OA21048A - Process for the removal of heavy metals from a phosphoric acid containing composition. - Google Patents

Abstract

The present disclosure provides improved methods for the removal of heavy metals, in particular cadmium, from an aqueous phosphoric acid containing composition, using an organothiophosphorous heavy metal precipitating agent to said composition, wherein the reaction between the heavy metals, in particular cadmium, and the organothiophosphorous precipitating agent is performed at a pH ranging between 1.6 and 2.0 measured after a 13-fold dilution by volume. Advantageously, an ionic polymer, particularly a cationic and/or an anionic poly(meth)acrylamide copolymer may be used to promote heavy metal precipitation and/or to facilitate the removal of the precipitates from the composition. More in particular, the phosphoric acid containing composition is obtained by the acid digestion of phosphate rock, preferably by nitric acid, sulfuric acid, or a combination thereof.

Description

PROCESS FOR THE REMOVAL OF HEAVY METALS FROM A PHOSPHORIC ACID CONTAINING COMPOSITION

Field

The present disclosure relates to the field of removing heavy métal ions, including but not limited to cadmium, from wet-process acidic compositions. More in particular, the present disclosure relates to removing heavy métal ions, such as cadmium, from phosphoric acid containîng process streams.

Backqround

Heavy metals such as cadmium, copper, nickel, lead, zinc and mercury are considered unacceptabie above a certain level, depending on the application, because of their toxicity and they thus hâve to be either completely removed or their levels hâve to be reduced significantly. Many processes hâve been deveioped over the years for their removal.

In this context, the phosphate rock extracted from phosphate mines typically contains heavy métal impurities, such as cadmium, copper, arsenic, or mercury. For instance, cadmium typically is present at levels between 0.15 to 507 mg/kg of phosphate rock having an average phosphorous (P2O5) content of about 30 weight% (Swe Swe Mar & Masanori Okazaki, Microchemical Journal 104 (17-21), September 2012). Unless the heavy metals are removed from the phosphate rock prior to or during its digestion with acid, such as prior to or during the nitro-phosphate process, the resulting phosphate-based products and fertilizers will contain cadmium and other heavy metals. Some forms of heavy metals, such as cadmium, can be taken upby plants and, thereby, end up in thefood chain. For instance, cadmium can cause damage to lungs, kidneys, and bones. Therefore, it is essential to limit the level of heavy mêlais, such as cadmium, in fertilizers. The European Union is now considering a limit of 60 mg cadmium per kilogram of phosphorous (expressed as P2O5). However, Finland is applying an even lower limit such as 21.5 mg of cadmium per kilogram of P2O5. The level of the heavy meta! impurities thus has to be significantly reduced.

The précipitation of heavy metals, such as cadmium, in the nitro-phosphate process or in other processes comprising the acid digestion of phosphate rock, has previously been reported.

US 4,378,340 discloses a method of removing heavy metals from an acid digest of phosphate rock by partial neutralization ofthe acids followed by précipitation ofthe heavy metals as sulphides. However, heavy métal (cadmium) précipitation as a sulphide compound benefits from a high pH, which will lead to unacceptable phosphor losses, as at higher pH values, both heavy métal sulphides and calcium phosphates precipitate.

US 4,986,970 discloses a method for removal of heavy metals, especially cadmium, primarily from a mother liquor made by the Odda process, using métal salts of dithiocarbonic acid-O-esters, referred to as xanthates, at a pH ranging from 1.4 and 2.0 measured after a 13-fold dilution by volume, and at températures ranging from 5 to 40°C. US2004/0179984A1 discloses a process and compositions to remove heavy métal ions, such as cadmium, copper, lead, nickel, arsenic, manganèse, zinc, and mercury ions from the wet phosphoric acid process. The process involves treating phosphoric acid prior to or after gypsum filtration with diorgano-dithiophosphinic acid (or alkali métal or ammonia salts thereof), a first diorgano-dithiophosphoric acid (or alkali métal or ammonia salts thereof) and optionally a second diorgano-dithiophosphoric acid (or alkali métal or ammonia salts thereof), precipitating metals such as cadmium, copper, lead, nickel, arsenic, manganèse, zinc and mercury at a température from about 10 to about 85°C and preferably in the range of about 50 to about 80°C, and separating the filtrate by either filtration or flotation. In this context, the examples only indicate that these compounds are effective in phosphoric acid, in particular at températures ranging from 60 to 80°C.

EP0091043 discloses the use of similar heavy métal removal agents as disclosed in US20040179984 for the removal of cadmium by précipitation from the Odda process. In EP0091043, Cd précipitation is performed at a mother liquor pH range of 0.5-1.5 (of the undiluted composition) with a desired pH range of 0.6-1.2.

WO2019071108 discloses the simultaneous use of organothiophosphorous compounds and surfactants, such as sulfosuccinate compounds and polyethyleneglycol esters for removing heavy métal ions from aqueous solutions containing phosphoric acid, in particular in various stages of wet process phosphoric acid production.

Nevertheless, despite the various approaches of the prior art, the removal of heavy metals, such as cadmium, from a phosphate rock digest by concentrated acid, such as nitric acid, remains challenging due to the very acidic and oxidizing conditions in the phosphate rock digest or in the mother liquor, and the presence of calcium, which may affect heavy métal précipitation as well. For instance, during the neutralization ofthe phosphate rock digest or mother liquor, heavy metals, such as cadmium, precipitate along with calcium phosphates. Indeed, at a pH value above 4 or 5, different phosphate species precipitate as well, which lead to unwanted losses of the valuable phosphorous. In addition, heavy métal contamination, especially cadmium, remains a concern to public health. In this context, as indicated above, regulatory agencies continue to impose lower limits on the acceptable level of heavy metals, in particular cadmium. There thus remains a need for improved methods for the efficient removal of heavy metals, such as cadmium, from phosphoric acid containing compositions. In particular, there remains a need for improved methods for the efficient removal of heavy metals, such as cadmium, while also maintaining phosphorous in solution and minimizing phosphorous loss by précipitation.

Summary

The present disclosure provides improved methods for the removal of heavy metals, in particular cadmium, from an aqueous phosphoric acid containing composition, which address the above identified needs in the art. The improved methods of the present disclosure provide heavy métal (Cd) précipitation conditions with high heavy métal (Cd) précipitation efficiency while keeping phosphorous in solution.

According to one aspect of the present disclosure, a method is disclosed for the removal of heavy metals from a phosphoric acid containing composition, comprising the steps of (a) providing a phosphoric acid containing composition comprising dissolved heavy metals, such as cadmium;

(b) precipitating the dissolved heavy metals by adding a heavy métal precipitating agent to composition of step (a), thereby obtaining a heavy métal precipitate in a phosphoric acid containing composition, wherein the heavy métal precipitating agent comprises a diorgano-dithiophosphinic acid or an alkali métal or ammonia sait thereof, represented by Formula 1

S ^R ||

--SM

Formula 1 wherein R is a linear or branched hydrocarbon group selected from alkyl, aryl, alkylaryl, or aralkyl, and wherein the hydrocarbon group contains 3 to 20 carbon atoms, and M is H, alkali métal or ammonia; and (c) separating the heavy métal precipitate from the phosphoric acid containing composition;

wherein the precipitating step (b) is performed at a pH of 1.6 to 2.0 (measured after a 13fold dilution by volume). Advantageously, at these pH values, the heavy métal précipitation and extraction efficiency is very high and the loss of phosphor by précipitation of phosphor containing compounds is very limited.

According to an embodiment ofthe présentdisclosure, R in Formula 1 is selected from the group consisting of cyclohexyl, isopropyl, isobutyl, n-propyl, octyl, hexyl, phenylethyl and 2,4,4-trimethyipentyl, particularly wherein the heavy metal-precipitation agent is sodium diisobutyldithiophosphinate. Advantageously, precipitating agents according to Formula 1 hâve a good cadmium extraction efficacy and are less hazardous compared to inorganic sulphides and xanthates. In particular, precipitating agents according to Formula 1 resuit in lower (if any) H₂S, COS or CS₂ émissions compared to inorganic sulphides and xanthates.

According to an embodiment of the présent disclosure, step (a) further comprises the steps of (i) adjusting the pH of a phosphoric acid containing composition comprisîng dissolved heavy metats to a pH of 1.6 to 2.0 measured after a 13-fold dilution by volume, thereby obtainîng a neutraiized phosphoric acid composition comprisîng a sludge fraction;

(ii) optionally adding a first flocculating agent to the composition of step (i);

(iii) separating the sludge fraction from the composition of step (i) or (ii).

Advantageously, removing the sludge fraction prior to step b) results in the addition of the heavy métal precipitating agent to an acid composition with a reduced amount of particles and/or sludge, thus facilitating the heavy métal précipitation and yielding a more concentrated heavy métal precipitate.

According to an embodiment of the présent disclosure, the pH adjustment is performed by the addition of ammonia.

According to an embodiment ofthe présent disclosure, step (b) comprises the steps of ( b1 ) precipitating the dissolved heavy metaîs by adding the heavy métal precipitating agent to the composition of step (a), thereby obtaining a heavy métal precipitate in a phosphoric acid containing composition; and (b2) adding a second flocculating agent to the composition obtained in step (b1), thereby obtaining agglomérâtes comprisîng the heavy meta! precipitate.

More in particular, the first and/or the second flocculating agent is an anionic polymer, a cationic polymer, or a mixture thereof. Advantageously, the flocculating agent promûtes the formation of agglomérâtes of the heavy métal précipitâtes, thereby facilitating the séparation of the heavy métal précipitâtes from the phosphoric acid containing composition.

According to an embodiment of the présent disclosure, in step (b), an anionic polymeric surfactant, a cationic polymerîc surfactant, or a mixture thereof is added in combination with the heavy métal précipitation agent to the composition of step (a). Advantageously, the ionic polymeric surfactant promûtes the précipitation of the heavy métal by the heavy métal precipitating agent.

In particular, the ionic polymeric surfactant or flocculating agent is an ionic (meth)acrylamide copolymer. More in particular, the ionic polymeric surfactant or flocculating agent is

- a cationic copolymer of (meth)acrylamide, such as a cationic copolymer of (meth)acrylamide and a chloro-methylated monomer,

- an anionic copolymer of (meth)acryiamide and (meth)acrylic acid, or

- a mixture thereof.

In certain embodiments of the présent disclosure, either one or both of the surfactant or flocculating agent is added in a dose of 5 to 30 g/m³ acid composition, particularly in a dose of 5 to 20 g/m³ acid composition.

According to an embodiment ofthe présent disclosure, the précipitation and/orflocculation steps are performed at a température of 10 to 50 °C.

According to an embodiment of the présent disclosure, the phosphoric acid containing composition is an acid digest of phosphate rock, preferably by nitric acid, sulfuric acid and/or a mixture thereof. More in particular, the phosphoric acid containing composition is an acidic aqueous composition comprising from 25 to 33 wt% phosphoric acid, from 621wt% nitric acid, from 3.5 to 5 wt% calcium and dissolved heavy metals, such as cadmium, with wt% being based on thetotal weight ofthe composition.

According to an embodiment ofthe present disclosure, the heavy metals are selected from cadmium, copper, nickel, mercury, zinc, arsenic, manganèse and/or lead; preferably the heavy métal is cadmium.

Description ofthe Figures

FIG. 1 schematically represents a particular embodiment of the method according to the present disclosure, comprising the steps, in sequence, of neutralizing a phosphoric acid composition to a pH 1.6-2.0 measured after a 13-fold dilution by volume, adding a precipitating agent, and separating the precipitate from the filtrate/supernatant.

FIG. 2 schematically represents a particular embodiment of the method according to the present disclosure, comprising the steps, in sequence, of neutralizing a phosphoric acid composition to a pH 1.6-2,0 measured after a 13-fold dilution by volume, adding a first flocculating agent and removing a sludge fraction, adding a precipitating agent, and separating the precipitate from the filtrate/supernatant

FIG. 3 schematically represents a particular embodiment of the method according to the present disclosure, comprising the steps, in sequence, of neutralizing a phosphoric acid composition to a pH 1.6-2.0 measured after a 13-fold dilution by volume, adding a first flocculating agent, removing a sludge fraction, adding a precipitating agent, adding a second flocculating agent and separating the precipitate from the filtrate/supernatant.

Detailed description

Beforethe present System and method ofthe invention are described, it is to be understood that this invention is not limited to particular Systems and methods or combinations described, since such Systems and methods and combinations may, of course, vary. It is also to be understood that the terminology used herein is not intended to be limiting, since the scope ofthe present invention will be limited only by the appended claims.

As used herein, the singular forms a, an”, and the include both singular and plural referents unless the context clearly dictâtes otherwise.

The terms comprising, comprises and comprised of as used herein are synonymous with including, includes or containîng, contains, and are inclusive or open-ended and do not exclude additional, non-recited members, éléments or method steps. It will be appreciated that the terms comprising, comprises and comprised of as used herein comprise the terms consisting of, consists and consists of'.

The recitation of numerical ranges by endpoints includes ail numbers and fractions subsumed within the respective ranges, as well as the recited endpoints.

The term about or approximately” as used herein when referring to a measurable value such as a parameter, an amount, a temporal duration, and the like, is meant to encompass variations of +/-10% or less, preferably +/-5% or iess, more preferably +/-1% or less, and still more preferably +/-0.1% or less of and from the specified value, insofar such variations are appropriate to perform in the disclosed invention. It is to be understood that the value to which the modifier about or “approximately” refers is itself also specifically, and preferably, disclosed.

Whereas the terms “one or more” or “at least one”, such as one or more or at least one member(s) of a group of members, is clear perse, by means offurther exemplification, the term encompasses inter alla a reference to any one of said members, or to any two or more of said members, such as, e.g., any £3, £4, £5, >6 or £7 etc. of said members, and up to ail said members.

Unless otherwise defined, ail terms used in disclosing the invention, including technical and scientifîc terms, hâve the meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. By means of further guidance, term définitions are included to better appreciate the teaching of the présent invention.

In the following passages, different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

Reference throughout this spécification to “one embodiment” or “an embodiment means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the présent invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment in various places throughout this spécification are not necessarily ail referring to the same embodiment, but may be. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as woutd be apparent to a person skilled in the art from this disclosure, in one or more embodiments. Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope ofthe invention, and form different embodiments, as would be understood by those ordinary skilled in the art. For example, in the appended claims, any of the claimed embodiments can be used in any combination.

In the présent description of the invention, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration only of spécifie embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilised and structural or logical changes may be made without departing from the scope of the présent invention. The foilowing detailed description, therefore, is not to be taken in a limiting sense, and the scope of the présent invention is defined by the appended claims.

In the présent disclosure, the concentration ofthe components comprised in a composition, when indicated as a percentage, is given by weight with respect to the total weight of the composition, unless otherwise stated.

In the présent application, unless otherwise stated, the pH values are measured after a 13 fold dilution by volume with water. Stated differently, the pH value is measured after mixing one volume of a non-diluted sample with 13 volumes of water.

In the présent disclosure, unless explicitly stated otherwise, the terms “ionic polymer” or ionic polymeric” as they relate to the flocculating agent or surfactant considered herein, are in the meaning of macromolecules comprising multiple charged or ionic subunits. More specifically, the term “ionic polymer or “ionic polymeric” as they relate to the flocculating agent or surfactant considered herein is used synonymousiy for the terms “poiyelectrolyte or “polyelectrolytic”, i.e. polymers, in particular polycations or polyanions, whose repeating units bear an electrolyte group. In the présent disclosure, ionic poly(meth)acrylamides, such as cationic or anionic poly(meth)acrylamides are particularly preferred.

The présent disclosure provides improved methods for the removal of heavy metals, in particular cadmium, from an aqueous phosphoric acid containing composition, wherein a heavy métal precipitating agent is added to an aqueous phosphoric acid containing composition, at a pH of about 1.6 to about 2.0 measured after a 13-fold dilution by volume.

As used herein, the term heavy métal generally refers to those éléments of the periodic table having a density of more than 5 g/cm³. Such heavy métal (or heavy métal ions) include, for example, one or more of cadmium, copper, nickel, mercury, zinc, arsenic, manganèse and iead. The présent disclosure is particularly directed for the removal of at least cadmium from compositions containing phosphoric acid. The term “phosphoric acid containing composition may be any aqueous acidic solution or composition containing unrefined phosphoric acid, digestion slurries, filtered acid, and/or concentrated acid, as further discussed below.

According to one aspect of the présent disclosure, a method is disclosed for the removal of heavy metals from a phosphoric acid containing composition, comprising the steps of (a) providing a phosphoric acid containing composition comprising dissolved heavy metals, such as cadmium;

(b) precipitating the dissolved heavy metals by adding a heavy métal precipitating agent to composition of step (a), thereby obtaining a heavy métal precipitate in a phosphoric acid containing composition, wherein the heavy métal precipitating agent comprises a organodithiophosphorous acid, in particular a diorgano-dithiophosphinic acid or an alkali métal or ammonia sait thereof, and (c) separating the heavy métal precipitate from the phosphoric acid containing composition;

wherein the precipitating step (b) rs performed at a pH of 1.6 to 2.0 (measured after a 13fold dilution by volume using water). At these pH values, heavy métal précipitation, particularly cadmium précipitation, using an organothiophosphorous acid or an alkali métal or ammonia sait thereof, in particular a diorgano-dithiophosphinic acid or an alkali métal or ammonia sait thereof as envisaged herein is especially effective. In addition, at these pH conditions, précipitation of phosphorous salts, in particular dicalcium phosphate (CaHPO₄) is minimized, thereby minimizing phosphorous losses and maintaining the content of phosphorous in the phosphoric acid composition and, hence, in the final product.

!n particular, the heavy métal is cadmium. In the context of the present disclosure, the organothiophosphorous heavy métal precipitating agent comprises a diorganodithiophosphinic acid or an alkali métal or ammonia sait thereof, represented by Formula 1

S ^R\ll p--_SM

Formula 1 wherein R is a linear or branched hydrocarbon group selected from alkyl, aryl, alkylaryl, or aralkyl, and wherein the hydrocarbon group contains 3 to 20 carbon atoms, and M is H, alkali métal or ammonia. Preferred examples of the groups R in the diorgano dithiophosphinic acid (or alkali métal or ammonia salts thereof) according to formula 1 include, but are not limited to, hydrocarbons containîng 3 to 20 carbon atoms in which the hydrocarbon group is linear or branched alkyl, cycloalkyl, alkylaryl, aralkyl. More preferably, suitable hydrocarbon groups include, but are not limited to, cyclohexyl, isopropyl, isobutyl, n-propyl, octyl, hexyl, phenylethyl, and 2,4,4-trimethyl pentyl. Even more preferably, the diorgano-dithiophosphinic acid (or sait thereof) used in the present invention as heavy métal précipitation agent is di-isobutyl dithiophosphinate. In a preferred embodiment, the precipitating agent is sodium di-isobutyl dithiophoshinate.

Generally, the heavy métal precipitating agent can be prepared according to the procedure described in US 4308214 and the correspondÎng examples by heating 67,2 parts of sulfur 114.8 to 284.8 parts of water to a température of about 70 C. To the mixture are then steadily metered in 29.5 to 64.5 of the commercially available di-phosphine. After the diphosphine has been metered, an addîtional 67,5 to 193.5 parts additional parts of diethyl phosphine are metered in at a rate such that within the time necessary to meter in ail of the diethylphosphine, 80.0 parts of a 50% solution of sodium hydroxide are also metered in at a constant rate to neutralize the corresponding dithiophosphinic acid that forms.

In certain embodiments, the heavy métal precipitating agent is added in an amount ranging from 10 pg to 1 mg per g of the phosphoric acid containîng composition, particularly from 50 pg to 0.75 mg per g of the phosphoric acid containîng composition, more particularly ranging from 0.2 to 0.6 mg or from 0.3 mg to 0.6 mg per g ofthe phosphoric acid containîng composition.

According to an embodiment of the present disclosure, the reaction with the heavy métal precipitating agent as envisaged herein may be performed for 3 minutes to 1.5 hour, for 5 minutes to one hour, or for 10 to 30 minutes. The skilled person understands that the reaction with the heavy métal precipitating agent as envisaged herein is particularly performed under vigorous mixing conditions, in particular at mixing speeds of 500 to 700 rpm. According to an embodiment of the présent disclosure, the reaction with the heavy métal precipitating agent as envisaged herein may be performed at température ranging from 5 °C to 80 °C, in particular at a température from 5 °C to 50 °C, more particularly are performed at a température of 5°C to 40°C. As the heavy métal precipitate may be less stable at températures above 40 °C, it may be desirable to perform the reaction with the heavy métal precipitating agent for less than 10 minutes at higher températures. Stated differently, at températures of 40 °C to 50 °C or higher, step c) is preferably performed 3 to 10 min after step b) to prevent unwanted dégradation of the precipitate at higher températures. Performing the précipitation at lower températures is bénéficiai for the stability of the precipitating agent, but may require more time for the precipitate to form.

In the context ofthe present disclosure, the phosphoric acid containing composition from which the heavy metals, in particular cadmium is to be removed, may be obtained by digesting a phosphate rock, a phosphate ore or a phosphate minerai with an acid. Such phosphate rock may contain high amounts of heavy metals, in particular cadmium, e.g. from 10 to 300 mg Cd/kg PîOs- The acid used in the digesting step may be nitric acid, sulfuric acid or a combination thereof.

In certain embodiments, the phosphoric acid containing composition comprises from 1 to 85wt% phosphoric acid, particularly from 1 to 60 wt% phosphoric acid, more particularly from 10 to 60 wt%, such as from 20 to 60 wt% phosphoric acid, even more particularly from 10 to 40 wt% phosphoric acid, most particularly from 20 to 35wt% or from 25 to 30 wt% phosphoric acid and dissolved heavy metals, such as cadmium. The phosphoric acid containing composition may comprise from 1 to 500 mg/l, more in particular from 1 to 250 mg/l, more in particular 1 to 100 mg/l dissolved cadmium.

In certain embodiments, the phosphoric acid containing composition is obtained by digesting a phosphate rock, a phosphate ore or a phosphate minerai with nitric acid at 65°C. In particular, the phosphoric acid containing composition comprises from 18 to 21 weight% nitric acid, from 25 to 29 weight% phosphoric acid and dissolved heavy metals, such as dissolved cadmium. More in particular, the phosphoric acid containing composition is obtained by the nitrophosphate process. More in particular, the phosphoric acid containing composition is the mother liquor obtained in the nitrophosphate process. In the nitrophosphate process, in a first step or digestion step, phosphate rock is digested in nitric acid at a température of 65°C, yîelding a digestion liquor. In a second step or crystallization step, calcium nitrate tetrahydrate is crystallized out of the digestion liquor yielding a crystal slurry. In a third step or séparation step, the crystallized calcium nitrate is separated by a technique such as filtration or centrifugation, resulting in calcium nitrate tetrahydrate crystals being separated from the liquid of the crystal slurry, referred to as the mother liquor. In certain embodiments, the phosphoric acid containing composition is obtaîned by a mixed acid process, wherein nitric acid is used for acidulation of a phosphate rock, a phosphate ore or a phosphate minerai. Sulfuric acid is typically added to precipitate the calcium as calcium sulphate (gypsum), which is generally left in the slurry and acts as a diluent. Phosphoric acid may be added in order to adjust the water soluble phosphorous, depending on the grade being produced.

In certain embodiments, the phosphoric acid containing composition may be subjected to one or more pretreatments, prior to the addition of the precipitating agent, as indicated in FIG. 1, FIG. 2 and FIG. 3. Such pretreatments include pH adjustment (FIG. 1 - FÎG. 3) and/or separating an insoluble fraction (sludge) (FIG. 2 and FIG. 3) from the phosphoric acid containing composition.

In certain embodiments, removal of the insoluble fraction (sludge) may comprise the addition of a first floccuiating agent to the neutralized phosphoric acid containing composition to facilitate the séparation of the insoluble fraction from the phosphoric acid containing composition (FIG.2 and 3).

In particular, according to an embodiment of the present disclosure, step (a) further comprises the steps of (i) adjusting the pH of a phosphoric acid containing composition comprising dissolved heavy metais to a pH of 1.6-2.0 (measured after a 13-fold dilution by volume using water), thereby obtaining a phosphoric acid containing composition comprising a sludge fraction;

(ii) optionally adding a first floccuiating agent to the composition of step (i);

(iii) separating the sludge fraction from the composition of step (i) or (ii).

In the context of present disclosure, the pH of the phosphoric acid containing composition is adjusted prior to the addition of the heavy métal precipitating agent to a pH ranging between 1.6 and 2.0 (measured after a 13-fold dilution by volume using water), thereby obtaining a so-cal!ed neutralized phosphoric acid containing composition, having a higher pH value that the pH value for the phosphoric acid containing composition before the pH adjusting step (i). In certain embodiments, the pH of the aqueous phosphoric acid containing composition is adjusted using gaseous ammonia. Advantageously, particularly when the phosphoric acid containing composition comprises nitric acid, no other Chemical éléments are introduced other than nitrogen and hydrogen aiready present in the nitric acid, such that a very pure NP-end product may be obtained.

As defined herein, a first flocculating agent is a compound that is added before step a -(ii). In certain embodiments, at least part of the insoluble components or sludge present in the phosphoric acid containing composition, particularly the neutralîzed phosphoric acid containing composition, may be removed prior to the addition of the heavy métal precipitating agent. More in particular, a first flocculating agent may be added to the phosphoric acid containing composition, particularly the neutralîzed phosphoric acid containing composition, to promote the agglomération and précipitation of the insoluble components or sludge fraction (FIG. 2 and 3). Surprisingly, the removal of insoluble components or sludge before the addition of the heavy métal precipitating agent did not affect the heavy métal précipitation efficiency of the method. Moreover, the séparation of part of the sludge and insoluble components, such as aided by flocculation, prior to heavy métal précipitation, in particular cadmium précipitation, facilitâtes the cadmium extraction from the composition comprising phosphoric acid. Advantageously, in this way, a smaller amount of the heavy métal precipitating agent as envisaged herein may be added to the phosphoric acid containing composition and the resulting heavy métal containing précipitâtes comprise a higher concentration of heavy metals, in particular cadmium.

The first flocculating agent may be any flocculating agent suitable for the agglomération and flocculation of the sludge fraction.

The séparation of the sludge may be accomplished by any standard technology for séparation such as, but not limited to, filtration, centrifugation, sédimentation, flotation or décantation. In certain embodiments, the séparation of the precipitated insoluble or sludge fraction due to the addition of the first flocculating agent is performed by centrifugation. In particular embodiments, the precipitated insoluble or sludge fraction is subject to a preconcentration step prior to centrifugation, wherein at least part of the liquid is separated from the precipitated sludge fraction. For instance, such pre-concentration step may be a settling step, wherein the sludge agglomérâtes settle, so that the liquid can be separated, such as by décantation, prior to centrifugation. Advantageously, this way, the amount of the composition to be centrifuged, particularly the amount of liquid, is reduced and the centrifugation step is rendered more efficient, as the solids/liquid séparation in the centrifugation is more easily achieved.

Accordîng to an embodiment of the present disclosure, heavy métal précipitation is promoted by the addition of a second flocculating agent, particularly an ionic polymeric flocculating agent, to the composition comprising heavy métal précipitâtes (FIG. 3). Stated differently, the second flocculating agent is added after the addition of the heavy métal precipitating agent and after heavy métal précipitâtes hâve been formed. According to an embodiment of the présent disclosure, step (b) comprises the steps of (b1 ) precipitating the dissolved heavy metals by adding the heavy métal precipitating agent to the composition of step (a), thereby obtaining a heavy métal precipitate in a phosphoric acid containing composition; and (b2) adding a second flocculating agent to the composition obtained in step (b1 ), thereby obtaining agglomérâtes comprising the heavy métal precipitate.

As defined herein, a second flocculating agent is a compound that is added after the heavy métal precipitating agent is added to the composition in step b) and during the separating step c).

The flocculating agent induced formation of larger agglomérâtes promotes the séparation ofthe précipitâtes comprising the heavy métal (cadmium) complexed with the heavy métal précipitation agent, from the aqueous phosphoric acid containing composition. The skilled person understands that the flocculating agent induced agglomerate formation is best performed under gentle mixing conditions, in particular at mixing speeds of 100 rpm to 300 rpm. This way, sufficient shearforces are applied to build agglomérâtes by collision ofthe métal précipitâtes and ionic charge attraction. Too high shear forces may overcome ionic charge attraction and thus not allow agglomération. In certain embodiments, the flocculation step c) is performed ai the same température and pH conditions as the précipitation step b).

Additionally or alternatively, heavy métal précipitation may be promoted by the addition of an ionic polymeric surfactant, which is added together with the di-organo dithiophosphinic acid according to formula 1, or an alkali métal or ammonia sait thereof as heavy métal precipitating agent. As defined herein, a surfactant is a compound that is added together with the heavy métal precipitating agent. Accordingly, in certain embodiments of the présent disclosure, step (b) comprises the step of precipitating the dissolved heavy metals by adding a heavy métal precipitating agent together with a ionic polymeric surfactant as envisaged herein, particularly a cationic polymeric surfactant, an anionic polymeric surfactant, or a mixture thereof, to the composition of step (a), at a pH between 1.6 and 2.0 (measured after a 13-fold dilution by volume using water), thereby obtaining a heavy métal precipitate in a phosphoric acid containing composition, wherein the heavy métal precipitating agent comprises an diorgano-dithiophosphinic acid according to Formula 1 as envisaged herein, or an alkali métal or ammonia sait thereof.

More in particular, either one or both ofthe second flocculating agent and the surfactant is a cationic polymer, an anionic polymer or a mixture thereof.

In particular embodiments, either one or both of the second flocculating agent and the surfactant is a cationic polymer, an anionic polymer or a mixture thereof, wherein either one or both of the cationic and the anionic polymer has an ionic charge ranging from 10% to 80%, i.e. wherein 10% to 80% ofthe moieties or subunits making up the polymer are ionic or charged moieties or subunits. For anionic or cationic polymerîc flocculating agents or surfactants considered herein, the ionic charge value may also be referred to as the degree of anionicity or the degree of cationicity, respectively. In certain embodiments, either one or both ofthe second flocculating agent and the surfactant is an ionic acrylamide copolymer or an ionic methacrylamide copolymer. As used herein, the term “ionic acrylamide copolymer, “ionic polyacrylamide”, “ionic methacrylamide copolymer or “ionic polymethacrylamide” refers to a polymer comprising acrylamide or methacryl amide subunits and additionally comprising subunits comprising an ionic charge. Cationic acrylamide copolymers or cationic methacrylamide copolymers comprise subunits having a cationic charge, particularly comprising a quaternary nitrogen atom, such as comprising ADAM or MADAM subunits, i.e. dimethylaminoethyl acrylate or dimethylaminoethel metacrylate, respectively. Anionic acrylamide copolymers or anionic methacrylamide copolymers comprise subunits having an anionic charge, particularly comprising a carboxylate or sulphonate functional group, such as comprising acrylic acid or methacrylic acid subunits, or styrene sulphonate subunits.

In certain embodiments, either one or both of the second flocculating agent and the surfactant is:

- a cationic polymer, particularly a cationic (meth)acrylamide copolymer, with a cationic charge ranging from 10% to 80%, particularly ranging from 20% to 60% or from 20% to 50%, more particularly ranging from 30% to 50% or from 35% to 45%;

- an anionic polymer, particularly an anionic (meth)acrylamide copolymer, with an anionic charge ranging from 10% to 50%, particularly ranging from 10% to 40%, more particularly ranging from 15% to 30%; or

- a mixture thereof.

A particularly preferred cationic flocculating agent or surfactant is a copolymer of acrylamide or methacrylamide monomers and a chloro-methylated monomer, such as dimethylaminoethyl methacrylate or dimethylaminoethyl acrylate monomers. A particularly preferred anionic flocculating agent is a copolymer of acrylamide or methacrylamide monomers and acrylic acid or methacrylic acid monomers.

In certain embodiments, the second polymeric flocculating agent or surfactant has a MW ranging from 3 x 10⁶ Dalton to 14 x 10⁶ Dalton, particularly from 4 x 10⁶ Dalton to 12 x 10⁶ Dalton, more particularly from 4 x 10^e Dalton to 8 x 10⁶ Dalton. The polymeric flocculating agent or surfactant may be a linear molécule or a branched molecuie.

In certain embodiments, either one or ail of the first flocculating agent, the second flocculating agent and the surfactant is added in a dose of 3 to 30 g/m³ acid composition, particularly in a dose of 3 to 20 g/m³ acid composition, such as in a dose of 5 to 20 g/m³ or 10 to 20 g/m³ acid composition.

In certain embodiments, the first flocculating agent may be the same or different than the second flocculating agent. In particular embodiments, the first flocculating agent is a cationic polymer, an anionic polymer or a mixture thereof. More in particular, the first flocculating agent is a cationic polymer, an anionic polymer or a mixture thereof, wherein the cationic and/or anionic polymer has an ionic charge ranging from 10% to 80%, i.e. wherein 10% to 80% of the moieties making up the polymer are ionic or charged moieties. The first flocculating agent may be a cationic polymer with a cationic charge ranging from 20% to 80%, particularly ranging from 20% to 60% or from 20% to 50%, more particularly ranging from 30% to 50% or from 35% to 45%; an anionic polymer with an ionic charge ranging from 10% to 50%, particularly ranging from 10% to 40%, more particularly ranging from 15% to 30%; or a mixture thereof. In particular embodiments, the first flocculating agent is an ionic copolymer of acrylamide or methacrylamide.

Accordingly, in particular embodiments, a method is provided for the removal of heavy metals dissolved in a phosphoric acid containîng composition, wherein the method comprises the steps of (a)(i) providing a phosphoric acid containîng composition comprising dissolved heavy metals, such as cadmium, and adjusting the pH of a phosphoric acid containîng composition comprising dissolved heavy metals to a pH between 1.6 to 2.0 (measured after a 13-fold dilution by volume using water), thereby obtaînîng a neutralized phosphoric acid containîng composition comprising a sludge fraction;

(a)(ii) adding a first flocculating agent, particularly an ionic polymeric flocculating agent as envisaged herein to the composition of step (a)(i), thereby obtaînîng sludge agglomérâtes, and, optionally, removing part of a liquid fraction;

(a)(iii) removing the sludge agglomérâtes from the composition of step (a)(ii), particularly by centrifugation;

(b1) precipitating the dissolved heavy metals by adding the heavy métal precipitating agent to the composition of step (a)(iii), thereby obtaining a heavy métal precipitate in a phosphoric acid containing composition;

(b2) adding a second flocculating agent, particularly an ionic polymeric flocculating agent as envisaged herein to the composition obtained in step (b1), thereby obtaining agglomérâtes comprising the heavy métal precipitate; and/or adding an ionic polymeric surfactant as envisaged herein concurrently together with the heavy métal precipitating agent in step (b1).

(c) separating the heavy métal precipitate from the phosphoric acid containing composition of step (b2), particularly by centrifugation.

According to an embodiment of the present disclosure, either one or both of the précipitation step (b) and the flocculation step (a)(ii) are performed at a température of 5°C to 50 °C, particularly are performed at a température of 5°C to 40°C. Particular good results were obtained at a température ranging from 10°C to 35°C or 10 to 30°C. These températures in combination with the above indicated pH conditions of 1.6 to 2.0 measured after a 13-fold dilution by volume, benefit the stability of the heavy métal precipitating agent in the phosphoric acid containing composition as envisaged herein. These conditions were also found to be optimal for flocculation and précipitation of the non-phosphate insoluble or sludge fraction in the (neutralized) phosphoric acid containing composition. Certain embodiments of the present disclosure include adjusting the température of the phosphoric acid containing composition to a température of 5°C to 50 °C, particularly to a température of 5°C to 40°C, more particularly to a température of 10°C to 35°C or 10°C to 30°C, by natural cooling or by heat exchangers.

In the context of the present disclosure, the séparation in step c) or step a)(iü) may be accomplished by State of the art technology for liquid-solid séparation such as, but not limited to, either one or both of centrifugation and décantation. Séparation by centrifugation îs particularly preferred. Although some of the agglomérâtes hâve been found to be quite fragile, séparation of the flocculants by centrifugation was surprisingly effective.

Another aspect of the present disclosure provides a method for preparing a fertilizer, particularly a nitrogen fertilizer, comprising the steps of

Digesting phosphate rock with nitric acid, thereby obtaining a composition comprisîng phosphoric acid and calcium nitrate;

Removing heavy metals from the composition comprisîng phosphoric acid according to any embodiment of the methods envisaged herein; in particular comprisîng the steps of optionally removing a sludge fraction by flocculation with a (first) flocculating agent and précipitation; precipitating the dissolved heavy metals, such as cadmium, by adding a heavy métal precipitating agent to the phosphoric acid containing composition, at a pH of 1.6 to 2.0 (measured after a 13-fold dilution by volume using water) wherein the heavy métal precipitating agent comprises a diorgano-dithiophosphinic acid according to formula 1 or an alkali métal or ammonia sait thereof; subsequently adding a first flocculating agent to the composition comprisîng heavy métal précipitâtes, particularly under gentle mixing conditions, such as at mixing speeds of 100 to 300 rpm, thereby obtaining agglomérâtes comprisîng the heavy métal precipitate in a phosphoric acid containing composition; and separating the agglomérâtes comprisîng the heavy métal precipitate from the phosphoric acid containing composition.

Further adjusting the pH of the phosphoric acid containing composition to approximately pH 5.8 (measured after a 13-fold dilution by volume using water) using gaseous ammonia;

optionally, adding potassium salts to the phosphoric acid containing composition with pH 5.8 (measured after a 13-fold dilution by volume using water);

particulating the phosphoric acid containing composition with pH 5.8 and optionally comprisîng potassium salts, and, further, optionally, coating and/or coloring the particles. In this manner, it is possible to obtain, from the nitro-phosphate process, coated or noncoated, colored on non-coîored NP or NPK particles with reduced amounts of heavy metals, such as cadmium. It will be évident to the person skilled in the art that the method of the disclosure can be applied on the total aqueous composition resulting from the digestion step or only on part ofthe digestion liquor. In the latter case, the part ofthe digestion liquor which is not treated according to a method of the présent disclosure is mixed with or diluted with the part of the digestion liquor which has been treated according to a method of the présent disclosure, such that the heavy métal (cadmium) levels of the combined composition is below a desired value, particularly remains within the regulatory limits.

Examples

Exampie 1 - Heavy métal précipitation at different pH values

Experimental setup

Mother liquor obtained from the Odda process based on 100 % uncaictned Khourigba 20 (K20) rock was neutralised to different pH values (pH 1.1-2.4), measured after a 13-fold dilution by volume using water (by mixing 1 part mother liquor and 13 parts water), to study the effect of pH on the Cd-extraction efficiency and potential phosphorous précipitation occurring during neutralisation. The pH range of 1.1-2.4 (measured after a 13-fold dilution by volume using water) corresponds to a pH range for the undiluted composition ranging of pH -0.95 to 0.68 (négative pH values may be measured in very acidic solutions). pH measurements were performed at room température (22-24°C). Ail the tests were performed in laboratory scale (150-250 g).

The used mother liquor comprised 4.8 wt% Ca (as measured by Atomic Absorption Spectroscopy), 8.1 wt% P (as measured by gravimetry P), 34wt% H^O (as measured by Karl Fisher titration) and 8.5 ppm Cd ( as measured by ICP-OES). It is understood that the percentages and amounts as they relate to the ML composition are merely an indication of such composition and are not limiting for the process considered herein.

After neutralization to the desired pH (ranging from pH 1.1-2.4, measured after a 13-fold dilution by volume using water), the neutralized mother liquor was cooled to approximately 5Û°C and centrifuged. The thus separated sludge was discarded.

Cd précipitation was performed with sodium diisiobutyl dithiophosphinate (DTPINa) as precipitating agent at not more than 30 °C to avoid dégradation of the precipîtating agent in the acidic media. In particular, to the supernatant was added DTPINa (0.016 g/g mother liquor 3.56 % - corresponding to 570 ppm DTPINa) under intense mixing using a magnetic stirrer (600rpm) for 3 min, then a cationic polyacrylamide flocculating agent was added (FO 4490 SS H, 0.1 wt % concentration, 0.003 g/g NP.-liq.), and, after addition ofthe flocculating agent, the stirring speed was reduced to 150 rpm for the flocculation step. After 3 min the suspension was centrifuged. The sludge and supernatant were analysed using ICP-OES (Thermo Scientific, iCAP 7400 Duo, in an axial mode for the heavy metals: Cd, Zn, Cu, Ni, Pb, Mn and As) for the heavy métal concentration and gravimétrie P to assess the potential P-ioss during the different steps.

Res u Its

The concentration and total amount of Cd in the neutralized mother liquor (before and after sludge removai) and in the supernatant/fiïtrate after Cd précipitation is shown in Table 1.

Table 1. Cd précipitation efficiency at different pH values

pHa	Cd in ML after neutralization (in mg)	Cd in ML after sludge removal (in mg)	Cd in supematant after Cd précipitation (in mg)	Cd removal efficiency (%)
1.1 (-0.95)	1.6	1.4	1.3	7.2%
1.3(-0.69)	1.6	1.4	1.3	5.6%
1.4 (-0.53)	1.8	1.6	1.5	7.6%
1.5(-0.27)	1.7	1.5	1.4	8.9%
1.6(-0.1)	1.5	1.3	0.06	96.6%
1.8(0.18)	1.8	1.6	0.07	96.3%
1.9(0.27)	1.6	1.4	0.04	97.7%
2.0 (0.31)	1.7	1.4	0.07	96.2%
2.2 (0.68)	2.6	2.3	0.2	89.2%

^a measured after a 13-fold dilution by volume using water; the values between brackets correspond to the pH of the undiluted sample.

These results show that sludge fraction contains very little Cd: the amount of Cd in the sample before sludge removal is similar to the amount Cd remaining in solution after sludge removal. Indeed, the sludge fraction merely consists of acid insoluble components which typically follows the liquor throughout the different process steps.

Table 1 further shows that the efficiency of the Cd extraction (by précipitation) dépends on the pH. Below pH 1.6 (measured after a 13-fold dilution by volume using water), only about 7-9% of the Cd was precipitated by the DTPINa compound. In contrast, at pH values above pH 1.6 (measured after a 13-fold dilution by volume using water), the Cd removal efficiency increased to about 96%. At pH 2.2 (measured after a 13-fold dilution by volume using water), the Cd removal is still very high (ca. 90%). However, the séparation step becomes the limiting factor, as the încreasing viscosity renders the séparation by centrifugation less efficient.

In addition, under the conditions of the présent experimental setup, neutralizing the mother liquor to a pH of 2.4 (measured after a 13-fold dilution by volume using water), the liquor became solid upon cooling and no Cd précipitation by DTPINa could be performed.

Furthermore, as demonstrated in Table 2, at pH values between 1.1 and 2.0 (measured after a 13-foId dilution by volume using water), about 90-95% of the phosphorous présent in the samples after sludge removal, remains in solution upon addition of the heavy métal precipitating agent, with about 5-10% of the P being lost in the Cd containing precipitate (waste). However, at pH 2.2 (measured after a 13-fold dilution by volume using water), only about 2/3 of the phosphorous présent în the samples after sludge removal remains in the supernatant, after removal of the heavy métal (Cd) précipitâtes, indicating significant phosphorous losses in the precipitated fraction. Stated differently, at pH values above 2.0 (measured after a 13-fold dilution by volume using water), such as at pH 2.2 (measured after a 13-fold dilution by volume using water), phosphorous compounds (e.g. calcium phosphates) start to precipitate as a resuit of the elevated pH. Minimizing the loss of P in the process is important as phosphorous is a valuable component in the mother liquor.

Table 2. P recovery (by gravimétrie P analysis) at different pH values

pHa	P in supernatant after Cd précipitation (in g)	P in Cd precipitate (waste) (in g)	P in supernatant (% total P)
1.1 (-0.95)	8.60	0.80	91.5%
1.3(-0.69)	11.61	0.63	94.9%
1.4 (-0.53)	13.87	0.90	93.9%
1.5(-0.27)	13.43	1.11	92.4%
1.6 (-0.1)	11.28	0.75	93.8%
1.8(0.18)	13.59	1.35	91.0%
1.9(0.27)	11.91	0.92	92.8%
2.0(0.31)	11.99	1.26	90.5%
2.2 (0.68)	14.87	7.87	65.4%

³ measured after a 13-fod dilution by volume using water; the values between brackets correspond to the pH of the undiluted sample.

In conclusion, below pH 1.6 (measured after a 13-fold dilution by volume using water), the Cd extraction (by précipitation) was negligible (about 7-9 %), whereas at pH values ranging from 1.6-2.0 (measured after a 13-fold dilution by volume using water), the Cd extraction efficiency was about 96 %. Advantageously, at pH values ranging from 1.6 to 2.0 (measured after a 13-fold dilution by volume using water), P losses in the heavy métal precipitate were minimized as well (about 5-10%). At pH 2.2(measured after a 13-fold dilution by volume using water) and above, Cd extraction was still high, but viscosity issues affected the efficient séparation of the Cd waste, !n addition, more P was lost in the Cd waste as well due to P précipitation.

Claims (11)

1. Process for the removal of heavy metals from a phosphoric acid containing composition, comprising the steps of (a) providing a phosphoric acid containîng composition comprising dissolved heavy metals, such as cadmium;

(b) precipitating the dissolved heavy metals by adding a heavy métal precipitating agent to composition of step (a), thereby obtaînîng a heavy métal precipitate in a phosphoric acid containîng composition, wherein the heavy métal precipitating agent comprises a diorganodithiophosphinic acid or an alkali métal or ammonia sait thereof, represented by Formula 1

S ^R ||

--SM

R^

Formula 1 wherein R is a linear or branched hydrocarbon group selected from alkyl, aryl, alkylaryl, or aralkyl, and wherein the hydrocarbon group contains 3 to 20 carbon atoms, and M is H, alkali métal or ammonia; and (c) separating the heavy métal precipitate from the phosphoric acid containing composition;

characterized in that the precipitating step (b) is performed at a pH of 1.6 to 2.0 (measured after a 13-fold dilution).

2. The process according to claim 1, wherein step (a) further comprises the steps of (i) adjusting the pH of a phosphoric acid containing composition comprising dissolved heavy metals to a pH of 1.6 to 2.0 measured after a 13-fold dilution by volume, thereby obtaînîng a phosphoric acid composition comprising a sludge fraction;

(ii) optionally adding a first flocculating agent to the composition of step (i) wherein said first flocculating agent is an anionic polymer, a cationic polymer, or a mixture thereof that is suitable for the agglomération and flocculation of the sludge fraction;

(iii) separating the sludge fraction from the composition of step (i) or (ii).

3. The process according to claim 2, wherein the pH adjustment is performed by the addition of ammonia.

4. The process according to any of claims 1 to 3, wherein step (b) comprises the steps of (b1 ) precipitating the dissolved heavy metals by adding the heavy métal precipitating agent to the composition of step (a), thereby obtaining a heavy métal precipitate in a phosphoric acid containing composition, and (b2) adding a second flocculating agent to the composition obtained in step (b1 ), thereby obtaining agglomérâtes comprising the heavy métal precipitate, wherein said second flocculating agent is an anionic polymer, a cationic polymer or a mixture thereof that is suitable for the agglomération and flocculation of composition obtained in step (b1).

5. The process according to any of claims 1 to 4, wherein, in step (b), an anionic polymeric surfactant, a cationic polymeric surfactant, or a mixture thereof is added in combination with the heavy métal précipitation agent to the composition of step (a) and wherein either one or both of the polymeric surfactant and the flocculating agent is a copolymer of (meth)acryiamide, particularly a cationic copolymer of (meth)acrylamide and a chloro-methylated monomer, an anionic copolymer of (meth)acrylamide and (meth)acrylic acid, or a mixture thereof.

6. The process according to any one of claims 4 to 5, wherein either one or both of the polymeric surfactant and the flocculating agent is added in a dose of 3 to 30 g/m³ acid composition, particularly in a dose of 3 to 20 g/m³ acid composition.

7. The process according to any one of claims 1 to 6, wherein either one or both of the précipitation and the flocculation steps are performed at a température of 10 to 50 ’C.

8. The process according to any one ofthe preceding claims wherein the phosphoric acid containing composition is an acid digest of phosphate rock, preferably by nitric acid, sulfuric acid or a mixture thereof.

9. The process according to any one of the preceding claims, wherein the phosphoric acid containing composition is an acidic aqueous composition comprising from 25 to 33 wt% phosphoric acid, from 6-21wt% nitric acid, from 3.5 to 5 wt% calcium and dissolved heavy metals, such as cadmium, with wt% based on the total weight ofthe composition.

10. The process according to any one ofthe preceding claims, wherein R in Formula 1 is selected from the group consisting of cyclohexyl, isopropyl, isobutyl, n-propyl, octyl, hexyl, phenylethyl and 2,4,4-trimethylpentyl, particularly wherein the heavy metal-precipitation agent is sodium diisobutyldithiophosphinate.

11. The process according to any one of the preceding claims, wherein the heavy métal is selected from cadmium, copper, nickel, mercury, zinc, arsenic, manganèse and/or lead;

preferably wherein the heavy métal is cadmium.