OA21049A - Process for the removal of heavy metals from a phosphoric acid containing composition using a flocculating agent. - Google Patents

Process for the removal of heavy metals from a phosphoric acid containing composition using a flocculating agent. Download PDF

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OA21049A
OA21049A OA1202200434 OA21049A OA 21049 A OA21049 A OA 21049A OA 1202200434 OA1202200434 OA 1202200434 OA 21049 A OA21049 A OA 21049A
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métal
phosphoric acid
heavy
agent
composition
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OA1202200434
Inventor
Patrycja KITA
Tanja Vojnovic
Tom Rames JØRGENSEN
Per Arne JORDBRÆK
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Yara International ASA
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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 ionic polymeric flocculating agent is added to a phosphoric acid containing composition subsequent to the addition of an organothiophosphorous heavy metal precipitating agent to said composition, particularly under gentle mixing conditions, such as between 100 and 300 rpm. The flocculating agent promotes the formation of agglomerates of the heavy metal containing precipitate, thus facilitating their removal from the composition. More in particular, the phosphoric acid containing composition is obtained by the acid digestion of phosphate rock; Dreferably by nitric acid, sulfuric acid, or a combination thereof.

Description

PROCESS FOR THE REMOVAL OF HEAVY METALS FROM A PHOSPHORIC ACID CONTAINING COMPOSITION USING A FLOCCULATING AGENT
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 containing process streams.
Backqround
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 present at levels between 0.15 to 507 mg/kg of phosphate rock having an average phosphorcus (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 prier to or during its digestion with acid, such as prior to or during the nitro-phosphate process, the resulting phosphate-based products and fertiiizers 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 thefood 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 fertiiizers. 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 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 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.
US 4,986,970 discloses a method for removal of heavy metals, especiaîly 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.
US2O040179984 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 U220040^ 79984 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, despitethe 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 liquor, and the presence of calcium, which may affect heavy métal précipitation as well. In addition, heavy métal contamination, especiaîly cadmium, remains a concern to public health. In this context, as indicated above, regulatory agencies continue to impose lower limits on the acceptable ievel 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.
Summarv
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, a floccuiating agent is added to a phosphoric acid containing composition, particularly under gentle mixing conditions, such as between 100 and 300 rpm, subsequently to the addition of a heavy métal precipitating agent.
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. Advantageously, by using an anionic flocculating agent as envisaged herein, there is no need for combining a heavy métal precipitating agent with a surfactant, such as a sulfosuccinate compound or a polyethyleneglycol ester. The methods ofthe present disclosure are particularly suited for the removal of heavy metals, such as cadmium, from an aqueous composition ofthe nitrophosphate process, comprising both phosphoric acid and nitric acid, without hydrogen sulphîde or NOx formation resulting from side reactions of the heavy métal precipitating agent.
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, wherein the method comprises 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 the composition of step (a), at a pH of at least 1.6 measured after a 13-fold dilution by volume using water, thereby obtaînîng 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, représenter! 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;
wherein the method further comprises the step of (c) adding a first flocculating agent to the composition obtained in step (b), wherein said first flocculating agent is a cationic polymer wherein 20% ta 80%, particularly
20% to 60% or 30% to 50%, more particularly 35% to 45% of the moieties making up the cationic polymer 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 polymer are anionic charged moieties, or a mixture thereof, thereby obtaining agglomérâtes comprisîng the heavy métal precipitate in a phosphoric acid containing composition; and (d) separating the agglomérâtes comprisîng the heavy métal precipitate from the phosphoric acid containing composition obtained in step (c).
According to an embodiment ofthe 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 diisobutyidithiophosphinate. 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 te 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 metals to a pH of at least pH 1.6 measured after a 13-foid dilution by volume using water, particularly by addition of ammonia; thereby obtaining a phosphoric acid containing composition comprisîng a sludge fraction;
(ii) optionally adding a second flocculating agent to the composition of step (i), wherein said second flocculating agent is a cationic 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 polymer are cationic charged moieties, an anionic polymer wherein 10%to 50%, particularly 10%to 40%, more particularly 15% to 30% ofthe moieties making up the anionic polymer are anionic charged moieties, or a mixture thereof;
(iii) separating the sludge fraction from the composition of step (î) 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 first and/or second flocculating agent is a copolymer of (meth)acrylamide, particularly a copolymer of (meth)acrylamide and a chloro-methylated monomer, such as dimethylaminoethyl (meth)acrylate, or a copolymer of (meth)acrylamide and (meth)acrylic acid.
According to an embodiment of the présent disclosure, the molecular weight (MW) of the polymeric flocculating agent ranges from 3 x 10e to 14 x 10s Daiton.
According to an embodiment of the présent disclosure, 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, such as in a dose of 5 to 20 g/m3 or 10 to 20 g/m3 acid composition.
According to an embodiment of the présent disclosure, the précipitation and/or flocculation steps are performed at a température of 5 to 50 °C. Advantageously, lower températures promote the stability of the heavy métal précipitâtes.
According to an embodiment of the présent disclosure, the phosphoric acid containing composition îs an acid digest of phosphate rock, preferably by nitric acid, sulfuric acid, or a combination thereof. In particular embodiments, 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.
According to an embodiment of the présent disclosure, the heavy metals are selected from cadmium, coppsr, nickel, mercury, zinc, arsenic, manganèse and/or lead; preferably the heavy metals are cadmium, copper and/or zinc; even more preferably the heavy métal is cadmium.
Another aspect ofthe présent disclosure provides for the use of a flocculating agent, for flocculating heavy métal précipitâtes in a phosphoric acid containing composition, wherein the heavy métal precipitate is a heavy métal precipitated from a phosphoric acid composition with a diorganodithlophosphinic acid or an alkaii métal or ammonia sait thereof, represented by Formula 1 above, ss the heavy métal precipitating agent, 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, wherein the flocculating agent is a cationic polymer wherein 20% to 80%, particularly 20% to 60% or 30% to 50%, more particularly 35% to 45% of the moieties makîng up the cationic polymer are cationic charged moieties, an anionic polymer wherein 10% to 50%, particularly 10% to 40%, more particularly 15% to 30% ofthe moieties making up the anionic polymer are anîonic charged moieties, or a mixture thereof. More in particular, the flocculating agent is a cationic or anionîc polymer with an ionic charge ranging from 10% to 60%, or a mixture thereof. More in particular, the flocculating agent is a copolymer of (meth)acrylamide, particularly a copolymer of (meth)acrylamide and a cnloro-methylated monomer, such as dimethylaminoethyl (meth)acrylate, a copolymer of (meth)acrylamide and (meth)acrylic acid, or a mixture thereof.
Description of the,Figures
FIG. 1 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, aading a precipitating agent, adding a (first) fiocculating agent and separating the precipitate from the fiItrate/supematant.
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, adding a (first) flocculating agent and separating the precipitate from the fiItrate/supematant.
FIG. 3 schematically represems a particular embodiment ofthe 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 second flocculating agent, removing a sludge fraction, adding a precipitating agent, adding s first flocculating agent and separating the precipitate from the filtrate/supernatant.
Detailed description
Before the présent System and method of the 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 présent 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 additions!, non-recited members, éléments or method steps. It will be apprecîated thaï 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 weil 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 stil! more preferably +/-0.1% or less of and from the specified value, insofar such variations are appropriate to perfcrm 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 per se, by means of further 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 dîsclosing the invention, including technical and scientific terms, hâve the meaning as commonly understood by one of ordinary skill in the artto which this invention belongs. By means of further guidance, term définitions are included to better appreciate the teaching of the present invention.
In the following passages, different aspects ofthe 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 s 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 characteristics may be ccmbined 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 ofthe
S 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 present 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 în which the invention may be practiced. It is to be understood that other embodiments may be utiiised, and structural or logical changes may be made without départir,g from the scope of the présent invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.
In the present disclosure, the concentration of the components comprised 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 present 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 present disclosure, unless explicitly stated otherwise, the terms “ionic polymer or “ionic polymeric” as they relate to the flocculating agents 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 agents considered herein is used synonymously for the terms “polyelectrolyte or “poiyelectrolytic”, i.e, polymers, in particular polycations or polyanions, whose repeating uniis bear an electrolyte group. In the present disclosure, ionic poly(meth)acrylamide copolymers, such as cationîc or anionic poly(meth)acrylamide copolymers, 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 a (first) flocculating agent is added to a phosphoric acid containing composition comprising heavy métal précipitâtes, or stated differently, is added to a phosphoric acid containing composition subsequently to the addition of a heavy métal precipitating agent. As used herein, the term heavy métal generaily 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 mere 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 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 present disclosure, a method is disclosed for the removal of heavy metals dissolved in a phosphoric acid containing composition, wherein the method comprises 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 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 métal precipitating agent comprises an organodithiophosphorous acid, in particular a diorgano-dithiophosphinic acid, or an alkali meta! or ammonia sait thereof;
(c) adding a first flocculating agent to the composition obtained in step (b), wherein said first flocculating agent is a catîonic 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 polymer 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 polymer are anionic charged moieties, or a mixture thereof, thereby obtaining agglomérâtes comprising rhe heavy métal precipitate in a phosphoric acid composition; and (d) separating the agglomérâtes comprising the heavy métal precipitate from the phosphoric acid containing composition.
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 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 S0 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 particuiar, 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 crystai lizati on step, calcium nitrate tetrahydrate is crystallized out of the digestion liquor yieiding a crystai siurry. In a third step or séparation step, the crystallized calcium nitrate îs separated by a technique such as filtration or centrifugation, resulting in calcium nitrate tetrahydrate crystals being separated from the liquid ofthe crystai siurry, 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. Sulfurîc 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 order to adjust the water soluble phosphorous, Oepending on the grade being produced.
In the context of the présent disclosure, the organothiophosphorous heavy métal precipitating agent comprises a diorgano-dithiophosphinic acid or an alkali métal or ammonia sait thereof, represented by Formula 1
S
Ml p j
M
Formula 1 wherein R is a linear or branched hydrocarbon group selected from alkyl, aryl, alkylaryl, or aralkyl, and wherein the hydrocarbcn group contains 3 to 20 carbon atoms, and M is H, alkali métal or ammonia. Preferred examples ofthe hydrocarbon groups R in the diorgano dithiophosphinic acid (or alkali métal or ammonia salts thereof) according to formula 1 include, but are not limitée to, linear or branched alkyi, cycloalkyl, alkylaryl, aralkyl having from 3-20 carbon atoms. More preferably, suitable hydrocarbon groups include, but are not limited to, cyctohexyl, isopropyl, isobutyl, n-propyl, octyl, hexyl, phenylethyl, and 2,4,4trimethyl pentyl. Even more preferably, the diorgano-dithiophosphinic acid (or sait thereof) used în the présent invention as heavy métal précipitation agent is di-isobutyl dithiophosphînaie. In a preferred embodiment, the precipitating agent is sodium di-isobutyl dithiophoshinate.
In particularly preferred embodiments, the heavy métal precipitating agent consists of a diorgano-dithiophosphinic acid or an alkali métal or ammonia sait thereof, represented by Formula 1, such as consiste of di-isobutyl dithiophosphînaie, î.e. no surfactants or other compounds are added simultaneously with the organodithiophosphorous heavy métal precipitating agent as envisaged herein.
Generalty, the heavy métal precipitating agent can be prepared according to the procedure described in US 4308214 and the corresponding examples by heating 67.2 parts of sulfur 114.8 to 284.8 pans of water to a température of about 70 °C. To the mixture are then steadily metered in 29.5 to 64.5 parts of the commercially available di-phosphine. After the di-phosphine has been metered, an additional 67.5 to 193.5 additional parts of diethyl phosphîne 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 thaï forms.
In certain embodiments, the heavy métal precipitating agent is added în an amount ranging from 10 pg to 1 mg per g ofthe 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.
According to an embodiment of the présent 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 réaction with the heavy métal precipitating agent as envisaged herein is particularly performed under vigorous mixing conditions, in particular at mixîng speeds of 500 to 700 rpm. According to an embodiment ofthe présent disclosure, the réaction 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 5aC to 40°C. As the heavy métal precipitate may be less stable ar températures above 40 C, it may be désirable 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 présent disclosure, heavy meta! précipitation is promoted by the addition of a first flocculating agent, particularly an ionic polymeric flocculating agent, to the composition comprising heavy métal précipitâtes. Said first flocculating agent is a cationic polymer, an anionic polymer or a mixture thereof, wherein 20 % to 80 %, particularly 20 % to 60% or 30% to 50%, more particularly 35% to 45% ofthe moieties making up the cationic polymer are cationic charged moieties. and wherein 10% to 50%, particularly 10% to 40%, more particularly 15% to 30% ofthe moieties making up the anionic polymer are anionic charged moieties. Stated differently, the first flocculating agent is added after the addition ofthe heavy métal precipitating agent and after heavy métal précipitâtes hâve been formed. 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 aggiomerate formation is best performed under gentle mixing conditions, in particular at mixing speeds of 100 rpm to 300 rpm. This way, sufficient shear forces are applied to build agglomérâtes by collision of the 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 at the same température and pH conditions as the précipitation step b).
For the anionic or cationic polymeric flocculating agents consldered herein, the ionic charge value may also be referred to as the degree of anionicity or the degree of catîonicity, respectively. In certain embodiments, the first flocculating agent 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 poly methacrylamide 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 méthacrylate subunits. 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.
A particularly preferred cationic flocculating agent is a copolymer of acrylamide or methacrylamide and a chloro-methyiated monomer, such as dimethylaminoethyl méthacrylate (MADAM) or dimethylaminoethyl acrylate (ADAM) monomers. A particularly preferred anionic flocculating agent is a copolymer of acrylamide or methacrylamide and acrylic acid or methacrylic acid.
In certain embodiments, the polymerîc flocculating agent has a MW ranging from 3 x 10e Dalton to 14 x 10e Dalton, particularly from 4 x 10s Dalton to 12 x 1O0 Dalton, more particularly from 4 x 105 Dalton to 8 x 106 Dalton. The polymerîc flocculating agent may be a linear molecuie or a branched molécule.
In certain embodiments, 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, such as in a dose of 5 to 20 g/m3 or 10 to 20 g/m3 acid composition.
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 first flocculating agent, as represented in FIG. 1, FIG. 2 and FIG. 3. Such pretreatments include pH adjustment (FIG. 1 - FIG. 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 second flocculating agent to the phosphoric acid containing composition after the pH adjustment step, in order to facilitate the séparation of the insoluble fraction from the phosphoric acid containing composition (FIG.3).
In particular, according to an embodiment ofthe present disclosure, step (a) further comprises the steps of (i) adjusting the pH of a phosphoric acid containing composition comprising oissolved heavy metals to a pH of at ieast pH 1.6 measured after a 13-fold dilution by volume using water, thereby obtaîning a phosphoric acid containing composition comprising a sludge fraction;
(ii) optionaliy adding a second flocculating agent to the composition of step (i), wherein said second flocculating agent is a cationic 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 polymer are cationic charged moieties, an anionic polymer wherein
10% to 50%, particularly 10% to 40%, more particularly 15% to 30% ofthe moieties making up the anionic polymer are anionic charged moieties, or a mixture thereof;
(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 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 measured after a 13-fold dilution by volume using water, 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 acîd according to Formula 1 as envisaged herein, or an alkali mata! or ammonia sait thereof, is especially effective. In addition, ai these pH conditions, précipitation of phospherous salis, 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. In certain embodiments, the pH ofthe 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 orsludge present in the phosphoric acid containing composition, particularly the phosphoric acid containing composition after pH adjustment may be removed prior to the addition of the heavy métal precipitating agent. More in particular, a cationic 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 polymer 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 polymer are anionic chargea moieties, or a mixture thereof as a second flocculating agent may be added to the phosphoric acid containing composition, particularly the phosphoric acid containing composition after pH adjustment, to promote the agglomération and précipitation ofthe insoluble components or sludge fraction (FIG. 3). Surprîsingly, 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 cf 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 envîsaged 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.
Tne second flocculating agent may be the same or different than the first flocculating agent.
In particular embodiments, the second flocculating agent is a copolymer of acrylamîde or methacrylamide.
According to particular embodiments ofthe présent disclosure, cationic polymers, more in particular cationic acrylamîde or methacryl amide based copoîymers, with a positive charge ranging from 30% to 50%, are especially 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 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 4,8 to 7,6 *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 *10® Dalton or 40 % linear cationic powder polyacrylamide (that is a polyacrylamide having 40% of its moieties positively charged) with molecular weight ranging from 9,5 to 12,1 *10® Dalton.
According to particular embodiments of the présent disclosure, anionic polymers, more in particular anionic acrylamîde or methacryl amide based copoîymers, with a anionic charge ranging from 15% to 30%, are especially suitable for the purpose of agglomerating and separating both the sludge and insoluble components priorto the addition ofthe 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)acrylamîde and (meth)acrylic acid may be used. Examples of suitable anionic polymers include, but are not limited to, 20 % linear anionic émulsion polyacrylamide (that is a polyacrylamide having 20% of its moieties negatively charged) with molecular weight ranging from 9,7 to 17,5 *10® Dalton or 20 mole % linear anionic powder pclyacrylamide (that is a polyacrylamide having 20% of its moieties negatively charged) with moiecular weight ranging from 12,2 to 17,5 *10® Dalton.
The séparation ofthe sludge may be accomplîshed by any standard technology for séparation such as, but not limited to. filtration, centrifugation, sédimentation, flotation or décantation. In certain embodiments, the séparation ofthe precipitated insoluble orsludge fraction due ta the addition ofthe second 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 tne 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 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 measured after a 13-fold dilution by volume using water, in particular a pH of 1.6 to 2.0 measured after a 13-fold dilution by volume using water, or a pH 1.6 to 2.2 measured after a 13-fold dilution by volume using water, thereby obtaining a phosphoric acid containing composition comprising a sluoge fraction;
(a)(ii) adding a cationic polymer wherein 20% to 80%, particularly 20% to 60% or 30%· to 50%, more particularly 35% to 45% ofthe moieties making up the cationic polymer are cationic charged moieties, an anionic polymer wherein 10% to 50%, particularly 10% to 40%, more particularly 15% to 30% ofthe moieties making up the anionic polymer are anionic charged moieties, or a mixture thereof as a second flocculating agent, particularly an cationic polymeric flocculating agent wherein 35% to 45% ofthe moieties making upthe cationic polymer are cationic charged moieties, 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 to the composition of step (a), at a pH of at least 1.6 measured after a 13-fold dilution py volume using water. in particular a pH cf 1.6 to 2.0 measured after a 13 fold dilution by volume using water, or a pH 1.6 to 2.2 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;
(c) adding a cstionic 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 polymer are cationic charged moieties, an anionic polymer wherein 10% to 50%, particularly 10% to 40%, mors particularly from 15% to 30% of the moieties making up the anionic polymer are anionic charged moieties, or a mixture thereof as a first floccuiating agent, particularly an cationic polymer 35% to 45% of the moieties making up the cationic polymer are cationic charged moieties, to the composition obtaîned in step (b), thereby obtaining agglomérâtes comprising the heavy métal précipitais in a phosphoric acid containing composition; and (d) separating the agglomérâtes comprising the heavy métal precipitate from the phosphoric acid containing composition of step (c), particularly by centrifugation.
According to an embodiment of the present disclosure, the précipitation step (b) and/or flocculation steps (c) and (a)(ii) are performed ai a température of 5°C to 50 °C, particularly are performed at a température of 5°C to 40°C. Particular good results were obtaîned at a température ranging from 10°C to 35°C or 10 to 30°C. These températures in combination with the abovs indicated pH conditions of 1.6 to 2.0 measured after a 13fold dilution by volume using water, 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 phosphoric acid containing composition prior to and after the précipitation step (b). 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 l0°C to 30°C, by ratura! cooling or by heat exchangers.
In the context of the present disclosure, the séparation in step d) or step a)(Hi) may be accomplished by state of the ad 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.
Another aspect ofthe 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 according to any embodiment of the methods envisaged herein; in particular comprising the steps of optionally removing a sludge fraction by flocculation with a second 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 at least 1.6 measured after a 13-fold dilution by volume using water, în particular a pH of 1.6 to 2.0 measured after a 13-fold dilution by volume using water, or a pH 1.6 to 2.2 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 cr an alkali meta! or ammonia sait thereof; subsequently adding a first flocculating agent to the composition comprising heavy métal précipitâtes, particularly under gentle mixing conditions, such as at mixing speeds of 100 to 300 rpm, thereby obtaining agglomérâtes comprising the heavy métal precipitate in a phosphoric acid containing composition; and separating the agglomérâtes comprising 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 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 non-coated, colored on non-coiored 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 of the digestion liquor. In the latter case, the part of the Digestion liquor which is not treated according to a method of the present disclosure is mixed with or diluted with the part ofthe digestion liquor which has been treated according to a method of the present disclosure, such that the heavy métal (cadmium) levels of the combined composition is below a desired value, particularly remains within the regulatory limits.
Another aspect of the présent disclosure provides for the use of a flocculating agent as envisaged in the présent disclosure, for flocculating heavy métal précipitâtes in a phosphoric acid containing composition, wherein the heavy métal precipitate is a heavy métal precipitated from a phosphoric acid composition with a diorgano-dîthiophosphinic acid or an alkali métal or ammonia sait thereof, represented by Formula 1 above, as the heavy métal precipitating agent.
Examples
Exemple 1 ~ Effect of a polymeric flocculating agent
The pH of cadmium-containing mother liquor (ML), freshly produced from the Odda nitrophospnare process, was adjusted to pH=1.8, using gaseous ammonia (NH3). The pH was measured after a 13-fold dilution by volume using water.
The used mother liquor comprised 5.14 wt% Ca (as measured by Atomîc Absorption Spectre s copy), 7.4 wt% P (as measured by gravimetry P), 34.2 wt% H2O (as measured by Karl Fisher titration) and 6 ppm Cd, and 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 diisiobuty! diihiophosphinate (DTPINa, 3.56 %, 0.336 g) was added to the mother liquor at pH=1.8 (60 g) under vigorous stirring (600 rpm). The mixture was stirred for 3 min, after which a polymeric flocculating agent (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 *108 Dalton, 0.1 wt% concentration, 0.078 g, 600 rpm) was added during intense stirring. Immediately after addition of the flocculating agent, stirring was reduced to 150 rpm in order to allow agglomérats formation. After 3 min of fiocculatîon/agglomeraie formation, the mixture was centrifuged (2440 rpm, 2 min, 1000 G) ana the supernatant and sludge/precipitate were separated.
In addition, a sampie of the supernatant was taken using a syringe with 5 pm filter in order to investigate the effect of séparation efficiency using a centrifuge. The cadmium and heavy métal content was analysed by ICP-OES (Thermo Scientific, iCAP 7400 Duo, wavelength:
226.502 nm Cd, 204.379 nm for Cu and 206.200 nm for Zn ail in axial mode). The Cdextraction efficiency of Cd-precipitation by DTPINa followed by flocculation was calculated to be 84 %. The Cu-extraction efficiency was also calculated to be 84 %.
Example 2 - Comparative example no polymeric flocculating agents
In a comparative example, no flocculating agent was added after the addition of DTPINa. More in particular, the pH of a cadmium-containing mother liquor freshly produced from the Odda nitro-phosphate process (same composition as above) was adjusted to 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 ai 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 supematant and precipitate were separated. The cadmium and heavy meta! content were analyzed by ICP-OES (Thermo Scientific, iCAP 7400 Duo, wavelength: 226.502 nm for Cd 204.379 nm; and 206.200 nm for Cu). The addition of DTPINa without a flocculant resulted in a Cd-extraction efficiency of about 73 %, and a Cu-extraction efficiency of 78 %.
The addition of a flocculating agent after the neavy métal precipitating agent thus increased the heavy métal (Cd and Cu) extraction from the mother liquor at pH 1.8.
Example 3 - Extraction of heavy metals from another composition
The pH of cadmium-containing mother liquor (ML), freshly produced from the Odda nitrophosphate process, was aciiusted te pH=1.8, using gaseous ammonia (NH3). The pH was measured after a 13-fold dilution by volume using water.
The used mother liquor 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 Kari 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 merely an indication of such composition and are not limiting for the process consîdered herein.
Trie resulting composition was centrifuged for (2440 rpm, 2 min, 1000 G) to reduce the amount of sludge.
Sodium diisiobutyl dithiophosphinate (DTPINa, 3.56 %, 0.34 g) was added to the mother liquor at pH=1.8 (60 g) under vigorous stirring (600 rpm). The mixture was stirred for 3 min, after which a polymeric flocculating agent (40 % cationic polyacrylamide (that is a polyacrylamide having 40% of its moieties positively charged) with molecular weight of from 8-0.5 10s Dalton, 0.1 wt% concentration, 0.078 g, 600 rpm) was added during intense stirring. Immediately after addition of the flocculating agent, stirring was reduced to 150 rpm in order to allow agglomérats formation. After 3 min of flocculation/aggiomerate formation, the mixture was centrifuged (2440 rpm, 2 min, 1000 G) and the supernatant and sludge/precipitats were separated.
In addition, a sampie of the supernatant was taken using a syringe with 5 pm filter in order to investigatethe effect of séparation efficiency using a centrifuge. The cadmium and heavy métal content was analysée! by ICP-OES (Thermo Scientific, iCAP 7400 Duo, wavelength: 226 502 nm for Cd; 204.379 nm for Cu and 206,200 nm for Zn ail in axial mode). The Cdextracticn efficiency of Cd-precipitation by DTPINa followed by flocculation was calculated to be 63 %. The Cu-extraction efficiency was calculated to be 77 % and the Zn-extraction efficiency was calculated to be 22 %.
Example 4 - Effect of no sludge removal from neutralised Irquor “ne pH of cadmium-containing mother liquor (ML), freshly produced from the Odda nitrophosphate proeess, was adjusted to pH=1.8, using gaseous ammonia (NHs). The pH was measured after a 13-fold dilution by volume using water.
The used mother liquor comprised 3.9 wt% Ca (as measured by ICP-OES), 8.5 wt% P (as measured by ICP-OES), 32.7 wt% H2O (as measured by Karl Fisher titration) and 1.4 ppm Cd, 12.4 ppm Cu, 34.1 ppm Zn,( 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 limiiîng for the proeess considered herein.
Trie resulting composition was then used as is.
Sodium diisiobuiyl diîhiophosphinate (DTPINa, 3.56 %, 8 g) was added to the mother liquor at pH=1.8 (500 g) under vigorous stirring (600 rpm). The mixture was stirred for 3 min, after which a polymeric flocculating agent (40 % cationic polyacrylamide (that is a poiyacrylamide having 40% of its moieties positively charged) with molecular weight of from 8-9.5 10e Dalton, 0.1 wî% concentration, 1.5 g, 600 rpm) was added during intense stirring. Immediately after addition of the flocculating agent, stirring was reduced to 150 rpm in order ta allow agglomerate formation. After 3 min of flocculation/aggiomerate formation, the mixture was centrifuged (2440 rpm, 2 min, 1000 G) and the supernatant and sludge/precipitate were separated.
la addition, a sample of the supernatant was taken using a syringe with 5 pm filter in order to investigate the effect of séparation efficiency using a centrifuge. The cadmium and heavy métal content was analysed by ICP-OES (Thermo Scientific, iCAP 7400 Duo, wavelength: 226.502 nm for Cd, 204.379 nm for Cu and 206.200 nm for Zn ail in axial mode). The Cd extraction efficiency of Cd-precipitation by DTPINa followed by flocculation was calculated to be 92 %, the Cu-extraction efficiency was calculated to be 92 % and the Zn-extraction efficiency was calculated to be 82.5 %.
Example 5 -Effect of sludge removal
The pH of 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 after a 13-fold dilution by volume using water.
The used mother liquor comprised 3.9 wt% Ca (as measured by ICP-OES), 8.5 wt% P (as measured by ICP-OES), 32.7 wt% H2O (as measured by Karl Fisher titration) and 1.4 ppm Cd, 12.4 ppm Cu, and 34.1 ppm Zn ( 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 diisiobutyI dithiophosphinate (DTPINa, 3.56 %, 5.5 g) was added to the mother liquor at pH=1.8 (343.6 g) under vigorous stirring (600 rpm). The mixture was stirred for 3 min, after wmlch a polymeric flocculating agent (40 % cationic polyacrylamide (that is a polyacrylamide having 40% of its moieties positîvely charged) with molecular weight of from 8-9.5 10s Dalton, 0.1 wt% concentration, 1.06 g, 600 rpm) was added during intense stirring. Immediately after addition of the flocculating agent, stirring was reduced to 150 rpm in order to allow agglomérats formation, After 3 min of flocculation/agglomerate formation, the mixture was centrifuged (2440 rpm, 2 min, 1000 G) and the supernatant and sludge/precipiiate were separated, in addition, a sample of the supernatant was taken using a syringe with 5 pm filter in order to investigate the effect of séparation efficiency using a centrifuge. The cadmium and heavy métal content was analysed by ICP-OES (Thermo Scientific, iCAP 7400 Duo, wavelength: 226,502 nm Cd, 204.379 nm for Cu and 206.200 nm for Zn ail in axial mode). The Cdextraciion efficiency of Cd-precipitation by DTPINa followed by flocculation was calculated to be 78 %, me Cu-extraction efficiency was calculated to be 66 % and the Zn-extraction efficiency was calculated to be 78 %.
Exampîe δ - Effect of the pH
To study the effect of pH on the heavy métal (in particular Cd, Cu and Zn) extraction efficiency, tne pH of s mother liquor was adjusted to different pH values (pH 1.1-2.4) measured after a 13-folo dilution by volume using water. Below pH 1.6 (measured after a
13-fold dilution by volume using water), only about 7-9 % ofthe Cd, 8-14 % of Cu and 7 % of Zn 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%. Cu-extraction increased to 88 % and Zn-extraction increased to 22 % . At pH 2.2 (measured after a 13-fold dilution by volume using water), the Cd removal is still very high (ca. 90%), and Cu-extraction is 81 % and Zn-extraction is 41 %, However, the séparation step becomes the limiting factor, as the increasing viscosity renders the séparation by centrifugation less efficient.
Example 7 - Different polymeric flocculating agents
Different polymers were screened to find the most efficient flocculation agent to separate the precipitated heavy metals, in particular Cd, from a mother iiquor composition after pH adjustment. The tested polymers differed in their charge (cationic, non-ionic, anionic (low and high %), molecular weight (middle and high MW), type (émulsion, powder) and structure (linear, branched).
The tested flocculants include:
- cationic polyacrylamîde (PAM) copolymers with charge of :
o 20 % linear cationic émulsion polyacrylamîde (that is a polyacrylamîde having 20% of its moieties positive!y charged) with molecular weight ranging from 5,9 to 9,9 *10e Dalton:
« 30 % linear cationic émulsion polyacrylamîde (that is a polyacrylamîde having 30% of its moieties positiveiy charged) with molecular weight ranging from 5,9 to 9,9 *10® Dalton;
o 30 % structured catîonio émulsion polyacrylamîde (that is a polyacrylamîde having 30% of its moieties positiveiy charged) with low molecular weight;
« 30 % structured cationic émulsion polyacrylamîde (that is a polyacrylamîde having
30% of its moieties positiveiy charged) with fairly high molecular weight;
e 35 % linear cationic powder polyacrylamîde (that is a polyacrylamîde having 35% of its moieties positiveiy charged) with molecular weight ranging from 7,5 to 9,2 *106 Dalton;
« 40 % linear cationic émulsion polyacrylamîde (that is a polyacrylamîde having 40% of its moieties positiveiy charged) with molecular weight ranging from 5,9 to 9,9 *106 Dalton:
* 40 % linear cationic powder polyacrylamîde (that is a polyacrylamîde having 40% of its moieties positiveiy charged) with molecular weight ranging from 4,8 to 7,6 *10s
Dalton:
« 40 % linear cationic powder polyacrylamide (that is a polyacrylamide having 40% of its moieties positively charged) with molecular weight ranging from 9,5 to 12,1 *10s 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 *10® Dalton;
® 45 % linear cationic powder polyacrylamide (that is a polyacrylamide having 45% of its moieties positively charged) with molecular weight ranging from 4,2 to 7,1 *10® Dalton;
« 50 % iinear cationic émulsion polyacrylamide (that is a polyacrylamide having
50% of its moieties positively charged) with molecular weight ranging from 6,4 to 9:5 *10® Dalton;
® 60 % linear cationic émulsion polyacrylamide (that is a polyacrylamide having 50% of its moieties positively charged) with molecular weight ranging from 6,4 to 9,6 *10® Dalton.
-anionic polyacrylamide copolymers with charge of:
® 5 % iinear anionic émulsion polyacrylamide (that is a polyacrylamide having 5% of its moieties negatively charged) with molecular weight ranging from 6,8 to 13,9 *10® Dalton;
» 10 % linear anionic émulsion polyacrylamide (that is a polyacrylamide having 10% of its moieties negatively charged) with molecular weight ranging from 8,5 to 16,1 *10e Dalton;
® 20 % linear anionic émulsion polyacrylamide polyacrylamide (that is a pci y acrylamide having 20% of its moieties negatively charged) with molecular weight ranging from 9,7 to 17,5 ”10® Dalton;
« 30 % linear anionic émulsion polyacrylamide (that is a polyacrylamide having 30% of its moieties negatively charged) with molecular weight ranging from 17,5 to 23,0 *'i0s Dalton;
« 50 % linear anionic émulsion polyacrylamide polyacrylamide (that is a polyacrylamide having 50% of its moieties negatively charged) with very high molecular weight.
The experimental procedure consisted ofthe steps of:
1. Adjustment of the pH of the mother liquor, particularly prepared according to the Odda or nitro phosphate process, to a pH of 1.8 (measured after a 13-fold dilution by volume using water) by ammonia addition and cooling ofthe resulting mother liquorto a température of 24 °C;
2. Centrifugation to remove the sludge fraction présent in the mother liquor at pH 1.8 (2440 rpm; for 2 min);
3. Addition ofthe heavy métal precipitating agent (1200 ppm of 5 % sodium di-isobutyl dithiophosphinate solution (1.2 g)) to the mother liquor at pH 1.8 (50 g) under vigorous stirring using a magnetic stirrerfor 3 min;
4. Addition of the flocculating agent (10-30 ppm) and agglomerate formation under gentle stirring (with plastic spoon);
5. Séparation ofthe precipitated and agglomerated material by centrifugation;
6. Analysis of the heavy métal content (Cd) of the supernatant by ICP-ΟΕΞ ((Thermo Scientific, iCAP 7400 Duo, wavelength: 226.502 nm in an axial mode).
7.1 Cationic polymers
The Cd extraction efficiency of polyacrylamide based cationic polymeric flocculants with a cationic charge between 20 % and 60 % (that is a polyacrylamide having 20-60 % of its moieties positively charged, in émulsion (EM)) and between 35 % and 45 % (that is a polyacrylamide having 35-45 % of its moieties positively charged, in powder form (FO)) were investigated Both the EM and FO type flocculating agents were prepared as a 0.5 wt % solution and were further diluted to a 0.1 wt % solution before addition. After settlîng, the settling behavior was assessed visually and via turbidity measurements ofthe clarity ofthe liquid phase after settling of the agglomérâtes.
In a first sériés of experiments, the best results were obtained with 40 % cationic charge polymer flocculating agent at a dosage of 10-30 ppm by weight. The émulsion type 40 % cationic charge polymer visually performed better than the powder type 40 % charge polymer, showing stronger and bigger agglomérâtes, with quite good settling rate (about 30 min).
A variant procedure was also considered wherein a second flocculating agent was added to the mother liquor atpH 1.8 (obtained in step 1), before centrifugation ofthe sludge fraction (step 2). This way the precipitating agent could be added to a mother liquor with r-educed amount of sludge/particles.
7.2 Anionic polymeric flocculating agents
Similar as in example 2.1 above, the Cd extraction efficiency of polyacrylamide based anionic polymeric flocculants with an anionic charge between 5 % and 50 % (that is a polyacrylamide having 5-50 % of its moieties negatively charged, in émulsion (EM)) were invesrigaied, with a non-ionic polymer (that is an uncharged poiyacrylamide)and 40 % cationic polymer (that is a polyacrylamide having 40 % of its moieties positively charged) as a reference, at a dosage of 10-60 ppm by weight.
The 20 % anionic charge polymeric flocculating agent (that is a polyacrylamide having 20 % of its groups negatively charged) at a dosage of 10-30 ppm by weight performed best of ail anionic polymers tested, albeit not as good as the 40 % cationic charge polymer, the latter creating stronger and bigger agglomérâtes.
The non-ionic polymer did not create any agglomérâtes in the mother liquor at pH 1.8, even at higher dosages.
7.3 Cationic oolymerlc flocculating agents with different structure
Similar as in example 7.1 above, the Cd extraction efficiency of structurally different ^0% cationic charge polymer was investigated (differing in molecular weight and their branched nature) and compared to the linear polymeric 40 % cationic charge polymer of example 7.1.
The differently structured 40 % cationic charge polymers ail gave similar results as the linear type of 40 % cationic charge polymer of example 7.1. When it cornes to polymers with higher molecular weight, the dosage which was used was smaller in comparison to tne linear cationic type of example 7. Ί and they created bigger fioccs and settling rate was faster. However. when added to the mother liquor, a more viscous composition was obtained.
7.4 Combination of cationic and a.^ionic polymeric flocculants
The Cd extraction efficiency of different combinations (concentrations) ofa 10 % anionic émulsion polyacrylamide (that is a polyacrylamide having 10 % of its moieties negatively charged) with moiecular weight ranging from 8,5 to 16,1 NO6 Dalton and a 40 % linear cationic powder poiyacrylamide (that is a polyacrylamide having 40 % of its moieties positively charged) with molecular weight ranging from 7,1 to 9,5 *106 Dalton was investigated, in particular 10 % anionic émulsion polyacrylamide (that is a polyacrylamide having 10 % of its moieties negatively charged) with molecular weight ranging from 8,5 to 16,1 NO3 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
21049 b
9,5 ’10s Dalton ): 30 ppm + 30 ppm and 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 *10s Dalton + 10 % anionic émulsion polyacrylamide (that is a polyacrylamide having 10 % of its moieties negatively charged) with molecular weight 5 ranging trom 8,5 to 16,1 *10s Dalton): 20 ppm + 10 ppm, and compared to the 40% cationic polymer used in example 7.1.
Visual observation indicates that using two type of charge of polymers can give stronge:' floccs, but at the same time the observed settling rate was slower in comparison to the polymer with 40% cationic charge.
Based on the above examples, it can be concluded that the addition of an ionic polymeric flocculating agent as an extra stage to a heavy métal (Cd) removal procedure is a promising method of liquids/solids séparation in a phosphoric acid containing composition at pH 1.8.

Claims (14)

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 to ihe 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 métal precipitating agent comprises a diorganodithiophosphinic acid or an alkali métal or ammonia sait thereof, represented by Formula 1
S
R, || p--SM
Formula 1 wherein R is a linear or branched hydrocarbon group selected from alkyl, aryl, alkylaryl, or aralkyl, and wherein the hydrocarbcn group contains 3 to 20 carbon atoms, and M is H, alkali métal or ammonia;
characterized in that the process further comprises the step of (c) adding a first flocculating agent to the composition obtained in step (b), wherein said first flocculating agent is a cationic 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 poiymer are cationic charged moieties, an anionic polymer wherein 10 % to 50 %; particularly 10 % to 40 %; more particularly 15 % to 30 %, ofthe moieties making up the anionic polymer are anionic chargée moieties, or a mixture thereof, thereby obtaining agglomérâtes comprising the neavy métal precipitate in a phosphoric acid containing composition; and (d) separating the agglomérâtes comprising the heavy métal precipitate from the phosphoric acid containing composition.
2. The process according to claim 1, wherein step (a) further comprises the steps of (i) adjusting ihe pH cfa 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, particularly by addition of ammonia; thereby obtaining a phosphoric acid containing composition comprising a sludge fraction;
(ii) optionally adding a second floccutating agent to the composition of step (i), wherein said second floccuiating agent is a cationic polymer wherein 20 % to 80 %; particularly 20 % te 60 % or 30 % to 50 %; more particularly 35 % to 45 %, ofthe moieties making up the cationic polymer 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 polymer are anionic charged moieties, or a mixture thereof;
(iii) separating the sludge fraction from the composition of step (i) or (ii).
3. The process according to any one of daims 1 to 2, wherein the first and, optionally, the second floccuiating agent is a copolymer of (meth)acryl amide, particularly a copolymer of (meth)acrylamide and a chloro-methylated monomer, such as dimethylaminoethyl (meth)acrylaie, or a copolymer of (meth)acrylamide and (meth)acrylîc acid.
4. The process according to any cne of daims 1 to 3, wherein the MW of the polymeric floccuiating agent ranges from 3 x 106 to 14 x 106 Dalton.
5 The process according to any one of daims 1 to 4, wherein the floccuiating 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 composmon.
6. The process according to any one of daims 1 to 5, wherein the précipitation and/or flocculation steps are performec at a température of 5 to 50 aC.
7. The process according to any one ofthe preceding daims wherein the phosphoric acid containing composition is an acid digest of phosphate rock, preferably by nitrîc acid, sulfuric acid, or a combination thereof.
8. The process according to any one of the preceding daims, wherein the phosphoric add containing composition is an acidic aqueous composition comprising from 6-21wt% nitric acici, 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.
9. The process according to any one ofthe preceding daims, wherein R in Formula 1 is selected from the group consisting of cydohexyl, isopropyl, isobutyl, n-propyl, octyl, hexyl, phenyiethyl and 2,4,4-trimethylpentyl, particularly wherein the heavy métalprécipitation agent is sodium diisobutyldithiophosphinate.
10. The process according to any one of the preceding claims, wherein the heavy metals 5 are selected from cadmium, copper, nickel, mercury, zinc, arsenic, manganèse and/or lead;
preferably wherein the heavy metals are cadmium, copper and/or zinc; even more preferably wherein the heavy métal is cadmium.
11. Use of a floccuiating agent, for floccuiating heavy métal précipitâtes in a phosphoric 10 acid containing composition, wherein the heavy métal precipitate is a heavy métal preclpîtated from a phosphoric acid 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
S. H J^P—SM ÏU
15 Formula 1 wherein R is a linear or branched hydrocarbon group selected from alkyl, aryl, alkylaryl, or aralkyi, and wherein the hydrocarbon group contains 3 to 20 carbon atoms, and M is H, alkali métal or ammonia, characterized in that the floccuiating agent is a cationic polymerle floccuiating agent, an 20 anionic polymeric floccuiating agent, or a mixture thereof.
12. Use according to claim 11, wherein the floccuiating agent is a cationic polymer wherein 20 % to 80 %; particularly 20 % to 30 % or 30 % to 50 %; more particularly 35 % to 45 %. of the moieties making up the cationic polymer are cationic charged moieties, an 25 anionic polymer wherein 10 % to 50 %; particularly 10 % to 40 %; more particularly 15 % to 30 %. of the moieties making up the anionic polymer are anionic charged moieties, or a mixture thereof.
13. Use according to claim 11 or '12, wherein the floccuiating agent is a copolymer of 30 (meîh)acrylamide, particularly a copolymer of (meih)acrylamide and a chloro-methylated monomer, such as dimethylaminoethyl (meth)acrylate, a copolymer of (meth)acrylamide and (meth)acrylic acid, or a mixture thereof.
14. Use according to any one of claims 11 to 13, wherein the heavy metals are selected from cadmium, copper, nickel, mercury, zinc, arsenic, manganèse and/or lead; preferably wherein the heavy metals are cadmium, copper and/or zinc; even more preferably wherein 5 the heavy métal is cadmium.
OA1202200434 2020-06-16 2021-06-16 Process for the removal of heavy metals from a phosphoric acid containing composition using a flocculating agent. OA21049A (en)

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