Process for Working Up Heavy Metal-Containing Residues Originally From the Decontamination of Crude Phosphoric Acid

This invention relates to a process of working up, i.e. concentrating, heavy metal-containing residues obtained on subjecting crude phosphoric acid to extractive decontamination using a dialkyldithiophosphoric acid ester and an adsorbent.

The removal of heavy metals, especially cadmium, from phosphoric acid is gaining increasing interest for ecological reasons as phosphoric acid derivatives, especially mineral fertilizers, make it possible for them to get into the chain of food.

Various such processes have already been described, wherein crude phosphoric acid is treated using a dialkyldithiophosphoric acid ester in combination with one or more fine particulate adsorbents and phosphoric acid substantially free from heavy metals is ultimately obtained (cf. EP-0 085 344 A2, EP-0 099 514 A1, DE-32 12 675 A1, corresponding to U.S. Pat. Nos. 4,452,768, 4,466,948 and 4,479,924, respectively).

In these processes, a heavy metal-containing residue, e.g. a filter cake, very variable in composition depending upon the following typical parameters is obtained:

(1) Content of organic impurities in the crude phosphoric acid,

(2) content of heavy metals in crude acid,

(3) quantity of dialkyldithiophosphoric acid ester used,

(4) nature and quantity of adsorbent used,

(5) extent of scrubbing filter cake and hence quantity of phosphoric acid adhering thereto.

The quantitative composition of these residues may vary within wide limits and depends on the particular method used to decontaminating the crude phosphoric acid.

Residues with relatively important proportions of heavy metal and organic substances are obtained, for example, in all those cases in which the crude acid is extracted with an undilute dialkyldithiophosphoric acid ester, the heavy metal-containing ester phase is separated, and the phosphoric acid is subsequently freed from minor residual quantities of extractant by admixing it naturally with a minor quantity of adsorbent, filtering and admixing the filter cake with the ester phase initially separated with formation of a relatively solid mixture.

The object of this invention is to provide a process of working up heavy metal-containing residues customarily obtained on subjecting crude phosphoric acid to extractive decontamination using a dialkyldithiophosphoric acid ester in combination with an adsorbent, the residues consisting substantially of the ester and adsorbent. More particularly, the invention provides for the heavy metals to be separated from the residues, to be subsequently concentrated and used for the commercially attractive recovery of metals, or for deposition in fairly small spaces on special deposition sites; the invention also provides for residue constituents other than the heavy metals to be recycled into the crude phosphoric acid extractive decontamination stage.

This object can unexpectedly be achieved in accordance with this invention by separating the residue into its essential constituents.

To this end, the invention provides for two routes A or B to be followed.

Route A provides for the residue to be suspended in water resulting in the formation of a liquid aqueous phase, organic phase and solid phase consisting of the adsorbent. Next, the adsorbent is (1) either separated from the suspension prior to separating the two liquid phases from one another or (2) the organic phase is separated from the altogether 3 phases obtained and the adsorbent is subsequently separated from the remaining aqueous phase. Next, in operational step (3) the aqueous phase obtained as under (1) or (2) in the form of dilute phosphoric acid is recycled into the crude phosphoric acid extractive decontamination stage and the organic phase consisting of the heavy metal-containing dialkyldithiophosphoric acid ester is either (a) directly used as such, i.e. for metallurgical purposes, or (b) converted by thermolysis or oxidation into a heavy metal concentrate or (c) mixed with a small quantity of pulverulent solid matter and made into an easy-to-handle solid product.

Route B provides for the residue to be (1) treated for as long as necessary to destroy the heavy metal/dialkyldithiophosphoric acid ester-complex with an oxidant--which may be used in the form of an aqueous solution--and for the adsorbent to be separated from the aqueous salt solution obtained. If deemed desirable or necessary, it is (2) possible for the salt concentration of the aqueous salt solution containing free adsorbent to be increased by circulating it while removing just a portion thereof, consumed oxidant being simultaneously replaced. A step (3) ultimately provides for heavy metal hydroxides or salts to be recovered by neutralizing or evaporating the salt solution separated or removed.

As provided for in (C), the adsorbent separated as under (A) or (B) is ultimately deposited as ecologically reliable material or recycled into the crude phosphoric acid extractive decontamination stage.

The step of oxidatively working up the organic phase can be carried out using oxidants permitting the heavy metal/thioester-complex to be destroyed, e.g. nitric acid, chlorine, hypochlorite, chlorate or also a peroxo-compound.

The following statements are intended to summarize the basic idea underlying the process of this invention and its beneficial effects.

It is desirable for heavy metal-containing filtered sludges obtained upon the removal of heavy metals from phosphoric acid by means of a dialkyldithiophosphoric acid ester and adsorbent to be converted to ecologically reliable solid matter and to be worked up into a heavy metal concentrate.

The present process enables the bulk of the heavy metals initially present to be converted to a reliable deposition product or to material which can be recycled and separated. The heavy metal concentrate with its high metal content can be used as a raw material in the metal-producing industries.

Solid matter and heavy metal concentrate are separated successively, namely mechanically and chemically. The sequential order of these two operational steps can be inversed.

Thus, for example, the adsorbent and heavy metal fraction are first separated mechanically and then worked-up chemically or the original filter cake is first subjected to oxidative-chemical treatment and then separated mechanically, e.g. by filtration. It is also possible for it to be subjected to an additional precipitating or concentrating processing step.

The following Examples illustrate the invention, the percentages being by weight unless otherwise stated.

500 g filter cake (3.3% P.sub.2 O.sub.5 ; 6.8% C.sub.org. ; 0.06% Cd; 0.05% Cu; 0.07% Zn) obtained on subjecting Florida crude phosphoric acid (30% P.sub.2 O.sub.5) to demetallization with addition of perlite was suspended in 250 g water and chlorine gas was injected via a frit over a period of 5 hours at 80.degree. C. with thorough agitation. After cooling to room temperature, the whole was filtered.

320 g filter cake practically free from heavy metal with 10.5% C.sub.org. and 485 g filtrate in which all of the heavy metals and phosphate were dissolved (2.6% P.sub.2 O.sub.5 ; 0.06 g C.sub.org. ; 0.06% Cd; 0.05% Cu; 0.07% Zn; 11.8% Cl.sup.-) were obtained.

200 g filtrate of Example 1 was neutralized with 50% sodium hydroxide solution to pH 9 and heavy metal hydroxides and phosphates were found to precipitate. The whole was filtered while hot and washed with 10 ml water. The filter cake dried at 80.degree. C. weighed 29.4 g and contained 0.41% Cd and 0.34% Cu. The filtrate which was a dilute disodium phosphate solution was recycled into the crude phosphoric acid.

500 g washed filter cake (5.9% P.sub.2 O.sub.5 ; 11.4% C.sub.org. ; 0.15% Cd; 0.22% Cu) obtained on subjecting Morocco crude acid (50% P.sub.2 O.sub.5) to demetallizing treatment with addition of perlite, and 1500 g nitric acid (15% HNO.sub.3) were stirred over a period of 1.5 h at 40.degree. C.

229 g filter cake free from heavy metal, containing 24.1% C.sub.org. and 1750 g filtrate in which all of the heavy metals and phosphate were dissolved (1.6% P.sub.2 O.sub.5 ; 0.05% C.sub.org. ; 0.042% Cd; 0.062% Cu) were obtained.

500 g filtrate of Example 3 was neutralized with 50% sodium hydroxide solution to pH 8.5 and the suspension was filtered while hot using a pressure filter. The filtrate was free from heavy metal (less than 1 ppm Cu, less than 1 ppm Cd).

52 g moist filter cake was obtained. It was dried at 80.degree. C. and 11 g dry filter cake containing 1.9% Cd and 2.8% Cu remained behind.

200 g filter cake obtained on subjecting phosphoric acid to demetallization with perlite and 200 g water were thoroughly stirred over a period of 2 hours at 50.degree. C. Next, the whole was allowed to deposit. A three phase mixture consisting of an upper black organic phase, a middle aqueous phase and a solid phase was obtained.

The organic phase was skimmed off (24.6 g) and the mixture of dilute phosphoric acid and perlite was separated by filtration, 112 g filter cake (45% adhering water) and 262 g aqueous phase containing 4.3% P.sub.2 O.sub.5 were obtained.

The organic phase contained 3.3% Cu and 2.4% Cd. The filter cake was practically free from heavy metal and was used again for freeing crude phosphoric acid from cadmium. The dilute phosphoric acid filtrate was recycled into the crude phosphoric acid.

20 g phosphoric acid thioester-phase containing heavy metal (3.3% Cu; 2.4% Cd) precipitated as described in Example 5 and 10 g 20% nitric acid were thoroughly stirred over a period of 1 h at room temperature.

A two phase mixture consisting of 17.5 g organic phase free from heavy metal and 12.3 g aqueous phase in which all of the heavy metals were dissolved (5.3% Cu; 3.9% Cd) was obtained.

A heavy metal hydroxide concentrate was prepared therefrom by precipitation with an alkaline liquor.

20 g organic phase of Example 6 was placed in a dish, gradually heated to 400.degree. C. in a muffle furnace, and maintained at that temperature over a period of 2 h.

A residue of 3.8 g containing 17.4% Cu and 12.5% Cd remained behind.

20 g organic phase of Example 6 was kneaded with various pulverulent solid substances until a friable solid mass was obtained. The following substances were used in the following quantities:

An organic phase containing heavy metal obtained on subjecting phosphoric acid to extraction using a pure dialkyldithiophosphoric acid ester can be solidified in analogous manner.