NO823584L - PROCEDURE FOR SEPARATION OF HEAVY METAL INGREDIENTS FROM INTERMEDIATES FROM PHOSPHOROUS MANUFACTURING PRODUCTS - Google Patents

Description

The invention relates to a method for separating heavy metal constituents, in particular cadmium and mercury, from raw phosphoric acids which are produced by digesting raw phosphates with nitric acid and from which the main amount of the formed calcium nitrate was removed.

Phosphorus-containing fertilizers are often produced by acid digestion of phosphorite or apatite. Under the influence of mineral acid, the crude phosphate's sparingly soluble tricalcium phosphate is split. The calcium salts of the mineral acids used must then be separated to obtain the raw phosphoric acid formed as a product.

The raw phosphate contained metallic impurities such as Cd, Pb,

Hg and As remain to a large extent in the phosphoric acid. Then this one

acid is not suitable for all applications, numerous attempts have already been made to free it from unwanted metallic impurities. For the processing of raw phosphoric acid into fertiliser, these offered suggestions - however, so far no technically favorable path.

Thus mentions AMDEL Bull. 1976 (19) 1-10, from the removal of cadmium from raw phosphate by calcination, an economically very cumbersome process. According to DE-OS 24 22 902, cadmium can be removed by pressure precipitation with hydrogen sulphide. In order to carry out the process without the difficulty of having to work with hydrogen sulphide under pressure, it is necessary to comply with certain concentrations of P2(->5*According to Japanese offenlegungsschrift 1979/37 096, a method is proposed in which two steps by treatment with a water-soluble organic solvent and distillation and possibly by subsequent extraction phosphoric acid to be purified.

Cadmium - the separation by means of ion exchangers is also discussed (Japanese offenlegungsschrift 1978/56 190). Neither does fixed storage electrolysis (metalloberflache 34 (1980) 494 - 501) offer any way to solve the problems.

The proposed methods are particularly unsuitable for liberating

the digestion of raw phosphate with the help of 60% saltpetre

acid according to the known Odda method formed crude phosphoric acid for heavy metal ions.

In the Odda process, the highly nitric acid-containing wet phosphoric acid is cooled from the digestion to separate calcium nitrate, the solubility of which is strongly temperature-dependent, and calcium nitrate is separated as tetrahydrate. Heavy metal ions such as Cd, Pb, Hg and As present in the raw phosphate remain to a large extent in the solution of the digester, the so-called mother acid. The mother acid usually contains 15-20 wt% P2O5, 30-35 wt% HN03 and 7-10 wt% CaO.

The mother acid is then usually neutralized step by step with ammonia while evaporating water and the formed crystal slurry is formed with calcium salts and granulated into fertilisers.

The basis for the invention was thus the task of producing a method with the help of which heavy-metal-poor fertilizers could be produced according to the Odda process.

It has now surprisingly been found that this task can preferably be solved by adjusting the mother acid produced by the Odda method with ammonia to a pH value in the range of 0.5 - 1.5, especially 0.6 - 1.2 and extracts the thus formed phosphoric acid solution, preferably in countercurrent with a solvent that is not or only slightly miscible with water, the solvent being a phosphorus compound selected from the group dithiophosphoric acid diester, dithiophosphonic acid O-ester and dithiophosphinic acid and preferably used in a mixture with inert organic diluent.

Compliance with the above-mentioned pH value range is above all particularly important for the extraction and separation of cadmium.

In the range of acidic pH values, on the other hand, extraction of the mother acid is prevented by the simultaneous occurrence of complex precipitations of phosphates, silica gel, calcium, magnesium and

aluminum compounds.

For measuring the pH value in concentrated mineral acid solutions, glass electrodes are suitably used single-rod measuring chains with reference system Ag/AgCl-, reference electrolyte %m KCl+AgCl or 3.5m KC1. Particularly hydrogen sulphide-proof single-bar measuring chains can be used, such as e.g. is equipped with two electrolyte chambers connected one after the other, separated by diaphragms. The pH values must be measured in the concentrated solution without dilution and at room temperature.

It is known that dithiophosphoric acid dialkyl esters (compare US patent 2,523,147, 2,705,694 and 2,798,880) and dithiophosphinic acids as well as dithiophosphonic acid O-esters (French patent 1,396,093 and US patent 3,300,409) are easily oxidizable and can therefore be used as antioxidants. It was therefore to be expected that the presence of oxidizing agents such as high concentrations of nitrate ions in acidic solution is an obstacle to an extractive treatment of the above-mentioned mother acid using the extractant according to the invention. The mother acid also contains nitroxides (NO ) from the digestion.

According to the invention, the phosphorus compounds suitable as extractants have the general formula I

Wherein R1 and R2 are the same or different and mean saturated or unsaturated aliphatic, araliphatic or aromatic hydrocarbon residues with 1 to 18 C atoms which are optionally substituted and R1 and R2 together have 6 to 36 C atoms, preferably 8-24 C -atoms and possibly also together form

an optional double-bonded residue and n each time independently means the numbers 0 or 1.

As examples of extractants according to the invention with formula I, there shall be i.a. mentioned:'

dithiophosphoric acid di-(2-ethylhexyl) ester

dithiophosphoric acid diisotridecyl ester

dithiophosphoric acid di-sec.butyl ester

dithiophosphoric acid di-cresyl ester

cyclohexyl dithiophosphonic acid O-n-butyl ester

di-cyclohexyl-dithiophosphinic acid

di-phenyl-dithiophosphinic acid di-(tricyclo-(5,2,1,0 2 6)decenyl)-dithiophosphinic acid naphthalene-2-dithio-phosphonic acid O-ethyl ester.

The amount of extractant used according to the invention is not critical and can vary within wide ranges. Preferably, however, they are used in amounts of 0.05 - 50% by weight, especially in amounts of 0.1 - 10% by weight, referred to the amount of mother acid.

The extractants according to the invention can be used individually or as a mixture. In the case of the compounds of formula I with substituted residues R 1 and R 2 , substitutions with halogen, hydroxy, hydroxy, carboxy or NC 2 groups are preferably used, if the resulting residues together are sufficiently hydrophobic, prior to the compounds of formula I

to the desired extent are poorly soluble in water.

Preferred compounds of formula I are further those in which and R2 are equal to (Cq^-C-^q)-alkyl, (Q^~ c^)-cycloalkyl, phenyl-(C1~C2)-alkyl, phenyl, with (Cl -C 4 )-alkyl substituted phenyl, naphthyl which is each optionally substituted with halogen, hydroxy, alkoxy, carboxy or NO 2 or tricyclodecenyl, R 1 and R 2 together having 8 to 2 4 C atoms.

However, the term poorly soluble water used in relation to the phosphorus compound according to the invention does not mean that the compounds in question must be completely insoluble, but only that the compounds in relation to the phase of nitric acid-containing phosphoric acid must have a sufficient immiscibility to enable a separation of the liquids in two different phases.

Although the phosphorus compound according to the invention with formula I

in undiluted, liquid form is excellently suitable for extraction, it is preferable to use them in a mixture with inert organic diluents to achieve an even better handling and regulation of the extraction and i.a. also for economic reasons.

By the organic diluents suitable for the application

according to the invention, it concerns such organic solvents in which the compounds—with—formula I are soluble, which are also practically insoluble in the crude nitrate ion-containing phosphoric acid solutions adjusted to pH 0.5 to 1.5 and are chemically indifferent to the latter. A large number of different organic solvents come into question here, such as petroleum petrol, kerosene, extra-light fuel oil, decahydronaphthalene, tetrabromoethane or xylene. The quantity ratio between the compounds of formula I and inert organic solvents can be varied within wide limits. Weight ratios from 1:2 to 1:50, especially 1:5 to 1:20, are preferred.

The extraction method according to the invention as well as the back-extraction or removal method for eliminating the heavy metal constituents from the extractant for reuse of the latter can be carried out continuously or discontinuously, respectively in portions. In the case of a discontinuous method, it may be necessary to repeat the extraction as often until the desired degree of extraction effect is achieved.

In a continuous process, it can e.g. the phosphoric acid solutions are preferably worked in countercurrent to the extractant phase.

The extraction can be carried out in a wide temperature range

at low or too high temperature, preferably between 5 and 120°C, especially at 10 to 90°C. Particularly preferred are normal temperature and ambient temperature. It can also be worked at normal pressure or elevated pressure. Improving the phase separation, the two-phase extraction mixture can be centrifuged, e.g. a separator.

For the person skilled in the art, it could in no way be foreseen that the method according to the invention would succeed in extracting the heavy metal ions practically completely from concentrated phosphoric acid solutions whose property with heavy metal ions to form stable complexes with a high distribution coefficient is known . of cadmium. Thereby, the distribution coefficients are so large that even a phase ratio between aqueous and organic phase (e.g. a 10% solution of the dithiophosphoric acid diester in heavy petrol) of 50 : 1 in one-stage extraction achieves good separation of the elements Cd and Hg . This result is all the more surprising when extraction with comparable phosphoric acid diesters instead of dithiophosphoric acid diesters fails completely.

(compare comparative examples 1 - 2).

The metallic impurities that have been extracted in the organic extractant phase can generally be removed from this phase again by back-extraction with water or acidic aqueous solutions, e.g. diluted mineral acids. On

in this way, the organic extractant can be advantageously regenerated, while the heavy metal components pass into the aqueous phase. Thereby, the organic extractant-containing phase can be primarily extracted with an aqueous solution of an alkaline compound before treating it with dilute mineral acid, such as e.g. dilute hydrochloric acid. Alkaline extract and acidic extract can then be combined. By

in the first of the two sub-steps, arsenic and some cadmium are extracted, while in the second step, cadmium and mercury are quantitatively removed. By changing the sequence or increasing the number of individual steps in the re-extraction of the heavy metals from the organic phase, certain heavy metals can be enriched in the re-extraction solution. From the aqueous reextraction solutions, the heavy metals can be separated and isolated by precipitation, e.g. such as hydroxides or sulfides.

The invention will be explained in more detail with the help of some examples.

Example 1-16

By digesting raw phosphates with nitric acid according to the Odda method, mother acids formed and clarified are adjusted with gaseous ammonia to the pH values indicated in the following table (the phosphoric acid phase). In addition, the extractants according to the invention listed in table 1 are added. where indicated quantity ratio between inorganic phase and organic phase between 5 : 1 and 50 : 1 and is processed for 3-5 minutes along intense mixing with a stirrer. The weight % of compounds A refers to diluent B. The extraction temperature is 25°C, with the exception of examples 8 and 9. Here, a temperature of 70°C was chosen.

The samples are then centrifuged for 2-5 minutes and the two-phase mixture is separated in a separatory funnel. The phosphoric acid solutions formed are mixed with some concentrated hydrochloric acid and analyzed using atomic absorption. The heavy metal content found is converted to the ammonia-treated mother acid or referred to the organic extractant phase.

The following table 1 gives details and results of the carried out examples 1 - 16 in a summarized form.

Example 17 - 18

Analogous to examples 1-14, prepared and clarified mother acids are adjusted with ammonia to pH 1.1 and extracted with dithiophosphoric acid di-'(2-ethylhexyl)-ester (10% by weight in petroleum petrol, boiling point 100 - 140°C) as discussed in example 11. The resulting organic phase is separated and then re-extracted with concentrated hydrochloric acid in a weight ratio of 20:1 at room temperature. In addition, the mixture of organic phase and concentrated hydrochloric acid is mixed well for 5 minutes with intense stirring, then centrifuged. The separated organic phase has before and after re-extraction the reproduced Cd contents summarized in table 2:

Comparison example 1-3

Analogous to examples 1-16, prepared and clarified mother acids are adjusted with ammonia to pH 1.1 and extracted with phosphoric acid di-(2-ethylhexyl) ester (compound C) or 2-mercapto-benz-thiazole (compound D) in 10 weight-% organic solution in the organic diluents specified in table 3 at room temperature in a mixing ratio of inorganic :: organic phase = 10:1 (parts by weight) under intense mixing by stirring. After stirring and subsequent centrifugation, the phases are separated. The phosphoric acid phase is mixed with some concentrated hydrochloric acid and analyzed using atomic absorption. The solidified Cd content is converted to the ammonia-treated mother acid. The result is reproduced in table 3.

Claims (9)

1.' Process for separating heavy metal constituents from crude phosphoric acids which are produced by digestion of crude phosphate with nitric acid and from which the main amount of formed calcium nitrate has been removed, characterized by adjusting this crude phosphoric acid with ammonia to a pH value between 0.5 and 1.5 and extracting the formed phosphoric acid solution with an organic phosphorus compound of formula I in which R-j _ and R2 are the same or different and mean saturated or unsaturated aliphatic, araliphatic or aromatic hydrocarbon residues with 1 to 18 C atoms which are optionally substituted and R^ and R2 together have 6 to 36 C atoms and optionally also together form a optionally substituted double-bonded residue and n each time independently means 0 or 1 or a mixture of these. 2. Method according to claim 1 using phosphorus compounds of formula I according to claim 1, wherein R-^ and R2 (C1-C18)-alkyl, (C5-C6)-cycloalkyl, phenyl (C-^C3-alkyl phenyl, phenyl substituted by (C -C -C )-alkyl, naphthyl which is optionally substituted with halogen, hydroxy, carboxy, carboxy or NO 2 or tricyclodecenyl and R - and R 2 together have 8 to 24 C atoms. 3. Method according to claims 1 and 2, characterized in that the phosphorus compounds of formula I are used in amounts from 0.05 to 50% by weight, referred to the phosphoric acid phase to be extracted. 4. Process according to claims 1-3, characterized in that the compound of formula I is used in a mixture with inert organic diluents. 5. Method according to claim 1, characterized in that the weight ratio between the compound of formula I and inert organic diluent is L:2 to L:50. 6. Method according to claims 4 and 5, characterized in that petroleum petrol, kerosene, fuel oil EL, decalin, tetrabromoethane or xylene are used as inert organic diluent. 7. Method according to claims 1-6, characterized in that the extraction is carried out at a temperature between 5 and 120°C. 8. Method according to claims 1-7, characterized in that the extraction is carried out continuously or discontinuously. 9. Method according to claims 1-8, characterized in that the separated extractant is freed from heavy metal constituents and used again for extraction.