphoric acid are discolored by traces of heavy metals will. Vanadium in particular has a very negative effect here. In addition, heavy metals, like vanadium, decomposition catalysts for perborates, which today are increasingly related with alkali polyphosphates are added to the detergents.

So far, various means have been recommended to remove the vanadium from wet phosphoric acid solutions remove. In one case, iron vitriol, metallic, is used to reduce the vanadium of the acid Iron or zinc added.

By adding these substances, however, additional P ₂ O _{5 is lost} as iron or zinc phosphate _{with the neutralization precipitation, so that this process is uneconomical in terms of P 2} O ₅ yield, apart from the further contamination of the wet phosphoric acid solution by these additives.

Another method starts with trisodium phosphate _{solution, introduces CO 2} and introduces a sulfur-containing compound into the carbonized solution, with vanadium and sulfur-containing precipitates forming which are filtered off. In the filtrate, by cooling and inoculating with disodium phosphate, disodium phosphate crystallizes out, which is particularly low in vanadium.

This process is cumbersome because you don't get a sprayable filtrate immediately, but that Allow disodium phosphate to crystallize first, then separate and wash and then in your own Crystal water must melt again.

Attempts have also been made (cf. Austrian patent specification 151 948), those in the wet phosphoric acids to reduce the heavy metal ions contained in them with the help of hydrogen sulfide or alkali sulfides and to precipitate the heavy metals as sulphides. However, these modes of operation have the disadvantage that the Reduction takes place to a sufficient extent and becomes free only after relatively long reaction times Hydrogen sulfide not only causes serious nuisance, but also serious accidents has led.

It is also known to have sulfate, arsenic and silicofluoride ions present in the wet phosphoric acids Help of barium sulfide or mixtures of barium sulfide and barium carbonate to precipitate the formed Filter off precipitate, then neutralize the filtrate and the resulting essentially to remove precipitate consisting of iron and aluminum phosphate. According to this Method, the added barium salts according to the amounts present in the phosphoric acid Sulphate, arsenic and silicofluoride ions are dimensioned, and therefore there may not be any removal of the vanadium also present in the acid, since it had not previously been recognized that To remove the vanadium, a minimum amount of the iron content of the acid must be present in the divalent oxidation stage.

Apart from this, the dangers associated with the use of hydrogen sulphide are reduced according to this working method or alkali sulfides occur, not eliminated.

The object of the present invention is to produce alkali phosphates with vanadium contents of less than 10 ppm in a simple and safe manner from wet process phosphoric acids with vanadium contents of up to about 1000 ppm and iron contents of at least 3000 ppm, based on the P ₂ O ₅ contents, by using the in Heavy metal ions contained in these acids are reduced with the aid of a reducing agent, the acids are _{neutralized with alkalis up to the alkali oxide to P 2} O ₅ molar ratio desired in the end product, the precipitates are filtered off and the alkali phosphates are separated from the filtrates.

According to the invention, this object is achieved by using dithionites as reducing agents to convert at least 0.3 g, preferably more than 0.4 g, of the iron contained in the acids, based on 100 g of dissolved P ₂ O ₅ , to the divalent oxidation stage convicted.

If alkali phosphates with vanadium contents below 2 ppm are to be obtained, more than 0.8 g, based on 100 g of P ₂ O ₅ , of the iron contained in the acid must be converted into the divalent oxidation stage.

Another advantage of the procedure according to the invention is that the dithionites due to Due to their better flow behavior, it is easier to dose than the sulfides.

example 1

500, 210 and 215 g of wet phosphoric acid with 22.6% P ₂ O ₅ , a total iron content of 4950 ppm,

attracted to P ₂ O ₅ , and an iron (II) content of 660 ppm or 14% are _{adjusted to different iron (II) contents by treatment with H 2} S-GaS or by reduction with dithionite and to pH 6.5 neutralized and filtered. The vanadium is determined in the filtrates.

H ₂ S-Em- salary ppm V ppm V, based on Phos management in the gene on P ₂ O ₅ - phor length of time 17% Filtrate Salary of acid 47% Filtrates 20 minutes 100% 3.5 -20% P ₂ O ₅ 500 g 5 hours 1.8 17.5 210 g Encore 0.9 9.0 215 g from 3 g 4.5 Na ₂ S ₂ O ₄

Example 2

1200, 500 and 250 g of wet phosphoric acid with 24.95% P ₂ O ₅ , a total iron content of 15,000 ppm, based on P ₂ O ₅ , are neutralized with 50% NaOH to pH 7, filtered and the vanadium in the filtrates is determined. The Fe (II) content of the starting acids varied depending on the treatment or the standing time.

Example 3

1200, 500 and 250 g of wet phosphoric acid with 19.9% P ₂ O ₅ , a total iron content of 9000 ppm, based on P ₂ O ₅ , are neutralized to pH 7 with 50% NaOH. Depending on the treatment and standing time, the iron (II) content in the starting acids again varied.

treatment üa / TT \ ppm V ppm V, based on Phos Fe (II) -
salary in fil
kicked gene on P ₂ O ₅ - phor
acid 92 g BaS Salary of
Filtrates to the Ent 91% 0.3 -20% P ₂ O ₅ 1200 g distance 1.5 of sulfate for more weekly 30% 3.1 500 g Service life 15.5 after reduction 99% 0.5 250 g with 5 g 2.5 Dithionite

30th 1200 g 35 500 g 40 treatment T? O / TT \ ppm V ppm V, based on Phos Fe (II) -
salary in fil
kicked gene on P ₂ O ₅ - phor
acid BaS for Salary of
Filtrates ? 50g Ent 93% 0.8 ~ 20% P ₂ O ₅ distance 4.0 of sulfate for more weekly 29% 3.4 Service life 17.0 after reduction 99% 0.3 with 5 g 1.5 Dithionite

Claims (1)

Claim: Process for the production of low-vanadium alkali metal _{phosphates with vanadium contents less than 10 ppm, based on the P 2} O ₅ contents, from wet process phosphoric acids with vanadium contents up to about 1000 ppm and iron contents of at least 3000 ppm, based on the P ₂ O ₅ contents Reduction of the heavy metal ions contained in these acids with the aid of a reducing agent, neutralization of the acids with alkalis up to the alkali oxide to P ₂ O ₅ molar ratio desired in the end product, filtering off the precipitated precipitates and separating off the alkali metal phosphates from the filtrates, characterized in that with With the help of dithionites as a reducing agent, at least 0.3 g of the iron contained in the acids, based on 100 g of dissolved P ₂ O ₅ , is converted into the divalent oxidation stage.