OA21051A

OA21051A - Process for the removal of heavy metals from a phosphoric acid containing composition using an ionic polymeric surfactant and use of said surfactant in the precipitation of heavy metals in a phosphoric acid containing composition.

Info

Publication number: OA21051A
Application number: OA1202200436
Authority: OA
Inventors: Patrycja KITA; Tanja Vojnovic; Katrine Lie BØYESEN
Original assignee: Yara International Asa
Priority date: 2020-06-16
Filing date: 2021-06-16
Publication date: 2023-10-09

Abstract

The present disclosure provides improved methods for the removal of heavy metals, in particular cadmium, from an aqueous phosphoric acid containing composition, wherein an organothiophosphorous heavy metal precipitating agent and an ionic polymeric surfactant, particularly a cationic polyacrylamide copolymer surfactant, are both added to a phosphoric acid containing composition, particularly under vigorous mixing conditions, such as between 500 and 700 rpm. The ionic polymeric surfactant promotes the precipitation of the heavy metals. 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

The présent 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 présent disclosure relates to removing heavy métal ions, such as cadmium, from phosphoric acid containing process streams.

Background

Heavy metals such as cadmium, copper, nickel, lead, zinc and mercury are considered unacceptable 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 developed 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 présent 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 up by plants and, thereby, end up in the food chain. For instance, cadmium can cause damage to lungs, kidneys, and bones. Therefore, it is essential to limit the level of heavy metals, 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 Iower limit such as 21.5 mg of cadmium per kilogram of P2O5. The level of the heavy métal 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 prevîously 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. 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 and at températures ranging from 5 to 40°C.

US 2004/0179984 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-dithiophosphorrc 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.

EP 0091043 discloses the use of similar heavy métal removal agents as disclosed in US20040179984 for the removal of cadmium by précipitation from the Odda process.

WO2019071108 discloses the simultaneous use of organothiophosphorous compounds and surfactants, in particular 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 rn the liquor, and the presence of calcium, which may affect heavy métal précipitation as well. 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.

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. In the improved methods of the present disclosure, the heavy metals are precipitated by adding an organothiophosphorous heavy métal precipitating agent in combination with an ionic polymeric surfactant to the aqueous phosphoric acid containing composition.

Advantageously, the combination of an organothiophosphorous heavy métal precipitating agent and an ionic polymeric surfactant, particularly a cationic poly(meth)acrylamide copolymer surfactant, led to an efficient précipitation and removal of the heavy métal from the phosphoric acid containing composition. The methods of the present disclosure are particularly suited for the removal of heavy metals, such as cadmium, from an aqueous composition of the nitro-phosphate process, comprising both phosphoric acid and nitric acid, without hydrogen sulphide or NOx formation resulting from side reactions of the heavy métal precipitating agent.

According to one aspectofthe present disclosure, a method is disclosed for the removal of heavy metals, in particular cadmium, dissolved in 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 and a surfactant to the composition of step (a), at a pH of at least 1.6, measured after a 13fold dilution by volume using water, 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

M .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; and (c) separating the heavy métal precipitate from the phosphoric acid containing composition obtained in step (b);

wherein the surfactant is a cationic polymeric surfactant wherein from 20% to 80%, more particularly from 20% to 60%, even more particularly from 20% to 50% of the moieties making up the cationic polymeric surfactant are cationic charged moieties, an anionic polymeric surfactant wherein from 1% to 10% of the moieties making up the anionic polymeric surfactant are anionic charged moieties, or a mixture thereof.

According to an embodiment of the présent disclosure, the surfactant is a cationic polyacrylamide or a cationic polymethacrylamide, with a cationic charge ranging from 20% to 80%, particularly ranging from 20% to 60% or 20% to 50% or from 30% to 50%, more particularly ranging from 35% to 45%.

According to an embodiment of the présent disclosure, the surfactant is a copolymer of (meth)acrylamide, particularly a copolymer of (meth)acrylamide and a chloro-methylated monomer, such as dimethylaminoethyl (meth)acrylate.

According to an embodiment of the présent disclosure, the molecular weight (MW) of the polymeric surfactant agent ranges from 3 x 10⁶ to 14 x 10⁶ Dalton.

According to an embodiment of the présent disclosure, the surfactant 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 or 10 to 20 g/m³ acid composition.

According to an embodiment of the présent disclosure, the ratio of precipitating agent to surfactant ranges from 1:1 to 10:1, particularly ranges from 3:1 to 6:1.

According to an embodiment of the présent disclosure, 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. 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 present disclosure, 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 at least pH 1.6 measured after a 13~fold dilution by volume using water, such as a pH of 1.6 to 2.2 measured after a 13-fold dilution by volume using water, particularly by addition of ammonia; thereby obtaining a phosphoric acid containing composition comprising a sludge fraction; (ri) optionally adding a first flocculating agent, such as a cationic polymeric flocculating agent, an anionic polymeric flocculating agent, or a mixture thereof, to the composition of step (i);

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

According to an embodiment ofthe present disclosure, step (c) comprises the steps of (c1 ) adding a second flocculating agent, in particular a cationic polymeric flocculating agent, an anionic flocculating agent, or a mixture thereof, to the composition obtained in step (b), thereby obtaining agglomérâtes comprising the heavy métal precipitate in a phosphoric acid composition; and (c2) separating the agglomérâtes comprising the heavy métal precipitate from the phosphoric acid containing composition.

According to an embodiment of the present disclosure, the phosphoric acid containing composition is an acid digest of phosphate rock, preferably by nitric acid, sulfuric acid, or a mixture thereof. More in particular, the phosphoric acid containing composition is an acidic aqueous composition comprising from 6-21wt% nitric acid, from 25 to 33 wt% phosphoric acid, from 3.5 to 5 wt% calcium and dissolved heavy metals, such as cadmium, with wt% being based on the total weight ofthe composition.

Accordingly, in an embodiment ofthe present disclosure the heavy metals are selected from cadmium, copper, nickel, lead, zinc and/or mercury; preferably the heavy metals are cadmium, copper and/or zinc; even more preferably the heavy métal is cadmium.

Another aspect of the present disclosure provides the use of a polymeric surfactant in the précipitation of heavy metals in a phosphoric acid containing composition with a diorganodithiophosphinic acid or an alkali métal or ammonia sait thereof, represented by Formula 1 above, as the heavy métal precipitating agent, wherein the polymeric surfactant is a cationic polymeric surfactant, an anionic polymeric surfactant, or a mixture thereof. In particular, the polymeric surfactant is a cationic copolymer of (meth)acrylamide. More in particular, the polymeric surfactant is a cationic polymer with a cationic charge ranging from 10% to 80%. More in particular, the polymeric surfactant is a copolymer of (meth)acrylamide, particularly a copolymer of (meth)acrylamide and a chloro-methylated monomer, such as dimethylaminoethyl (meth)acrylate.

Description ofthe Figures

FIG. 1 schematically represents a particular embodiment of the method according to the present disclosure, comprising the steps, in sequence, of adjusting the pH of a phosphoric acid composition to 1.6-2.2 measured after a 13-fold dilution by volume using water, adding a precipitating agent in combination with an ionic polymeric surfactant, and separating the precipitate from the filtrate/supernatant.

FIG. 2 schematically represents a particular embodiment of the method according to the présent disclosure, comprising the steps, in sequence, of adjusting the pH of a phosphoric acid composition to 1.6-2.2 measured after a 13-fold dilution by volume using water, removing a sludge fraction, adding a precipitating agent in combination with an ionic polymeric surfactant and separating the precipitate from the filtrate/supernatant.

FIG. 3 schematically represents a particular embodiment of the method according to the présent disclosure, comprising the steps, in sequence, of adjusting the pH of a phosphoric acid composition to 1.6-2.2 measured after a 13-fold dilution by volume using water, adding a flocculating agent, removing a sludge fraction, adding a precipitating agent in combination with an ionic polymeric surfactant, and separating the precipitate from the f iltrate/su pernatant.

Detaîled description of invention

Before the 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 of the 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 containing, 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 less, 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 specificaily, 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 clearperse, by means of further exemplification, the term encompasses inter alia 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 scientific terms, hâve the meaning as commonly understood by one of ordinary skiil in the art to which this invention belongs. By means of further guidance, term définitions are included to better appreciate the teaching of the present invention.

In the foîlowing 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 present 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 characteristîcs may be combined in any suitable manner, as would 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 of the invention, and form different embodiments, as would be understood by those ordinary skilled in the art. For example, tn the appended claims, any of the claimed embodiments can be used in any combination.

In the present description of the invention, reference is madeto 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 present invention. The foîlowing detailed description, therefore, is not to be taken in a limiting sense, and the scope of the présent invention is defined by the appended daims.

In the présent application, the percentages are given by weight, unless otherwise stated.

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

in the présent disclosure, unless otherwise stated, the pH values are measured after a 13 times 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 floccuiating agent or surfactant considered herein, are in the meaning of macro molécules comprising multiple charged or ionic subunits. More specifically, the term “ionic polymer or ionic polymeric” as they relate to the floccuiating agent or surfactant considered herein is used synonymously for the terms “polyelectrolyte” 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 present disclosure provides improved methods for the removal of heavy metals, in particular cadmium, from an aqueous phosphoric acid containing composition, wherein an ionic polymeric surfactant is added together with a heavy métal precipitating agent to a phosphoric acid containing composition. As used herein, the term “heavy métal generally refers to those éléments ofthe 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 lead. The present 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 composition or solution containing unrefined phosphoric acid, digestion slurries, fïltered acid, and/or concentrated acid, in particular as further discussed below.

According to one aspect of the present disclosure, a method is disclosed for the removal of heavy metals, in particular cadmium, dissolved in 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 together with a surfactant to the composition of step (a), at a pH of at least 1.6 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 meta! precipitating agent comprises a an 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 of step (b);

As defined herein, a surfactant is a compound that is added together with the heavy métal precipitating agent, in step (b).

In the context of the 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 P₂O₅. 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 35 wt% or from 25 to 30 wt% phosphoric acid, as well as 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 minera! 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, yielding a digestion liquor. In a second step or crystallization step, calcium nitrate tetra hydrate 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 în 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 obtained by a mixed acid process, wherein nitric acid is used for acidulation of a phosphate rock, a phosphate ore or a phosphate minerai. Sulfurtc acid is typically added to precipitate the calcium as calcium sulphate (gypsum), which is generally left in the siurry and acts as a diluent Phosphoric acid may be added in orderto adjust the water soluble phosphorous, depending on the grade being produced.

In certain embodiments, the surfactant is a copolymer of acrylamide or methacrylamide. 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 dimethylaminoethyl 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 particular embodiments, the surfactant is a cationic polymer, particularly a cationic (meth)acrylamide copolymer, with a cationic charge ranging from 20% to 60%, particularly ranging from 20% to 50% or from 30% to 50%, more particularly ranging from 35% to 45%. A particularly preferred cationic surfactant is a copolymer of acrylamide or methacrylamide and a chloro-methylated monomer, such as dimethylaminoethyl méthacrylate or dimethylaminoethyl acrylate.

In certain embodiments, the polymeric surfactant has a MW ranging from 3 x 10⁶ Dalton to 14x10® Dalton, particularly from 4x10® Dalton to 12 x 10® Dalton, more particularly from 4 x 10® Dalton to 8 x 10® Dalton. The surfactant may be a linear molécule or a branched molécule.

In certain embodiments, 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 the context of the present disclosure, the organothiophosphorous heavy métal precipitating agent comprises a diorgano-dithiophosphinic acid or an alkaii métal or ammonia sait thereof, represented by Formula 1

S rJI .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, alkaii métal or ammonia. Preferred examples of the hydrocarbon groups R in the diorgano dithiophosphinic acid (or alkaii métal or ammonia salts thereof) according to formula 1 include, but are not limited to, linear or branched alkyl, cycloalkyl, alkylaryl, aralkyl having from 3-20 carbon atoms. More preferably, suitable hydrocarbon groups include, but are not limited to, cyclohexyl, isopropyl, isobutyl, n-propyl, octyl, hexyl, phenylethyl, and 2,4,4trimethyl 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 corresponding exampîes 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 parts ofthe commercially available di-phosphine. After the di-phosphine has been metered, an additional 67.5 to 193.5 additional parts of diethyl phosphine are metered in at a rate such that within the time necessary to meter in ail ofthe 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 rangîng from 10 pg to 1 mg per g of the phosphoric acid containing composition, particularly from 50 pg to 0.75 mg per g of the phosphoric acid containing composition, more particularly ranging from 0.2 to 0.6 mg or from 0.3 mg to 0.6 mg per g of the phosphoric acid containing composition.

In certain embodiments, the weight ratio of precipitating agent to surfactant ranges from 1:1 to 10:1, particularly ranges from 3:1 to 6:1.

In the context ofthe présent disclosure, the heavy meta! precipitating agent as envisaged in the présent disclosure is added in combination with an ionic polymeric surfactant as envisaged herein. Stated differently, the heavy métal precipitating agent as envisaged herein, particularly the diorgano-dithiophosphinic acid or an alkali métal or ammonia sait thereof represented by Formula 1, and the ionic polymeric surfactant as envisaged herein are added essentially simultaneously or concurrently together to the phosphoric acid containing composition comprising dissolved heavy metals, in particular under vigorous mixing conditions, such as at mixing speeds of 500 to 700 rpm. In certain embodiments, step b) corresponds to precipitating the dissolved heavy metals by adding a blend of a heavy métal precipitating agent and a surfactant to the composition of step (a), at a pH of at least 1.6 measured after a 13-fold dilution by volume using water, particularly under vigorous mixing conditions, for instance at a mixing speed of 500 to 700 rpm, thereby obtaining a heavy métal precipitate in a phosphoric acid containing composition, wherein the heavy métal precipitating agent comprises a an organodithiophosphorous acid, in particular a diorgano-dithiophosphinic acid according to formula 1 above, or an alkali métal or ammonia sait thereof; and wherein the surfactant is a cationic polymeric surfactant wherein from 20% to 80%, more particularly from 20% to 60%, even more particularly from 20% to 50% of the moieties making up the cationic polymeric surfactant are cationic charged moieties, an anionic polymeric surfactant wherein from 1% to 10% ofthe moieties making up the anionic polymeric surfactant are anionic charged moieties, or a mixture thereof. According to particular embodiments of the présent disclosure, cationic polymers, more in particular cationic acrylamide or methacryl amide based copolymers, with an ionic charge ranging from 20% to 60%, such as from 20% to 50%, are especially suitable as polymeric surfactant for use in combination with the heavy métal precipitating agent as envisaged herein. In particular, a polymer of (meth)acrylamide and a chloro-methylated monomer may be used. Examples of suitable cationic polymers include, but are not limited to, 40 % linear cationic powder polyacrylamide (that is a polyacrylamide having 40% of its moieties positively charged) with molecular weight ranging from 7,1 to 9,5 *10⁶ Dalton or 20 % linear cationic powder polyacrylamide (that is a polyacrylamide having 20% of its moieties positively charged) with molecular weight ranging from 8,7 to 11,0 *1O⁶ Dalton.

According to an embodiment ofthe présent disclosure, the reaction with the heavy métal precipitating agent and the surfactant 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 and the surfactant as envisaged herein is particularly performed under vigorous mixing conditions, for instance at a mixing speed of 500 to 700 rpm. According to an embodiment of the présent disclosure, the reaction with the heavy métal precipitating agent and the surfactant 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 tempera tu re of 5 °C to 40 °C. As the heavy métal precipitate may be less stable at températures above 40 °C, it may be désirable to perform the reaction with the heavy métal precipitating agent and the surfactant for less than 10 minutes at higher températures.

In certain embodiments, the phosphoric acid containing composition may be subjected to one or more pretreatments, prior to the addition of the precipitating agent and the ionic surfactant, as indicated in FIG. 1, FIG. 2 and FIG. 3. Such pretreatments include pH adjustment (FIG. 1 - FIG. 3) and 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 flocculating agent to the phosphoric acid containing composition at pH of at least 1.6 to facilitate the séparation of the insoluble fraction from the phosphoric acid containing composition (FIG.3).

In particular, 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 comprising dissolved heavy metals to a pH of at least pH 1.6, thereby obtaining a phosphoric acid containing composition comprising 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).

As defined herein, a first flocculating agent is a compound that is added before step (a)(ii).

In the context of présent disclosure, the pH of the phosphoric acid containing composition may be adjusted prior to the addition of the heavy métal precipitating agent to a pH of at least 1.6, in particular between 1.6 and 2.2 or between 1.6 and 2.0 (measured after a 13 fold dilution by volume using water), thereby obtaining a phosphoric acid containing composition comprising a sludge fraction. At this pH, heavy métal précipitation, particularly cadmium précipitation, using an organothiophosphorous acid according to Formula 1 as envisaged herein, or an alkali métal or ammonia sait thereof, is especially effective. In addition, at these pH conditions, précipitation of phosphorous salts, in particular dicalcium phosphate (CaHPCU) is minimized, thereby minimizing phosphorous losses and maintaining the content of phosphorous in the phosphoric acid composition and, hence, in the final product. 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 already present in the nitric acid, such that a very pure NP-end product may be obtained.

In certain embodiments, at least part ofthe insoluble components or sludge present in the phosphoric acid containing composition, particularly the phosphoric acid containing composition comprising a sludge fraction, may be removed priorto the addition ofthe heavy métal precipitating agent and the ionic polymerîc surfactant. More in particular, a first flocculating agent may be added to the phosphoric acid containing composition, particularly the phosphoric acid containing composition comprising a sludge fraction, particularly under gentle mixing conditions, such as at mixing speeds of 100 to 300 rpm, to promate the agglomération and précipitation of the insoluble components or sludge fraction (FIG. 3). Surprisingly, the removal of at least part of the insoluble components or sludge before the addition of the heavy métal precipitating agent and ionic polymeric surfactant drd not affect the heavy métal précipitation efficiency ofthe 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 ofthe sludge fraction. In particular embodiments, the first flocculating agent is acationic polymer wherein 20% to 80%, particularly 20% to 60% or 30% to 50%, more particularly 35% to 45% of the moieties making up the cationic polymeric surfactant are cationic charged moieties, an anionic polymer wherein 10% to 50%, particularly 10% to

40%, more particularly 15% to 30% of the moieties making up the anionic polymeric surfactant are anionic charged moieties, or a mixture thereof. In particular embodiments, the first floccuiating agent is a copolymer of acrylamide or methacrylamide. According to particular embodiments of the present disclosure, cationic acrylamide or methacryl amide based copolymers, with a cationic charge ranging from 30% to 50% or from 35% to 45%, are especiaîly suitable for the purpose of agglomerating and separating the sludge and insoluble components prior to the addition of the heavy métal precipitating agent and surfactant. In particular, a polymer of (meth)acrylamide and a chloro-methylated monomer may be used. Examples of suitable cationic polymers include, but are not limited to, 40 % linear cationic powder polyacrylamide (that îs a polyacrylamide having 40% of its moieties negatively charged) with molecular weight ranging from 7,1 to 9,5 *10⁶ Dalton, 40 % linear cationic powder polyacrylamide (that is a polyacrylamide having 40% of its moieties negatively charged) with molecular weight ranging from 4,8 to 7,6 *10^s Dalton, or 40 % linear cationic powder polyacrylamide with molecular weight ranging from 9,5 to 12,1 *10⁶Dalton. According to particular embodiments of the present disclosure, anionic acrylamide or methacrylamide based copolymers, with a anionic charge ranging from 1 to 10%, are especiaîly suitable for the purpose of agglomerating and separating both the sludge and insoluble components prior to the addition of the heavy métal precipitating agent, as well as the heavy métal précipitâtes formed after addition and mixing of the heavy métal precipitating agent to the phosphoric acid containing composition. In particular, a polymer of (meth)acrylamide and (meth)acrylic acid may be used. Examples of suitable anionic polymers include, but are not limited to linear anionic émulsion polyacrylamide with molecular weight ranging from 9,7 to 17,5 *10⁶ Dalton or linear anionic powder polyacrylamide with molecular weight ranging from 12,2 to 17,5 *10⁶ Dalton.

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 floccuiating agent is performed by centrifugation. In particular embodiments, the precipitated insoluble or siudge 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, priorto 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.

Accordingly, in particular embodiments, a method is provided for the removal of heavy metals dissolved in a phosphoric acid containing composition, wherein the method comprises the steps of (a)(i) providing a phosphoric acid containing composition comprising dissolved heavy metals, such as cadmium, and adjusting the pH of a phosphoric acid containing composition comprising dissolved heavy metals to a pH of at least pH 1.6, in particular a pH of 1.6 to 2.0 or pH 1.6 to 2.2, thereby obtaining a phosphoric acid containing 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 obtaining 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;

(b) precipitating the dissolved heavy metals by adding a heavy métal precipitating agent together with an ionic poiymeric surfactant as envisaged herein, particularly a cationic polymeric surfactant, to the composition of step (a), at a pH of at least 1.6, in particular a pH of 1.6 to 2.0 or pH 1.6 to 2.2, 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; and (c) separating the agglomérâtes comprising the heavy métal precipitate from the phosphoric acid containing composition of step (b), particularly by centrifugation.

According to an embodiment of the present disclosure, the précipitation step (b) and/or 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, 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 ofthe nonphosphate insoluble or sludge fraction in the phosphoric acid containing composition comprising a sludge fraction. 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)(üi) may be accomplished by State of the art technology for liquid-solid séparation such as, but not limited to, centrifugation and/or décantation. Séparation by centrifugation is particularly preferred. Although some of the agglomérâtes hâve been found to be quite fragile, séparation of the flocculants by centrifugation was surprisingly effective. In addition, in certain embodiments, the séparation of the heavy métal précipitâtes formed and obtained in step b) may be promoted by the addition of a second flocculating agent. As defined herein, a second flocculating agent is a compound that is added after the heavy métal precipitating agent is added to the composition after step (b) and during the separating step (c). The second flocculating agent may be any flocculating agent suitable for the agglomération and flocculation of the heavy métal précipitâtes. The second flocculating agent may bethe sarne or different than the first flocculating agent. In particularly preferred embodiments, the second flocculating agent is a compound accordîng to the ionic polymeric surfactant as described herein.

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

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

Removing heavy metals from the composition comprising phosphoric acid accordîng to any embodiment of the methods envisaged herein; in particular comprising 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 in combination with an ionic polymeric surfactant, in particular a cationic polymeric surfactant, an anionic polymeric surfactant, or a mixture thereof, to the phosphoric acid containing composition, at a pH of at least 1.6, particularly under vigorous mixing conditions, such as at mixing speeds of 500 to 700 rpm, wherein the heavy métal precipitating agent comprises a diorgano-dithiophosphinic acid accordîng to formula 1 or an alkali métal or ammonia sait thereof; and separating the heavy métal précipitâtes from the phosphoric acid containing composition.

Further adjusting the pH of the phosphoric acid containing composition to approximately pH 5.8 using gaseous ammonia;

optionally, adding potassium salts to the phosphoric acid containing composition with pH 5.8;

particulating the phosphoric acid containing composition with pH 5.8 and optionally comprising 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-colored 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 oronly on partofthe digestion liquor. Inthe lattercase, the part ofthedigestion liquorwhich is not treated according to a method of the present disclosure is mixed with or diluted with the part ofthe digestion liquorwhich has been treated according to a method ofthe present disclosure, such that the heavy métal (cadmium) levels of the combined composition is below a desired value, particularly remains within the regulatory limite.

Another aspect of the present disclosure provides for the use of an ionic polymeric surfactant as envisaged in the present disclosure, in the précipitation of heavy metals in a phosphoric acid containing composition with a diorgano-dithiophosphinic acid or an alkali métal or ammonia sait thereof, represented by Formula 1 above, as the heavy métal precipitating agent.

Examples

Example 1 - Effect of a cationic polymeric surfactant

The pH of a cadmium-containing mother liquor (ML), freshiy produced from the Odda nitrophosphate process, was adjusted to pH=1.8 using gaseous ammonia (NH₃). The pH was measured after mixing one volume of a non-diluted sample with 13 volumes of water.

The used mother liquor comprised 5.14 wt% Ca (as measured by Atomic Absorption Spectroscopy), 7.4 wt% P (as measured by gravimetry P), 34.2 wt% H₂O (as measured by Karl Fisher titration) and 6 ppm Cd, 15.2 ppm Cu (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.

The resulting composition was centrifuged for (2440 rpm, 2 min, 1000 G) to reduce the amount of sludge.

Sodium diîsiobutyl dithiophosphinate (DTPINa 3.56 %, 0.336 g) was added to the resulting NP-liquor (pH=1.8, 60 g) under vigorous stirring (600 rpm). Simultaneously with the addition of DTPINa, a cationic polyacrylamide copolymer as surfactant was added under intense stirring (40% linear cationic powder polyacrylamide (that is a polyacrylamide having 40% of its moieties positively charged) with a molecular weight ranging from 7,1 to 9,5 *10^s Dalton, 0.1 wt% concentration, 0.078 g, 600 rpm). The mixture stirred for a total of 3 min. Next, the resulting composition was centrifuge for (2440 rpm, 2 min, 1000 G) and the supernatant and precipitate were separated. In addition, a sample of the composition was taken using a syringe with 5 pm filter. This was to investigate the effect of séparation efficiency using a centrifuge. The cadmium and heavy métal content were analyzed by ICP-OES (Thermo Scientific, iCAP 7400 Duo, wavelength: 226.502 nm (Cd) and 204.379 nm (Cu) in an axial mode).

The combination of DTPINa and the cationic polyacrylamide copolymer surfactant resulted in a Cd-and Cu-extraction efficiency of about 85% following centrifugation or filtration. This demonstrates the very efficient removal of the heavy métal containing precipitate by centrifugation.

Example 2 - Comparative example: Cd précipitation without surfactant

The pH of a freshly produced cadmium-containing mother liquor (composition as in example 1) was adjusted to pH=1.8 (measured after a 13-fold dilution by volume using water) using gaseous ammonia (NH₃). The resulting composition was centrifuged for (2440 rpm, 2 min, 1000 G)

DTPINa (3.56 %, 0.336 g) was added to the mother liquor at pH=1.8 (60 g) under vigorous stirring (600 rpm). After 3 min of reaction the mixture, the resulting composition was centrifuged for (2440 rpm, 2 min, 1000 G) and the supernatant and precipitate were separated. In addition, a sample of the composition was taken using a syringe with 5 pm filter. The cadmium and heavy métal content were analyzed by ICP-OES (Thermo Scientific, iCAP 7400 Duo, wavelength: 226.502 nm (Cd) and 204.379 nm (Cu) in an axial mode). The addition of DTPINa without a surfactant resulted in a Cd-extraction efficiency of about 73 %, Cu-extraction efficiency of about 78 %.

Example 3 - Effect of different charged polymeric surfactants

Different surfactants were screened in combination with 200 ppm DTPINa as précipitation agent, using the experimental setup as in Example 1. The tested surfactants include cationic polyacrylamide (PAM) copolymers with charge of 20 % linear cationic powder polyacrylamide (that is a polyacrylamide having 20% of its moieties positively charged) with molecular weight ranging from 8,7 to 11,0 *10⁶ Dalton, 40 % linear cationic powder polyacrylamide (that is a polyacrylamide having 40% of its moieties positively charged) with molecular weight ranging from 7,1 to 9,5 *10⁶ Dalton or 80 % linear cationic powder 5 polyacrylamide (that is a polyacrylamide having 80% of its moieties positively charged) with molecular weight ranging from 5,0 to 7,1 *10⁶ Dalton, anionic polyacrylamide copolymers with charge of 3 % linear anionic powder polyacrylamide (that is a polyacrylamide having 3% of its moieties negatively charged) with molecular weight ranging from 13,4 to 16,0 *10^eDalton), 20 % linear anionic powder polyacrylamide (that is a polyacrylamide having 20% 10 of its moieties negatively charged) with molecular weight ranging from 12,2 to 17,5 *10®

Dalton or 50 mole % linear anionic powder polyacrylamide (that is a polyacrylamide having 50% of rts moieties negatively charged) with molecular weight ranging from 11,3 to 22,2 *10⁶ Dalton, a non-ionic PAM surfactant linear powder polyacrylamide (that is a polyacrylamide uncharged) with molecular weight ranging from 8,5 to 12,0 *10⁶ Dalton and 15 dioctyl sulfosuccinate. The results are presented in Table 1.

Table 1. Cd and Cu extraction efficiency of different surfactants in combination with DPTINa (200ppm) with séparation of the precipitate by centrifugation

Surfactant	Cd extraction (%)	Cu extraction (%)
20 mole % linear cationic powder polyacrylamide with molecular weight ranging from 8,7 to 11,0 *10⁶ Dalton	84	88
40 mole % linear cationic powder polyacrylamide with molecular weight ranging from 7,1 to 9,5 *10^s Dalton	85	85
80 mole % linear cationic powder polyacrylamide with molecular weight ranging from 5,0 to 7,1 *10⁶ Dalton	68	77
80 mole % linear cationic powder polyacrylamide with molecular weight ranging from 5,0 to 7,1 *10⁶Dalton	39	46
20 mole % linear anionic powder polyacrylamide with molecular	19	29

weight ranging from 12,2 to 17,5 *10⁶Dalton
50 mole % linear anionic powder polyacrylamide with molecular weight ranging from 11,3 to 22,2 *10⁶Dalton	25	65
50 mole % linear anionic powder polyacrylamide with molecular weight ranging from 11,3 to 22,2 *10⁶Dalton	23	37
Dioctyl sulfosuccinate	27	41

The présent resuits show that PAM surfactants are suitable alternatives for dioctyl sulfosuccinate (in combination with DTPINa as précipitation agent) for the removal of heavy metals in phosphoric acid containing compositions. Surprisingîy, the Cd and Cu removal of the combined/simultaneous use of DTPINA (200 ppm) as precipitating agent and cationic polymers having 20-80 % of their moieties positively charged, as surfactants, under vigorous stirring conditions (600 rpm or in the range of 500-70ppm and with removal ofthe précipitâtes by centrifugation) resulted in the highest heavy meta! efficiency (67-84%), compared to the use of other polyacrylamide polymers or dioctyl sulfosuccinate as surfactant. With anionic polyacrylamides, only 19-39% of Cd and 29-65% of Cu were removed. A similar low removal efficiency for Cd and Cu was obtained with the non-ionic PAM and with dioctyl sulfosuccinate. For the anionic and non-ionic polyacrylamide and dioctyl sulfosuccinate surfactants, the removal efficiency could be increased by using a 5pm nylon fiiter for the séparation of the heavy métal précipitâtes (e.g. to 44-66% (Cd) for the anionic and non-ionic polyacrylamides and 63% for dioctyl sulfosuccinate), but it generally remaîned inferior to the removal efficiency with the cationic polyacrylamides.

Cationic polymers used as surfactant thus were found to be particularly effective in removing heavy metals (in combination with DTPINa), using centrifugation as a séparation technique, without the need for an (additional) filtration step.

Example 4 - Effect of using another mother liquor

The pH of a cadmium-containing mother liquor (ML), freshly produced from the Odda nitrophosphate process, was adjusted to pH=1.8 using gaseous ammonia (NH3). The pH was measured measured after mixing one volume of a non-diluted samplewith 13 volumes of water.

The used mother iiquor comprised 6.52 wt% Ca (as measured by Atomic Absorption Spectroscopy), 11.6 wt% P (as measured by gravimetry P), 34.78 wt% H₂O (as .measured by Karl Fisher titration) and 3.5 ppm Cd, 12.3 ppm Cu, 97.4 ppm Zn (as measured by ICPOES). It is understood that the percentages and amounts as they relate to the ML composition are merety an indication of such composition and are not limiting for the process considered herein.

Sodium diisiobutyl dithiophosphinate (DTPINa 3.56 %, 0.34 g) was added to the resulting NP-liquor (pH=1.8, 60 g) under vigorous stirring (600 rpm). Simuitaneously with the addition of DTPINa, a cationic polyacrylamide copolymer as surfactant was added under intense stirring (40% linear cationic powder polyacrylamide (that is a polyacrylamide having 40% of its moieties positively charged) with a molecular weight ranging from 7,1 to 9,5 *10⁶ Dalton, 0.1 wt% concentration, 0.078 g, 600 rpm). The mixture was stirred for a total of 3 min. Next, the resulting composition was centrifuged for (2440 rpm, 2 min, 1000 G) and the supernatant and precipitate were separated. In addition, a sam pie of the composition was taken using a syringe with 5 pm filter. This was to investigate the effect of séparation efficiency using a centrifuge. The cadmium and heavy métal content were analyzed by ICPOES (Thermo Scientific, iCAP 7400 Duo, wavelength: 226.502 nm for Cd and 204.379 nm for Cu in an axial mode).

The combination of DTPINa and the cationic polyacrylamide copolymer surfactant resulted in a Cd- efficiency of about 50%, Cu-extraction efficiency of 62 % and Zn-extraction efficiency of 23 % foilowing centrifugation or filtration.

Claims

1. Process 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 and a surfactant to the composition of step (a), at a pH of at least 1.6, 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 II ^P-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 of step (b);

characterized in that the surfactant is a cationic polymeric surfactant wherein from 20% to 80%, more particularly from 20% to 60%, even more particularly from 20% to 50% ofthe moieties making up the cationic polymeric surfactant are cationic charged moieties, an anionic polymeric surfactant wherein from 1% to 10% ofthe moieties making up the anionic polymeric surfactant are anionic charged moieties, or a mixture thereof.

2. The process according to claim 1, wherein the surfactant is a (meth)acrylamide based anionic polymer, a (meth)acrylamide based cationic polymer, or a mixture thereof.

3. The process according to claim 2, wherein the surfactant is a copolymer of (meth)acrylamide and a chloro-methylated monomer, such as dimethylaminoethyl (meth)acrylate.

4. The process according to any one of claims 1 to 3, wherein the MWofthe surfactant ranges from 3 x 10® to 14 x 10® Dalton.

5. The process according to any one of claims 1 to 4, wherein the surfactant is added în a dose of 3 to 30 g/m³ acid composition, particularly in a dose of 3 to 20 g/m³ acid composition.

6. The process according to any one of claims 1 to 5, wherein the weight ratio of precipitating agent to surfactant ranges from 1:1 to 10:1 , particularly ranges from 3:1 to6:1.

7. The process according to any one ofthe preceding claims, 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 at least pH 1.6, particularly by addition of ammonia; thereby obtaining a phosphoric acid containing composition comprising a sludge fraction;

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

8. The process according to any one of the preceding claims, wherein step (c) comprises the steps of (c1) adding a second flocculating agent to the composition obtained in step (b), thereby obtaining agglomérâtes comprising the heavy métal precipitate in a phosphoric acid composition; and (c2) separating the agglomérâtes comprising the heavy métal precipitate from the phosphoric acid containing composition.

9. 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.

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

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

12. The process according to any one of the preceding claims wherein the heavy metals are selected from cadmium, copper, nickel, fead, zinc and/or mercury; preferably the heavy metals are cadmium, copper and/or zinc; even more preferably the heavy métal is cadmium.

13. Use of a polymeric surfactant in the précipitation of heavy metals in a phosphoric acid containing composition with a diorgano-dithiophosphinic acid or an alkali métal or ammonia sait thereof, represented by Formula 1, as the heavy métal precipitating agent,

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, characterized in that the polymeric surfactant is a cationic polymeric surfactant wherein from 20% to 80%, more particularly from 20% to 60%, even more particularly from 20% to 50% of the moieties making up the cationic polymeric surfactant are cationic charged moieties, an anionic polymeric surfactant wherein from 1% to 10% of the moieties making up the anionic polymeric surfactant are anionic charged moieties, or a mixture thereof.

14. Use according to claim 13, wherein the polymeric surfactant is a cationic copolymer of (meth)aorylamide, an anionic copolymer of (meth)acrylamide, or a mixture thereof.

15. Use according to claim 14, wherein the polymeric surfactant is a cationic copolymer of (meth)acrylamide and a chloro-methylated monomer.

16. Use according to any one of claims 13 to 15 wherein the heavy metals are selected from cadmium, copper, nickel, lead, zinc and/or mercury; preferably the heavy metals are cadmium, copper and/or zinc; even more preferably the heavy métal is cadmium.