WO2021106710A1

WO2021106710A1 - Anionic copolymers suitable as scaling inhibitors for sulfide-containing scale

Info

Publication number: WO2021106710A1
Application number: PCT/JP2020/042984
Authority: WO
Inventors: Jurgen MATHEIS; Florian Wolf; Wolfgang Hater
Original assignee: Kurita Water Industries Ltd.
Priority date: 2019-11-28
Filing date: 2020-11-18
Publication date: 2021-06-03
Also published as: WO2021106148A1; JP2023502826A; EP4065523A1

Abstract

The present invention relates to anionic copolymers, which are suitable as sulfide scaling inhibitors in water bearing systems, in particular as inhibitors for the formation of antimony(III) sulfide-containing scale. The present invention also relates to a method for reducing sulfide scaling in water bearing systems, in particular in water loops of geothermal power plants, in which the formation of sulfide-containing scale and especially the formation of antimony(III) sulfide-containing scale may occur. The anionic copolymers have the following characteristics: 1) The copolymers comprise a. polymerized units of a monomer M1, which is a primary amide of a monoethylenically unsaturated monocarboxylic acid having 3 to 6 C atoms and b. polymerized units of a monoethylenically unsaturated monomer M2, which is selected from monoethylenically unsaturated monomer having a sulfonate group and or a sulfate group and monoethylenically unsaturated monomer having a carboxyl group and mixtures thereof; 2) The copolymers have an anionic charge density in the range from 0.5 to 6.0 mol/kg, 3) The sodium salts of the copolymers have a weight average molecular weight Mw of at least 10000 g/mol, as determined by gel permeation chromatography.

Description

Anionic copolymers suitable as scaling inhibitors for sulfide-containing scale

FIELD OF THE INVENTION

The present invention relates to anionic copolymers, which are suitable as scaling inhibitors for sulfide-containing scale in water bearing systems, in particular as inhibitors for the formation of antimony(III) sulfide-containing scale. The present invention also relates to a method for reducing sulfide-containing scaling in water bearing systems, in particular in water loops of geothermal power plants, in which the formation of sulfide-containing scale and especially the formation of antimony(III) sulfide-containing scale may occur.

BACKGROUND OF THE INVENTION

The growing need for clean and renewable energy leads to a constant increase of investment in geothermal power generation. The efficient use of geothermal brines is generally limited by their dissolved salts, which are particularly troublesome to form deposits, which are also termed “scales”, in the surface zone of the geothermal fluid loop. The scaling or fouling induces a performance drop of components of a power plant, like pipes, pumps and heat exchangers, and inevitably results in a decrease of efficiency and can even cause failure of the whole power plant. Additional maintenance is required for removing scale. Using proper additives would inhibit their formation and promote smooth operations.

In water bearing systems, such as brine water loops of geothermal power plants, common scales based on silicates, calcium carbonate, calcium sulfate or barium sulfate are to be mitigated. Apart from that, less common scales like sulfides, in particular sulfides of antimony and arsenic, may cause fouling problems, in particular in the brine water loops of geothermal power plants. Despite of the low antimony concentrations in the water of most water bearing systems, the presence of antimony and sulfide in water may be very problematic with regard to fouling, as antimony (III) sulfide will quantitatively precipitate, because its solubility in water at pH 9 or lower is extremely low. In water bearing systems, antimony (III) sulfide will normally crystallize in the form of thin needles, which pack loosely on the internal surfaces of the water-bearing system, thereby forming a porous layer that can trap brine. As a consequence, heat transfer rates will be reduced and/or pipe clogging may occur - see e.g. K. Brown, Proceedings International Workshop on Mineral Scaling 2011 Manila, Philippines, 25-27 May 2011, pp. 103 - 106. The removal of stibnite-containing scale is difficult and can hardly be achieved by chemical means. Therefore, stibnite-containing scale will normally be removed mechanically, which is time consuming and requires maintenance for removing the stibnite scale.

Numerous anti-scaling additives have been proposed as useful additives to aqueous systems including certain polyphosphates, polyacrylic acids, polymethacrylic acids, lignin sulfonic acids and their salts, tannin, naphthalene sulfonic acid formaldehyde condensation products, polyphosphates, such as tripolyphosphate and hexameta-phosphate, phosphonic acids, polymaleic acids and hydrolysed copolymers and terpolymers of maleic anhydride and the salts of these acids. The use of these anti-scaling additives, particularly the polyacrylic acid and maleic acid polymers, is widely recognised as being effective in inhibiting the build-up of common scale, in particular build-up of silicate, calcium carbonate, barium sulfate, calcium phosphate and calcium sulfate scale.

While the formation of common scales can be mitigated with the abovementioned anti-scaling additives, the formation of sulfide-containing scale, in particular antimony(III) sulfide-containing scale, hereinafter also referred to as stibnite-containing scale, is still a serious problem in water bearing units, where water contains antimony and sulfide. As the solubility of antimony(III) sulfide decreases with decreasing pH and also with decreasing temperature, a particular risk for the formation of stibnite-containing scale exists at a pH of at most pH 9, in particular at most pH 8.5 and/or at temperatures of at most or below 120°C. Conditions favouring the formation of sulfide-containing scale, in particular stibnite-containing scale, are usually met in the brine loops of geothermal power plants. The problem of the formation of sulfide-containing scale, in particular stibnite-containing scale, may however also occur in other water bearing systems, wherein the water contains heavy metal salts, in particular antimony salts, and sulfide and which are operated under conditions that favour the formation of water insoluble sulfides, such as stibnite.

US 4,224,151 describes a method for preventing scale deposition from a hydrogen sulfide-containing geothermal fluid by injection of an oxygen containing gas into the geothermal fluid in such a way that the hydrogen sulfide contained in the geothermal fluid is partially oxidized to water-soluble oxidation products, wherein the sulfur has an oxidation number of less than +6.

J. S. Gill et al. in GRC Transactions, Vol 37, 2013 suggest that certain anionic copolymers having a high charge density and a low molecular weight will reduce the particle size of precipitated stibnite and impart anionic charge to the stibnite particles and thus may reduce the formation stibnite-containing scale by dispersing the stibnite particles. Similar results were disclosed by L. Muller et al. in Proceedings World Geothermal Congress 2015, Melbourne, Australia 19-25 April 2015, pages 1 to 11.

US 5,032,298 describes low molecular weight copolymers of acrylic acid and acrylamide having a molecular weight distribution such that at least 60% by weight of the copolymers have a molecular weight of below 500 Dalton. These copolymers are suggested to be suitable for preventing scaling on the internal walls of oil or geothermal equipment. The copolymers described therein are not particularly suitable for inhibiting the formation of sulfide scale.

US 5,403,493 discloses that copolymers having carboxylate groups and/or sulfonate groups may be suitable as scale inhibitors, which are mentioned to be suitable in geothermal dwells. The copolymers are used in combination with a carbohydrazide in order to achieve a better corrosion protection. The copolymers described therein are not suitable for inhibiting the formation of sulfide scale.

US 5,256,303 discloses a method for inhibiting calcium sulfate scale formation and deposition and/or dispersing iron from a feedstream passing through a reverse osmosis system. Even though the copolymers described therein comprise acrylamide and acrylic acid units, their application described therein is limited to sulfate scale inhibitor in a certain system, i.e. a reverse osmosis system. Moreover, this document does not provide any evidence or hint that the copolymers described therein are suitable for inhibiting the formation of sulfide scale.

US 4,801,388 discloses a method of controlling scale deposits by adding hydrocarbon copolymers or terpolymers of (meth)acrylic acid and sulfoalkyl(meth)acrylamide The copolymers or terpolymers described therein is especially suitable for calcium phosphate, calcium carbonate, iron phosphate, barium sulfate and magnesium phosphate. However, this document does not provide any evidence or hint that the copolymers described therein are suitable for inhibiting the formation of sulfide scale.

It is well accepted in the art that a universal antiscalant inhibiting all type of scales with similar performance is not available and most likely impossible to obtain. Rather, most known antiscalents are specific with regard to the type of scale. For example, Hater et al. in Proceedings of the EUROCORR conference, 2013 report that Aminotrimethylenphosphonate (ATMP) exhibits 100% inhibition against calcium carbonate but much lower inhibition of at most 20% against calium sulphate and calium phosphate. By contrast to ATMP, a homopolymer of aspartic acid (PASP) exhibits 80% inhibition against calcium sulphate but nearly no inhibition effect on calcium phosphate. Not only homopolymers, but also copolymers are not suitable for inhibiting all type of scales. For example, a copolymer of acrylic acid (AA) and 2-acrylamido-2-methylpropane sulfonic acid (AMPS) described by Hater et al. inhibits calcium sulphate, calcium phosphate or iron oxide dispersing much better (about 70% to 80% inhibition) than calcium carbonate scale (under 40% inhibition). The study of Amjad in Materials Performance, Volume 55, No. 6, 2016 (NACE International) also reports different performance of antiscalants against various scales. For instance, the inhibition of calcium carbonate by 1-hydroxyethylidine 1,1,-diphosphonic acid (HEDP) reaches nearly 100%, whereas calcium phosphate inhibition by HEDP is only about 30%. Such study results reveal that high inhibition effect of an antiscalant on certain scale cannot be transferred to other scale types. In other words, even if an antiscalant shows 100% inhibition of a specific scale, e.g. calcium carbonate, there is no guarantee that this antiscalant shows the same inhibition effect on other type of scale like zinc hydroxide. Therefore, inhibition effect on certain scale cannot be predicted or deduced from the inhibition effects on other scales.

US 2018/0327294 discloses a method for inhibiting the formation of sulfide and silica scale in the brine loops of geothermal power plants, which comprises the injection of an aqueous composition to the brine, which contains a sulfide scale inhibitor and a silica scale inhibitor. The sulfide scale inhibitor is a copolymer of acrylic acid and 2-acrylamido-2-methylpropane sulfonic acid (AMPS), while the silica scale inhibitor is a copolymer of acrylic acid and a hydroxypolyethoxy allyl ether. The copolymers described therein are not particularly suitable for inhibiting the formation of stibnite scale.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide anti-scaling agents, which efficiently reduce or even inhibit the formation of sulfide-containing scale, in particular stibnite-containing scale and/or arsenic sulfide containing scale, at low dosages.

It was surprisingly found that these and further problems are solved by anionic copolymers, which comprise
a. polymerized units of a monomer M1, which is a primary amide of a monoethylenically unsaturated monocarboxylic acid having 3 to 6 C atoms and
b. polymerized units of a monoethylenically unsaturated monomer M2, which is selected from monoethylenically unsaturated monomer having a sulfonate group and/or sulfate group and monoethylenically unsaturated monomer having a carboxyl group and mixtures thereof;
where the copolymers have an anionic charge density in the range from 0.5 to 6.0 mol/kg or 0.5 to 5.0 mol/kg, in particular in the range from 0.8 to 5.0 mol/kg or 0.8 to 4.0 mol/kg or 1.0 to 4.0 mol/kg and especially in the range from 0.5 to 3.0 mol/kg, 0.8 to 3.0 mol/kg or 1.0 to 3.0 mol/kg,
and where the copolymers in the form of their sodium salts have a weight average molecular weight Mw of at least 10000 g/mol, in particular at least 15000 g/mol, more particular at least 20000 g/mol and especially at least 22000 g/mol or at least 25000 g/mol, as determined by gel permeation chromatography.

These copolymers provide for inhibition of sulfide-containing scale, in particular very good inhibition of stibnite-containing scale and thus are particularly suitable for application in water bearing systems, in particular in water bearing systems, where the formation of sulfide-containing scale, in particular stibnite-containing scale and/or arsenic sulfide containing scale, is likely to occur.

Therefore, a first aspect of the present invention relates to a method for reducing the formation of sulfide-containing scale, in particular for reducing the formation of stibnite-containing scale and/or arsenic sulfide containing scale in a water bearing system, which comprises the addition of an anionic copolymer to the water in the water bearing system, where the copolymer comprises
c. polymerized units of a monomer M1, which is a primary amide of a monoethylenically unsaturated monocarboxylic acid having 3 to 6 C atoms and
d. polymerized units of a monoethylenically unsaturated monomer M2, which is selected from monoethylenically unsaturated monomer having a sulfonate group and monoethylenically unsaturated monomer having a carboxyl group and mixtures thereof;
where the copolymer in its deprotonated form has an anionic charge density in the range from 0.5 to 6.0 or 0.5 to 5.0 mol/kg, in particular in the range from 0.8 to 5.0 or 0.8 to 4.0 mol/kg or 1.0 to 4.0 mol/kg and especially in the range from 0.5 to 3.0 mol/kg, 0.8 to 3.0 or 1.0 to 3.0 mol/kg,
and where the copolymer in the form of its sodium salt has a weight average molecular weight Mw of at least 10000 g/mol, in particular at least 15000 g/mol, more particular at least 20000 g/mol and especially at least 22000 g/mol or at least 25000 g/mol, as determined by gel permeation chromatography.

A second aspect of the present invention relates to the use of the anionic copolymers, which comprise polymerized units of a monomer M1 and of a monomer M2 as defined herein and which in their deprotonated form have a charge density and a molecular weight as defined herein for reducing the formation of sulfide-containing scale, in particular for reducing the formation of stibnite-containing scale and/or arsenic sulfide containing scale in water bearing systems, in particular in the brine loops of geothermal power plants.

A third aspect of the present invention relates to anionic copolymers, which comprise polymerized units of a monomer M1 and of a monomer M2 as defined herein, where the copolymers in their deprotonated form have an anionic charge density in the range from 0.5 to 5.0 mol/kg, in particular in the range from 0.8 to 5.0 mol/kg or 1.0 to 5.0 mol/kg, more particular 0.5 to 4.0 mol/kg, 0.8 to 4.0 mol/kg or 1.0 to 4.0 mol/kg and especially in the range from 0.5 to 3.0 mol/kg, 0.8 to 3.0 mol/kg or 1.0 to 3.0 mol/kg, and where the copolymer in the form of its sodium salt has a weight average molecular weight Mw in the range from 15000 to 500000 g/mol, or frequently in the range from 20000 to 500000 g/mol, in particular in the range from 22000 to 300000 g/mol and especially in the range from 25000 to 250000 g/mol, as determined by gel permeation chromatography.

The present invention is associated with several benefits. First of all, the anionic copolymers as defined herein provide for efficient reduction or inhibition of the formation of sulfide-containing scale, in particular stibnite-containing scale and/or arsenic sulfide containing scale in water bearing systems, where formation of sulfide-containing scale is likely to occur. The copolymers defined herein do not require phosphorous containing anti-scaling agents that are normally required for achieving efficient reduction or inhibition of the formation of sulfide-containing scale. Moreover, the copolymers of the present invention are easily prepared from readily available and inexpensive monomers by standard copolymerization techniques.

DETAILED DESCRIPTION OF THE INVENTION

The following statements apply to any of the aspects disclosed herein.

Here and in the following, the term “scale” refers to deposits on internal walls of water bearing systems. The terms “scale” and “deposits” are used synonymously.

Here and in the following, sulfide-containing scale refers to scale, which contains solid metal sulfides, including in particular the sulfides of antimony and arsenic.

Here and in the following, stibnite-containing scale refers to scale, which contains solid antimony(III) sulfide, i.e. stibnite or Sb₂S₃ respectively. In particular, stibnite-containing scale predominately contains crystalline stibnite or essentially consists thereof. This crystalline stibnite is typically present in the form of fine needles.

Here and in the following, the phrase “reduction or inhibition of the formation of sulfide-containing scale” means that the formation of sulfide-containing scale, in particular stibnite-containing scale, but also antimony(V) sulfide-containing scale and/or arsenic sulfide containing scale, in a water bearing system is significantly reduced by at least 15%, in particular by at least 20% or at least 25% compared to a water bearing system without anti-scaling agent.

Here and in the following, the term “scale” and “scaling” are used synonymously.

Here and in the following, the term “anti-scaling agent” and “antiscalant” are used synonymously.

Here and in the following, the term “sulfide-containing scale” relates to scale, which contains or consists of inorganic sulfides, in particular scale which contains or consists of antimony(III)sulfide, antimony(V)sulfide and arsenic(III)sulfide. The term “sulfide-containing scaling” relates to the formation of a sulfide-containing scale

Here and in the following, the term “carboxyl group” refers to the group COOH. The term “sulfonate group” refers to the group SO₃ ^-, which is bound to a carbon atom. The term “sulfate group” refers to the group -O-SO₃ ^-, where the left oxygen atom is bound to a carbon atom of the monomer.

Here and in the following, the term “monoethylenically unsaturated monomer” means that the monomer has exactly one ethylenically unsaturated double bond, which is capable of undergoing a free radical copolymerization.

Here and in the following, the term “water bearing system” means any device or arrangement of devices, which has internal surfaces that are in contact with water, including tubes, heat-exchangers, boilers, steam generators, turbines, pumps, wells, fracking devices. These devices may be part of a plant, such as power plants, in particular geothermal power plants, desalination plants, paper mills, fracking plants and the like.

Here and in the following, the term “copolymer in its deprotonated form” means that the acidic groups of the copolymer, i.e. the carboxyl groups and the sulfonate groups of the polymerized units of the monomer M2, are completely deprotonated and thus present in the anionic form.

Under the conditions of their use for reducing or inhibiting sulfide-containing scaling the copolymers are usually present in their partly or completely deprotonated form. The carboxyl groups of the polymerized units of the monomers M2 will usually be present at least partly or even completely in their deprotonated form, i.e. as CO₂ ^- groups, while the sulfonate groups will usually be present in the completely deprotonated form, i.e. as SO₃ ^- groups. Thus, the polymerized units of the monomers M2 impart an anionic charge to the copolymer. If the carboxyl groups and the sulfonate groups of the polymerized units of the monomer M2 are completely deprotonated, the copolymers have an anionic charge density in the range from 0.5 to 5.0 mol/kg, in particular in the range from 0.8 to 5.0 mol/kg or 1.0 to 5.0 mol/kg, more particular in the range from 0.5 to 4.0 mol/kg, 0.8 to 4.0 mol/kg or 1.0 to 4.0 mol/kg and especially in the range from 0.5 to 3.0 mol/kg, 0.8 to 3.0 mol/kg or 1.0 to 3.0 mol/kg.

The anionic charge densities of the copolymers in their deprotonated form correlate with the relative molar amount of the polymerized units of the monomers M1. The anionic charge density Q_A of a copolymer, which contains polymerized units of monomers M1 and M2 and optionally one or more further neutral monomers M3 can be calculated by the following equation (1):

Q_A = 1000 x [M2] / ([M1] x Mw(M1) + [M2] x Mw(M2) + [M3] x Mw(M3)) (1)

In equation (1) the variables have the following meanings:
[M1] is the relative molar amount of the monomer M1 in mol-%;
[M2] is the relative molar amount of the monomer M2 in mol-%;
[M3] is the relative molar amount of the monomer M3 in mol-%;
Mw(M1) is the molecular weight of the monomer M1 in g/mol;
Mw(M2) is the molecular weight of the monomer M2 in g/mol; and
Mw(M3) is the molecular weight of the monomer M3 in g/mol.

A skilled person will immediately recognize that the term [M3] x Mw(M3) is 0, if the copolymer does not contain polymerized units of the monomer M3. A skilled person will also immediately recognize that the charge density of a copolymer, which contains two different monomers M2a and M2b can be calculated by a modified equation (1a):

Q_A = 1000 x A / ([M1] x Mw(M1) + B + [M3] x Mw(M3)) (1a)

where
A is [M2a] +[M2b];
B is [M2a] x Mw(M2a) + [M2b] x Mw(M2b);
[M2a] and [M2b] are the relative molar amount of the monomers M2a and M2b, respectively, in mol-%;
and
Mw(M2a) and Mw(M2b) are the molecular weights of the monomers M2a and M2b, respectively in g/mol.

If the copolymers in addition to the polymerized units of the monomers M1 and M2 and optional monomers M3 contain polymerized units of a cationic monomer M4 the copolymers are zwitterionic in their deprotonated form. However, the net anionic charge density will be in the range given above. The net anionic charge density can be calculated by the following equation (1b):

Q_A = 1000 x ([M2] - [M4]) / C (1b)

where
C = ([M1] x Mw(M1) + [M2] x Mw(M2) + [M3] x Mw(M3) + [M4] x Mw(M4));
[M4] is the relative molar amount of the monomer M4 in mol-%;
Mw(M4) is the molecular weight of the monomer M4 in g/mol; and where
[M1], [M2], [M3], Mw(M1), Mw(M2) and Mw(M3) are as defined for equation (1).

The anionic charge density of the copolymers in their deprotonated form can also be determined experimentally by titration. For example, the total charge of the polymer can be determined e.g. by a colloidal charge titration. That means by detecting the streaming potential and titration with a polyelectrolyte titrant of the opposite charge, here a cationic polyelectrolyte, such as polydadmac (polydiallyldimethyl ammonium chloride), to the point of zero charge. Colloidal charge titration is a well established technique for determining the surface charge of colloid particles and can be applied by analogy (see e.g. L.H. Mikkelsen, Wat. Res., (2003) Vol 37, 2458-2466 and K. Ueno et al., J. Chem. Edu. 62(7) (1985), 627 as well as J. Plank; B. Sachsenhauser Cement and Concrete Research, (2009) Vol 39 (1),1-5. Alternatively, the amount carboxyl groups can be determined by acid/base titration, while the amount of sulfonate groups can be determined spectrometrically, e.g. via ¹H-NMR spectroscopy.

With regard to their capability to reduce or inhibit the formation of sulfide-containing scale, it is beneficial, if the copolymers of the present invention have a weight average molecular weight M_W of at least 15000 g/mol or higher, in particular at least 20000 g/mol or higher, more particularly at least 22000 g/mol or higher, especially at least 25000 g/mol or at least 30000 g/mol, as determined by gel permeation chromatography of the sodium salt of the copolymer, in particular in buffered water at pH 7.0. Frequently, the weight average molecular weight M_W of the copolymers of the present invention does not exceed 10⁷g/mol and is preferably at most 5x10⁶ g/mol, in particular at most 10⁶ g/mol, more particularly at 500000 g/mol or at most 300000 g/mol and especially at most 250000 g/mol, as determined by gel permeation chromatography of the sodium salt of the copolymer, in particular in buffered water at pH 7.0. In particular, the copolymers of the present invention have a weight average molecular weight M_W in the range from 15000 to 10⁶ g/mol, more particularly in the range from 15000 to 500000 g/mol, 20000 to 500000 g/mol or in the range from 22000 to 300000 g/mol and especially in the range from 25000 to 250000 g/mol, as determined by gel permeation chromatography.

The number average molecular weight Mn of the copolymers of the present invention in the form of their sodium salts is frequently in the range from 2000 to 10⁶ g/mol, in particular in the range from 3000 to 500000 g/mol or 4000 to 300000 g/mol, more preferably in the range from 7000 to 250000 g/mol and especially in the range from 10000 to 200000 g/mol, as determined by gel permeation chromatography. Frequently, the ratio of the weight average molecular weight Mw to the number average molecular weight Mn of the copolymer in the form of its sodium salt, i.e. the ratio Mw/Mn is in the range from 1.1 to 10, in particular in the range from 2 to 6.

Both weight average molecular weights (Mw) and number average molecular weights (Mn) as referred herein relate to the molecular weight of the sodium salt of the respective copolymer, which is determined by gel permeation chromatography, in the following abbreviated as GPC. Gel permeation chromatography is usually carried out by using crosslinked acrylate copolymers of defined pore size as stationary phase, water buffered to pH 7 as the eluent and polyacrylic acid sodium salts as standards. Further details on GPC are given below in the experimental part.

With regard to the capability of the copolymers to reduce or inhibit the formation of sulfide-containing scale, the amount of polymerized units of the monomers M1 is preferably in the range from 55 to 95 mol-%, in particular in the range from 60 to 95 mol-% or 60 to 90 mol-%, especially in the range of 65 to 95 mol-% or 65 to 90 mol-% or 65 to 88 mol-% or 70 to 88 mol-%, based on the total molar amount of polymerized units in the copolymer.

Examples of suitable monomers M1 are acrylamide and methacrylamide as well as mixtures thereof. Preferably, the polymerized units of monomer M1 comprises polymerized units of methacrylamide. In particular, at least 50 mol-% or at least 80 mol-% or at least 100 mol-% of the polymerized units of monomers M1 are polymerized units of methacrylamide.

With regard to the capability of the copolymers to reduce or inhibit the formation of sulfide-containing scale, the amount of polymerized units of the monomers M2 is preferably in the range from 5 to 45 mol-%, in particular in the range from 5 to 40 mol-% or 10 to 40 mol-%, especially in the range of 5 to 35 mol-% or 10 to 35 mol-% or 12 to 35 mol-% or 12 to 30 mol-%, based on the total molar amount of polymerized units in the copolymer.

According to the invention, the monomers M2 are selected from the group consisting of monoethylenically unsaturated monomers having a sulfonate group or a sulfate group, hereinafter termed monomers M21, and monoethylenically unsaturated monomers having a carboxyl group, hereinafter termed monomers M22.

Suitable monomers M21 having a sulfonate group are, for example, vinylsulfonic acid, allylsulfonic acid, methallylsulfonic acid, allyloxy-2-hydroxypropane sulfonaic acid, vinylbenzene sulfonic acid, and monomers of the formula (I):

where
R is H or C₁-C₃-alkyl, such as methyl, ethyl or n-propyl, in particular H or methyl;
X is O or NH; and
Z is C₂-C₆-alkandiyl, such as 1,2-ethandiyl, 1,2-propandiyl, 1,3-propandiyl, 2-methyl-1,2-propnadiyl, 1,4-butandiyl, 1,2-butandiyl, 2-methyl-1,2-butandiyl etc.;
and the salts thereof, in particular the ammonium salts and the alkalimetal salts and especially the sodium salts thereof.

Suitable monomers M21 having a sulfate group are, for example, allylsulfate and allyl polyether sulfates, such as allylpolyethoxy sulfate, i.e. monomers of the formula (II):

CH₂=CH-CH₂-(O-CH₂-CH₂)_n-O-S(=O)-OH (II)

where n is in the range from 2 to 20,
and the salts thereof, in particular the ammonium salts and the alkalimetal salts and especially the sodium salts thereof.

Amongst the monomers M21, preference is given to monomers having a sulfonate group, in particular to the monomers of the formula (I), the salts thereof, in particular the alkalimetal salts and especially the sodium salts. Particular preference is given to monomers of the formula (I), wherein R is H or CH₃, X is N and Z is C₂-C₄-alkandiyl, which are also termed N-acrylamido-C₂-C₄-alkylsulfonic acids and N-methacrylamido-C₂-C₄-alkylsulfonic acids, and to wherein R is H or CH₃, X is N and Z is C₂-C₄-alkandiyl, which are also termed acryloxy-C₂-C₄-alkylsulfonic acids and methacryloxy-C₂-C₄-alkylsulfonic acids and to the salts thereof, in particular the alkalimetal salts and especially the sodium salts. Examples of monomers M21 of the formula (I) are 2-acryloxyethylethane sulfonic acid, 3-acryloxypropane sulfonic acid, 2-acryloxypropane sulfonic acid, 2-acrylamidoethane sulfonic acid, 2-acrylamidopropane sulfonic acid and 2-acrylamido-2-methylpropane sulfonic acid and the salts of the aforementioned monomers, in particular the alkalimetal salts and especially the sodium salts. Especially, the monomer M21 is 2-acrylamido-2-methylpropane sulfonic acid or a salt thereof, in particular the ammonium salt and the alkalimetal salts thereof, and especially the sodium salt thereof.

Suitable monomers M22 are in particular monoethylenically unsaturated monocarboxylic acids having 3 to 6 C atoms, such as acrylic acid, methacrylic acid, crotonic acid, isocrotonic acid, semiesters of maleic acid or fumaric acid, such as monomethylmaleate, monomethylfumarate, monoethylmaleate, monoethylfumarate and 2-acryloxyacetic acid. Suitable monomers M22 are also monoethylenically unsaturated dicarboxylic acids having 4 to 6 C atoms, such as fumaric acid, maleic acid, itaconic acid and citraconic acid. Preferred monomers M22 are monoethylenically unsaturated monocarboxylic acids having 3 to 6 C atoms, with particular preference given to acrylic acid, methacrylic acid and mixtures thereof.

Besides the units of polymerized monomers M1 and monomers M2, the copolymers may comprise polymerized units of neutral monoethylenically monomers M3. The amount of polymerized units of monomers M3 may be as high as 20 mol-%, based on the total molar amount of polymerized units in the copolymer, but is frequently at most 5 mol-%, especially at most 2 mol-% and at least at most 1 mol-%, or even 0 mol-%, based on the total molar amount of polymerized units in the copolymer. Examples of such monomers include, but are not limited to, the hydroxy-C₂-C₄-alkyl esters of monoethylenically unsaturated monocarboxylic acids having 3 to 6 carbon atoms, such as hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate, or hydroxypropyl methacrylate, N-vinyllactams, such as N-vinyl pyrrolidone and N-vinyl caprolactam.

Besides the units of polymerized monomers M1 and monomers M2, the copolymers may comprise polymerized units of cationic monoethylenically monomers M4. The amount of polymerized units of monomers M4 may be as high as 20 mol-%, based on the total molar amount of polymerized units in the copolymer, but is frequently at most 5 mol-%, especially at most 2 mol-% and at least at most 1 mol-%, or even 0 mol-%, based on the total molar amount of polymerized units in the copolymer. Examples of such monomers M4 include, but are not limited to, N-C₁-C₄-alkyl-N’-vinylimidazolium salts, such as N-methyl-N’-vinylimidazolium salts, N-C₁-C₄-alkyl-vinylpyridinium salts, such as N-methyl-vinylpyridinium salts, in particular the chlorides, sulfates, hydrogensulfates, methosulfates and ethosulfates thereof, and the monomers of the formula (III)

where
R’ is H or C₁-C₃-alkyl, such as methyl, ethyl or n-propyl, in particular H or methyl;
X’ is O or NH; and
Z’ is C₂-C₆-alkandiyl, such as 1,2-ethandiyl, 1,2-propandiyl, 1,3-propandiyl, 2-methyl-1,2-propnadiyl, 1,4-butandiyl, 1,2-butandiyl, 2-methyl-1,2-butandiyl etc.;
R^a, R^b, R^c are identical or different and C₁-C₃-alkyl, in particular methyl or ethyl;
A- is a counter anion equivalent, such as chloride, methosulfate, ethosulfate, hydrogen sulfate and 1/2 sulfate.

Besides the units of polymerized monomers M1 and monomers M2, the copolymers may comprise polymerized units of monomers M5, which have more than 1 ethylenically unsaturated double bond, e.g. from 2 to 5 ethylenically unsaturated double bonds. The amount of polymerized units of monomers M5 is frequently at most 1 mol-%, especially at most 0.5 mol-% and at least at most 0.1 mol-%, and in particular 0 mol-%, based on the total molar amount of polymerized units in the copolymer. Examples of such monomers M5 include, but are not limited to, divinyl benzene, ethylene glycol diacrylate, propanediol diacrylate, butanediol diacrylate, hexandiol diacrylate, diethylene glycol diacrylate, triethylene glycol triacrylate, trimethylolpropane triacrylate, di(trimethylolpropane) tetraacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate and the corresponding di-, tri- and tetramethacrylates.

Particular preference is given to copolymers, which consist of at least 95 mol-%, in particular at least 98 mol-% and especially at least 99 mol-%, based on the total molar amount of polymerized units in the copolymer, of units of polymerized monomers M1 and units of polymerized monomers M2.

Particular preference is given to copolymers, which do not contain more than 1000 ppm of phosphorous, in particular less than 500 ppm of phosphorous.

A preferred group 1 of embodiments relates to copolymers, which comprise polymerized units of monomers M21, i.e. monoethylenically unsaturated monomers having a sulfonate group, in particular polymerized units of at least one monomer of the formula (I), especially polymerized units of 2-acrylamido-2-methylpropane sulfonic acid. In these preferred groups of embodiments, the polymerized units of monomers M21 may be the sole polymerized units of monomers M2. In this preferred group 1 of embodiments the copolymers preferably consist of at least 95 mol-%, in particular at least 98 mol-% and especially at least 99 mol-%, based on the total molar amount of polymerized units in the copolymer, of units of polymerized monomers M1 and units of polymerized monomers M21.

Amongst the copolymers of group 1 of embodiments, particular preference is given to copolymers, which consist of at least 95 mol-%, in particular at least 98 mol-% and especially at least 99 mol-%, based on the total molar amount of polymerized units in the copolymer, of
- 55 to 95 mol-% or 60 to 95 mol-%, in particular 60 to 90 mol-%, especially 65 to 95 mol-% or 65 to 90 mol-% or 65 to 88 mol-% or 70 to 88 mol-%, based on the total molar amount of polymerized units in the copolymer, of polymerized units of at least one monomer M1, which is preferably selected from acrylamide, methacrylamide and mixtures thereof;
- 5 to 45 mol-% or 5 to 40 mol-%, in particular 10 to 40 mol-%, especially 5 to 35 mol-% or 10 to 35 mol-% or 12 to 35 mol-% or 12 to 30 mol-%, based on the total molar amount of polymerized units in the copolymer, of polymerized units of at least one monomer M21, which is preferably selected from monomers of the formula (I), and which especially is 2-acrylamido-2-methylpropane sulfonic acid.

Another preferred group 2 of embodiments relates to copolymers, which comprise polymerized units of monomers M22, i.e. monoethylenically unsaturated monomers having a carboxyl group, in particular polymerized units of monoethylenically unsaturated monocarboxylic acids having 3 to 6 C atoms, especially polymerized units of acrylic acid and/or methacrylic acid. In these preferred copolymers, the polymerized units of monomers M22 may be the sole polymerized units of monomers M2. In this preferred group 2 of embodiments, the copolymers preferably consist of at least 95 mol-%, in particular at least 98 mol-% and especially at least 99 mol-%, based on the total molar amount of polymerized units in the copolymer, of units of polymerized monomers M1 and units of polymerized monomers M22.

Amongst the copolymers of group 2 of embodiments, particular preference is given to copolymers, which consist of at least 95 mol-%, in particular at least 98 mol-% and especially at least 99 mol-%, based on the total molar amount of polymerized units in the copolymer, of
- 55 to 95 mol-% or 60 to 95 mol-%, in particular 60 to 90 mol-%, especially 65 to 95 mol-% or 65 to 90 mol-% or 65 to 88 mol-% or 70 to 88 mol-%, based on the total molar amount of polymerized units in the copolymer, of polymerized units of at least one monomer M1, which is preferably selected from acrylamide, methacrylamide and mixtures thereof;
- 5 to 45 mol-% or 5 to 40 mol-%, in particular 10 to 40 mol-%, especially 5 to 35 mol-% or 10 to 35 mol-% or 12 to 35 mol-% or 12 to 30 mol-%, based on the total molar amount of polymerized units in the copolymer, of polymerized units of at least one monomer M22, which is preferably selected from monoethylenically unsaturated monocarboxylic acids having 3 to 6 C atoms, especially from acrylic, methacrylic acid and mixtures thereof.

A particular preferred group 3 of embodiments relates to copolymers, which comprise polymerized units of both monomers M21 and monomers M22, i.e. a combination of polymerized units of at least one monomer M21 and at least one monomer M22, in particular polymerized units of a combination of
- at least one monomer M21 selected from monomers of the formula (I) and the salts thereof, especially polymerized units of 2-acrylamido-2-methylpropane sulfonic acid; and
- at least one monomer M22 selected from monoethylenically unsaturated monocarboxylic acids having 3 to 6 C atoms, especially from acrylic acid, methacrylic acid and mixture thereof.

In this preferred group 3 of embodiments, the copolymers preferably consist of at least 95 mol-%, in particular at least 98 mol-% and especially at least 99 mol-%, based on the total molar amount of polymerized units in the copolymer, of units of polymerized monomers M1, units of polymerized monomers M21 and units of polymerized monomers M22. In this particular group 3 of embodiments, the molar ratio of polymerized units of monomer M21 to polymerized units of monomer M22 is frequently in the range from 1:10 to 10:1, in particular in the range from 1:5 to 5:1 and especially in the range from 1:3 to 3:1.

Amongst the copolymers of group 3 of embodiments, particular preference is given to copolymers, which consist of at least 95 mol-%, in particular at least 98 mol-% and especially at least 99 mol-%, based on the total molar amount of polymerized units in the copolymer, of
- 55 to 95 mol-% or 60 to 95 mol-%, in particular 60 to 90 mol-%, especially 65 to 95 mol-% or 65 to 90 mol-% or 65 to 88 mol-% or 70 to 88 mol-%, based on the total molar amount of polymerized units in the copolymer, of polymerized units of at least one monomer M1, which is preferably selected from acrylamide, methacrylamide and mixtures thereof;
- 5 to 45 mol-% or 5 to 40 mol-%, in particular 10 to 40 mol-%, especially 5 to 35 mol-% or 10 to 35 mol-% or 12 to 35 mol-% or 12 to 30 mol-%, based on the total molar amount of polymerized units in the copolymer, of polymerized units of a combination of at least one monomer M21, which is preferably selected from monomers of the formula (I), and which especially is 2-acrylamido-2-methylpropane sulfonic acid; and at least one monomer M22, which is preferably selected from monoethylenically unsaturated monocarboxylic acids having 3 to 6 C atoms, especially from acrylic, methacrylic acid and mixtures thereof, where the molar ratio of polymerized units of monomer M21 to polymerized units of monomer M22 is frequently in the range from 1:10 to 10:1, in particular in the range from 1:5 to 5:1 and especially in the range from 1:3 to 3:1.

The deprotonated copolymers of groups 1, 2 and 3 of embodiments have anionic charge density as given above, in particular in the range from 0.8 to 4.0 mol/kg or 1.0 to 4.0 mol/kg and especially in the range from 0.5 to 3.0, 0.8 to 3.0 mol/kg or 1.0 to 3.0 mol/kg.

The sodium salts of the copolymers of groups 1, 2 and 3 of embodiments have weight average molecular weights M_W as given above and in particular weight average molecular weights M_W in the range from 15000 to 10⁶ g/mol, more particularly in the range from 15000 to 500000 g/mol or in the range from 20000 to 500000 g/mol or in the range from 22000 to 300000 g/mol and especially in the range from 25000 to 250000 g/mol, as determined by gel permeation chromatography. The sodium salts of the copolymers of groups 1, 2 and 3 of embodiments have number average molecular weights M_N as given above and in particular number average molecular weights M_N in the range from 3000 to 500000 g/mol or 4000 to 300000 g/mol, more preferably in the range from 7000 to 250000 g/mol and especially in the range from 10000 to 200000 g/mol, as determined by gel permeation chromatography.

While the copolymers of the present invention may have any molecular architecture, they are preferably statistical copolymers. Statistical copolymers are understood as copolymers, in which the distribution of the polymerized units of the different monomers, here monomers M1 and M2 and optionally one or more further monomers M3, M4 and/or M5, in the chain follows a statistical distribution. In other words, the ratio of the monomers in a section corresponds to the molar ratio of the monomers (random distribution). The copolymers may have a linear or branched structure, but preferably they are essentially linear. The copolymers are in particular statistical copolymers having a linear structure. However, the copolymers may have an alternating or a gradient structure.

The copolymers of the present invention can be prepared by copolymerization of at least one monomer M1 and at least one monomer M2 and optionally one or more further monomers M3, M4 and/or M5 by analogy to known methods for copolymerizing ethylenically unsaturated monomers as described e.g. in “Polymer Chemistry“ by S. Koltzenburg, M. Maskos, O. Nuyken (Springer-Verlag 2017) and D. Braun et al, “Praktikum der Makromolekularen Stoffe” and the literature cited therein. The copolymers of the present invention can also be prepared by polymerization of at least one monomer M1 and optionally one or more further monomers M3, M4 and/or M5 by analogy to known methods for copolymerizing ethylenically unsaturated monomers followed by partial hydrolysis of the polymerized units of the monomer M1. Thereby, the primary amide groups of the polymerized monomers M1 will be converted into carboxyl groups. The partial hydrolysis will be carried out, such that the degree of conversion corresponds to the desired anionic charge density of the copolymer. Partial hydrolysis can be carried out as described herein.

Frequently, the copolymers described herein are obtainable by a free-radical aqueous solution copolymerization of monomers M1 and M2 and optionally one or more further monomers M3, M4 and/or M5. Alternatively, the copolymers described herein are obtainable by a free-radical aqueous solution polymerization of at least one monomer M1 and optionally at least one monomer M2 and optionally one or more further monomers M3, M4 and/or M5 followed by a partial hydrolysis of the polymerized units of the monomer M1.

The term free-radical polymerization is understood that the polymerization of the ethylenically unsaturated monomers, here acrylic acid and the C₁-C₃ alkyl acrylate, is performed in the presence of a polymerization initiator, which, under polymerization conditions, forms radicals, either by thermal decomposition or by a redox reaction. A solution polymerization means that a solution of the monomers in a solvent, which is also capable of dissolving the copolymers, is polymerized by a free radical polymerization, i.e. in the presence of a polymerization initiator.

Suitable solvents for performing the solution polymerization include water and polar organic solvents and mixtures thereof with water. Suitable polar organic solvents are those, which are at least partially miscible with water and which preferably are miscible with water to an extent of at least 100 g/L at 20°C and ambient pressure. Suitable organic solvents include, but are not limited to, C₁-C₄-alkanols, such as methanol, ethanol, propanol, isopropanol (= 2-propanol), n-butanol, sec-butanol (= 2-butanol) or isobutanol, dimethyl sulfoxide, ethyl acetate. Preferred organic solvents are selected from C₁-C₄-alkanols, such as methanol, ethanol, propanol, isopropanol, n-butanol, sec-butanol or isobutanol, preferably isopropanol or sec-butanol. In particular, solvents for performing the solution polymerization are selected from water and mixtures of water and at least one polar organic solvent, preferably from water and mixtures of water and one or more C₁-C₄-alkanols. Especially, the solvent used for the free-radical polymerization of the monomers forming the copolymer or its precursor contains at least 50% by volume, in particular at least 70% by volume, based on the total amount of solvent.

If the polymerization of the monomers forming the copolymer or its precursor is conducted as a solution polymerization, the concentration of the monomers in the polymerization reaction may vary. In particular, the weight ratio of the monomers and the solvent will be in the range from 1:20 to 1.2:1, in particular in the range from 1:10 to 1:1 and especially in the range from 1:8 to 1:1.5.

The polymerization of the monomers forming the copolymer or its precursor is preferably a free-radical copolymerization and thus triggered by means of a free-radical polymerization initiator (free-radical initiator). These may in principle be peroxides or azo compounds. Of course, redox initiator systems may also be used.

Suitable peroxides may, in principle, be inorganic peroxides, such as hydrogen peroxide or peroxodisulfates, such as the mono- or di-alkali metal or ammonium salts of peroxodisulfuric acid, for example the mono- and disodium, -potassium or ammonium salts, or organic peroxides, e.g. peroxy acids and esters of peroxy acids, such as diisopropyl peroxydicarbonate, t-amyl perneodecanoate, t-butyl perneodecanoate, t-butyl perpivalate, t-amyl perpivalate, bis(2,4-dichlorobenzoyl) peroxide, diisononanoyl peroxide, didecanoyl peroxide, dioctanoyl peroxide, dilauroyl peroxide, bis(2-methylbenzoyl) peroxide, disuccinoyl peroxide, diacetyl peroxide, dibenzoyl peroxide, t-butyl per-2-ethylhexanoate, t-butyl-2-ethylhexanoate, bis(4-chlorobenzoyl) peroxide, t-butyl perisobutyrate, t-butyl permaleate, 1,1-bis(t-butyl peroxy)cyclohexane, t-butyl peroxyisopropylcarbonate, t-butyl perisononanoate, t-butyl peracetate, t-amyl perbenzoate, 3-(t-butylperoxy)-3-phenylphthalide or t-butyl perbenzoate, alkyl and cycloalkyl peroxides, such as 1,1-bis(t-butyl peroxy)-3,5,5-trimethylcyclohexane, 2,2-bis(t-butylperoxy)butane (di-t-butyl peroxide), 2,2-bis-10-(t-butylperoxy)propane, 2,5-dimethyl-2,5-bis(t-butylperoxy)hexane, di(t-amyl) peroxide, α,α′-bis(t-butylperoxyisopropyl)benzene, 3,5-bis(t-butylperoxy)-3,5-dimethyl-1,2-dioxolane, di(t-butyl) peroxide, 2,5-dimethyl-2,5-bis(t-butylperoxy)hexyne, 3,3,6,6,9,9-hexamethyl-1,2,4,5-tetraoxacyclononane, p-menthane hydroperoxide, pinane hydroperoxide, aromatic peroxides, such as dicumyl peroxide, diisopropylbenzene, mono-α-hydroperoxide or cumene hydroperoxide. A suitable peroxide may also be hydrogen peroxide.

Typical azo initiators are, for example, 4,4′-azobis-4-cyanovaleric acid (ACVA), 2,2′-azobis(2-methylpropionamidine) dihydrochloride, 2,2′-azobis(2-methylpropionitrile) (AIBN), 2,2′-azobis(2-methylbutanenitrile), 2,2′-azobis(2,4-dimethylvaleronitrile), 1,1′-azobis(cyanocyclohexane), 1,1′-azobis(N,N-dimethylformamide), 2,2′-azobis(2-methylbutyronitrile), 2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile), 2,2′-azobis(2,4,4-trimethylpentane), 2,2′-azobisisobutyronitrile, 2,2′-azobis(2-methylbutyronitrile), 2,2′-azobis(2,4-dimethylvaleronitrile), 2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile), 1,1′-azobis(1-cyclohexanecarbonitrile), 2,2′-azobis(isobutyramide) dihydrate, 2-phenylazo-2,4-dimethyl-4-methoxyvaleronitrile, dimethyl 2,2′-azobisisobutyrate, 2-(carbamoylazo)isobutyronitrile, 2,2′-azobis(2,4,4-trimethylpentane), 2,2′-azobis(2-methyl propane), 2,2′-azobis(N,N′-dimethyleneisobutyramidine), 2,2′-azobis(N,N′-dimethyleneisobutyramidine) hydrochloride, 2,2′-azobis(2-amidinopropane), 2,2′-azobis(2-amidinopropane) hydrochloride, 2,2′-azobis(2-methyl-N-[1,1-bis(hydroxymethyl)ethyl]propionamide), azobis(2-amidopropane) dihydrochloride or 2,2′-azobis(2-methyl-N[1,1-bis(hydroxymethyl)-2-hydroxyethyl]propionamide).

Typical redox initiators are, for example, mixtures of an oxidizing agent, such as hydrogen peroxide, peroxodisulfates or aforementioned peroxide compounds and a reducing agent. Corresponding reducing agents, which may be used are sulfur compounds with a low oxidation state, such as ammonium sulfite and alkali metal sulfites, for example potassium and/or sodium sulfite, ammonium bisulfite and alkali metal hydrogensulfites, for example potassium and/or sodium hydrogensulfite, ammonium metabisulfite and alkali metal metabisulfites, for example potassium and/or sodium metabisulfite, formaldehydesulfoxylates, for example potassium and/or sodium formaldehyde sulfoxylate, ammonium salts and alkali metal salts, specifically potassium and/or sodium salts of aliphatic sulfinic acids and alkali metal hydrogensulfides, for example potassium and/or sodium hydrogensulfide, salts of polyvalent metals, in particular Co(II) salts and Fe(II) salts, such as iron(II) sulfate, iron(II) ammonium sulfate or iron(II) phosphate, but also dihydroxymaleic acid, benzoin and/or ascorbic acid, and reducing saccharides, such as sorbose, glucose, fructose and/or dihydroxyacetone.

Preferably, the free-radical polymerization initiator comprises an inorganic peroxide, in particular a peroxodisulfate, such as the mono- or di-alkali metal or ammonium salts of peroxodisulfuric acid. In particular, the free-radical polymerization initiator is a redox initiator, which comprises an inorganic peroxide, in particular a peroxodisulfate, such as the mono- or di-alkali metal or ammonium salts of peroxodisulfuric acid as the oxidizing agent. In these redox initiators, the reducing agent is preferably selected from the group of sulfur compounds with a low oxidation state, such as alkali metal sulfites, for example potassium and/or sodium sulfite, alkali metal hydrogensulfites, for example potassium and/or sodium hydrogensulfite, alkali metal metabisulfites, for example potassium and/or sodium metabisulfite, formaldehydesulfoxylates, for example potassium and/or sodium formaldehydesulfoxylate, alkali metal salts, specifically potassium and/or sodium salts of aliphatic sulfinic acids and alkali metal hydrogensulfides, for example potassium and/or sodium hydrogensulfide. In these redox initiators, the molar amount of the reducing agent will exceed the molar amount of the oxidizing agent. In particular, the molar ratio of the reducing agent to the oxidizing agent is in the range from 1.5:1 to 100:1 and in particular in the range from 2:1 to 50:1.

The molecular weight of the copolymer can be adjusted by choosing a proper relative amount of the free-radical polymerization initiator with respect to the monomers to be polymerized. As a rule of thumb, increasing the relative amount of the free-radical polymerization initiator will result in a decrease of the molecular weight, while decreasing the relative amount of the free-radical polymerization initiator will result in an increased molecular weight. If the free-radical polymerization initiator is selected from redox initiators, an increase of the molar ratio of the reducing agent to the oxidizing agent will likewise result in a decreased molecular weight and vice versa. Typically, the amount of the free-radical polymerization initiator is in the range of 0.02 to 20 mmol, in particular 0.5 to 5.0 mmol per 1 mol of monomers to be copolymerized. In case of redox initiators, these ranges refer to the oxidizing agent.

The polymerization of the monomers forming the copolymer or its precursor is usually conducted at temperatures in the range from 25 to 150°C. Temperatures employed are frequently in the range from 40 to 120°C, in particular in the range from 50 to 110°C and especially in the range from 60 to 90°C.

The polymerization of the monomers forming the copolymer or its precursor can be carried out at a pressure of less than, equal to or greater than 1 atm (atmospheric pressure), and so the polymerization temperature may exceed 100°C and may be up to 150°C. Polymerization of the monomers is normally performed at ambient pressure, but it may also be performed under elevated pressure. In this case, the pressure may assume values of 1.1 to 15 bar (absolute) or even higher values. Frequently, the free-radical polymerization of the invention is conducted at ambient pressure (about 1 atm) with exclusion of oxygen, for example under an inert gas atmosphere, for example under nitrogen or argon.

The polymerization of the monomers forming the copolymer or its precursor can be carried out e.g. by a batch or semi-batch procedure or by a continuous procedure. In the batch procedure, the monomers to be polymerized and optionally the solvent used in the polymerization procedure is charged to a reaction vessel, while the majority or the total amount of the polymerization initiator is added to the reaction vessel in the course of the polymerization reaction. In a semi batch procedure, at least a portion of the total amount of the free-radical polymerization initiator and solvent and optionally a small portion of the monomers are charged to the reaction vessel and the majority of monomers to be polymerized are added to the reaction vessel in the course of the polymerization reaction. In a continuous process, the monomers, the polymerization initiator and the solvent are continuously added to a reaction vessel, and the obtained copolymer is continuously discharged from the polymerization vessel. Preferably, the polymerization of the monomers forming the copolymer or its precursor is conducted as a semi-batch procedure. In particular, at least 90% of the monomers to be polymerized are added to the reaction vessel in the course of the polymerization reaction.

The polymerization of the monomers forming the copolymer or its precursor may also include a step, where any residual monomers are removed e.g. by physical means, such as distillation or by chemical means, i.e. by forced radical polymerization, e.g. by using a second free-radical polymerization initiator, which is added to the polymerization reaction after at least 90% of the monomers to be polymerized have been reacted. Preferably, the second free-radical polymerization initiator is a hydroperoxide or a persulfate.

In case the monomers polymerized in the polymerization reaction do not comprise a monomer M2, a precursor polymer is obtained, which usually does not contain polymerized units of monomers M2 or only an insufficient amount thereof. These polymers will then be subjected to a partial hydrolysis in order to achieve a partial conversion of the carboxamide groups of the polymerized monomers M1 into carboxyl or carboxylate groups. Partial hydrolysis can be carried out by various methods known to the person skilled in the art. Typically, partial hydrolysis is carried out by dissolving the polymer in water and adjusting the pH to acidic or alkaline conditions. This may be accompanied by heating the polymer solution in order increase the rate of hydrolysis. Acidic hydrolysis is preferably conducted in the presence of strong acids, such as hydrochloric acid or sulfuric acid. The amount of acid is preferably chosen, such that the initial pH value is in the range of pH 4 to pH -1 and in particular in the range of pH 2 to pH 0. Alternatively, partial hydrolysis can be carried out in alkaline environment, preferably at a pH in the range of pH 8 to pH 14, in particular in the range of pH 9 to pH 12. The pH is then adjusted by the addition of a strong base, such as an alkalimetal hydroxide. The temperature required for hydrolysis of the aqueous polymer solution may be room temperature (20°C) or even below. Partial hydrolysis at room temperature may be considered as a prolonged product maturation step and may last multiple days, weeks or month. Once the desired properties have been achieved, the pH is adjusted to a neutral range, e.g. to pH 5 to pH 8. Preferably, hydrolysis is carried out at temperatures between 20 and 180°C, especially preferably between 30 and 100°C. The degree of conversion will depend on the temperature, the pH of the reaction medium and the reaction time. A skilled person will find out by routine the conditions for carrying out the partial hydrolysis for achieving the desired degree of conversion.

Partial hydrolysis may also occur under the conditions of the polymerization, if the polymerization of the monomers is carried out in water or a solvent, which contains water. However, if the monomers to be polymerized do not comprise a monomer M2, the amount of carboxyl groups formed during polymerization will usually not be sufficient to achieve an anionic charge density in the above ranges. It is also possible to subject copolymers, which already contain polymerized units of monomers M1 and monomers M2, to a partial hydrolysis. For example, a copolymer, which contains polymerized units of a monomer M1 and a monomer M21, to a partial hydrolysis in order to produce a copolymer, which contains polymerized units of a monomer M1, of a monomer M21 and also of a monomer M22.

The copolymer can typically be isolated from the resulting polymerization mixture or the reaction mixture obtained in the partial hydrolysis by means of relatively customary methods, e.g. by means of precipitation or distillation. However, it is also possible or even preferred to use the solution of the copolymers obtained by the polymerization reaction or in a subsequent partial hydrolysis, in particular in those cases, where the solvent is water or contains water. Frequently, the concentration of the copolymer in such solutions is in the range from 10 to 50 % by weight, in particular in the range from 15 to 40 % by weight, based on the total weight of the solution.

The copolymers of the present invention are particularly useful for reducing or inhibiting the formation of sulfide-containing scale, in particular of stibnite-containing scale, and also of arsenic sulfide (As₂S₃) containing scale in water bearing systems, in particular in water-treatment units, which are in permanent contact with antimony and sulfide-containing water and where thus a high risk exists that antimony sulfide-containing scale, in particular stibnite-containing scale, will form.

Therefore, the present invention also relates to a method for reducing or inhibiting the formation of sulfide-containing scale, in particular of stibnite-containing scale, in water bearing systems, which comprises the addition of a copolymer as defined to the water contained in the water bearing system. The present invention also relates to a method for reducing or inhibiting the formation of arsenic sulfide-containing scale, in water bearing systems, which comprises the addition of a copolymer as defined to the water contained in the water bearing system.

As mentioned above, a risk of sulfide-containing scale particularly exists in any water bearing system, in particular in water bearing systems, which contain sulfides, especially in water bearing systems, which contain both antimony and/or arsenic and sulfide, because in the latter the formation of stibnite-containing scale and or arsenic sulfide will likely occur.

As the solubility of antimony(III) sulfide decreases with decreasing pH and also with decreasing temperature, a particular risk for the formation of stibnite-containing scale exists, if the water in the water bearing system has a pH of at most pH 9, in particular at most pH 8.5, e.g. in the range of 4 to pH 9 or in the range of pH 5 to pH 8.5 as determined at 22°C, at and/or in the water bearing systems temperatures of the water of at most or below 120°C, e.g. in the range of 40 to 120°C, occur and/or temperature drops of at least 40 K may occur. Therefore, particular embodiments of the invention relate to the use and method of the invention, where at least one of the following conditions are met:
- The water to be treated contains antimony ions and sulfide, in particular antimony concentrations of at least 1 ppm, calculated as elemental antimony, and concentrations of sulfide ions of at least 1 ppm, calculated as elemental sulfur;
- the water bearing system is operated at pH values of at most pH 9, in particular of at most pH 8.5, e.g. in the range of 4 to pH 9 or in the range of pH 5 to pH 8.5 as determined at 22°C;
- in the water bearing system temperatures of the water contained therein of at most 120°C, in particular in the range of 40 to 120°C occur or are likely to occur,
- temperature drops of at least 40 K occur or are likely to occur.

The use of the copolymers and the method of the invention is not limited to particular water bearing systems, and thus they can be used in any water-bearing system, where formation of a sulfide-containing scale and in particular stibnite-containing scale or arsenic sulfide containing scale, is likely to occur.

Conditions favouring the formation of sulfide-containing scale, in particular stibnite-containing scale and/or arsenic sulfide containing scale are usually met in the brine loops of geothermal power plants, such as dry steam stations, flash steam stations, or binary cycle stations. The problem of the formation of stibnite-containing scale and/or arsenic sulfide containing scale may however also occur in other water bearing systems, wherein the water contains antimony salts and/or arsenic salts and sulfide and which were operated under conditions that favour the formation of stibnite and arsenic sulfide. Examples of further water bearing systems, where the formation of stibnite-containing scale and/or arsenic sulfide containing scale may occur are water bearing systems used in the fracking process.

Therefore, a particular group of embodiment of the present invention relates to the use and methods as described herein, where the copolymers are added to the brine contained in the brine loops of a geothermal power plant, e.g. the brine loops of a dry steam station, of a flash steam station or of a binary cycle station. The brine loops of a geothermal power plant may include different parts, where scale formation will likely occur, e.g. wells, steam pipes, condensers, separators, heat exchangers, pumps nozzles, tubes, etc. Scale formation, in particular stibnite formation, will likely occur in those parts of the brine loops, where the heat is removed from the brine, in particular above-ground parts, such as heat exchangers and condensers, but also other above ground parts, in which the brine cools down, including nozzles, pumps and tubes.

For effectively reducing or inhibiting the formation of sulfide-containing scale and in particular of stibnite-containing scale and/or arsenic sulfide containing scale, the copolymer will be usually added to the water in the water bearing system, such that a concentration of the copolymer in the water is at least 1 g/m³, in particular at least 2 g/m³, e.g. in the range from 1 to 100 g/m³ or in the range from 2 to 50 g/m³.

The copolymers may be added to the water as such, but are preferably added as an aqueous composition. Frequently, the concentration of the copolymer in such aqueous compositions is in the range from 1 to 40% by weight, in particular in the range from 5 to 30% by weight, based on the total weight of the composition. The aqueous composition may contain further additives conventionally used, such compositions including corrosion inhibitors, biocides, surfactants, gas-hydrate inhibitors, hydrogen sulfide scavengers, inert fluorescent tracer and also further anti-scaling agents.

The copolymer may be added to the water at any point of the water bearing system, in particular at a point, where the formation of sulfide-containing scale will likely occur, or upstream of this point. Likewise, in a geothermal power plant, the copolymer may be added to the brine at any point in the brine loops of a geothermal power plant. However, it is preferred to add the composition to the brine at a point upstream to a point, where scaling is likely to occur, e.g. upstream of a heat exchanger.

The copolymer as defined herein are compatible with the anti-scaling agents, which are commonly used for reducing or inhibiting the formation of scale in water-bearing systems, in particular in the brine loops of a geothermal power plant. Therefore, the copolymers as defined herein can be used in combination with a further anti scaling agent conventionally used for reducing or inhibiting the formation of common scale, such as scales based on silicates, calcium carbonate, calcium phosphate, calcium sulfate or barium sulfate. These anti-scaling agents including scale-inhibitors, i.e. compounds, which inhibit the formation of insoluble scale and scale dispersants, i.e. anti-scaling agents, which assist in dispersing insoluble inorganic material, which may form in the water-treatment unit.

Examples of such anti-scaling additives conventionally used for reducing or inhibiting the formation of common scale including phosphonates, such as ATMP (aminotris(methylenephosphonic acid)), PBTC (phosphonobutanetricarboxylic acid), DTPMP (diethylenetriamine penta(methylene phosphonic acid), HEBP (1-hydroxyethylidene)bisphosphonic acid), polyacrylic acids, copolymers of acrylic acid and AMPS, terpolymers of acrylic acid, AMPS and tert.-butylacrylamide, polymethacrylic acids, polymaleic acids and hydrolysed copolymers and terpolymers of maleic anhydride and the salts of these acids.

The use of the copolymers of the present invention may also be combined with conventional mechanical cleaning, such as sponge ball cleaning or cleaning by injection of water jets.

I. Abbreviations:
AA acrylic acid
AM acrylamide
AMPS-Na Sodium salt of 2-acrylamido-2-methylpropane sulfonic acid
APTAC 3-acrylamidopropyl trimethylammonium chloride
ATMP aminotris(methylenephosphonic acid)
b.w. by weight
DI-water: deionized water
DTPMP diethylenetriamine penta(methylene phosphonic acid)
GPC gel permeation chromatography
HEBP (1-hydroxyethylidene)bisphosphonic acid
M_W weight average molecular weight
M_N number average molecular weight
MAA methacrylic acid
MAM methacrylamide
MEHQ 4-methoxyphenol
NaBS Sodium bisulfite (NaHSO₃)
NaPS Sodium persulfate (Na₂S₂O₈)
PBTC phosphonobutanetricarboxylic acid
PP polypropylene
PTFE poly(tetrafluoroethene)
rpm rotations per minute

II. Polymer Analytics:
1) GPC:
GPC was performed by using an Agilent 1200er Series equipped with a PSS Security Degasser and a column oven adjusted to 35°C and a detection system measuring the refractive index. Columns used (in the direction of flow):
1. PSS Suprema Guard Column
2. PSS Suprema 3000 A°
3. PSS Suprema 100 A°
Flow rate was 1 ml/min.

As an eluent, ultrapure Water (Merck Millipore), which supplemented with 0.1 M NaNO₃ and 0.01 M NaH₂PO₄/Na₂HPO₄ buffer to adjust pH 7 was used. Prior to use, the eluent was filtered over a 0.45 μm cellulose acetate filter membrane.

Polymer samples were diluted in a ratio of 1/10 with deionized water and adjusted to pH 7 by addition of 10% sodium hydroxide solution.

Samples were diluted with the eluent containing the internal standard ethylene glycol to reach a polymer concentration of 3 to 5 g/liter. The samples are filtered with 0.45 μm cellulose acetate (or hydrophilized PTFE) syringe filters prior to measurement.

For calibration narrowly dispersed poly(methy acrylic acid) sodium salt standards of PSS (Polymer Standard Service GmbH Germany) with the following peak molecular weights in Dalton were used:
1. 1310
2. 3480
3. 8210
4. 34900
5. 72900
6. 163000
7. 326000
8. 549000
(All molecular weights refer to the sodium salt of poly(methacrylic acid.)

For calibration purposes, the GPC curves of the narrowly dispersed standard were analysed with regard to the retention volume of their refractive index intensity peak using the Win GPC UniChrom software from PSS (Polymer Standard Service GmbH Germany). A calibration correlation curve was created by means of polynomial fit (Polynomial 3).

2) Viscosity:
The viscosity of the polymer solutions was measured at 20°C ± 0.5°C by using a Brookfield digital viscosimeter (Brookfield Engineering Laboratories, Inc.) with spindle 61 or 62 at a rotational speed of 60 rpm.

3) pH measurement:
pH was determined by direct measurement at 20°C ± 0.5°C using a Mettler Toledo In lab routine pH electrode connected to a WTW 3210 pH meter (WTW Germany) calibrated by WTW buffer solutions.

4) Determination of the charge density:
4.1. Determination of the amount of carboxyl groups
The amount of carboxyl/carboxylate groups was determined by back titration of samples acidified with hydrochloric acid with 0.1 N NaOH solution.

Typically, 2 g of a 15% b.w. polymer solution were weighed into a 150 ml beaker. After addition of 55 ml of DI water and 150 to 200 μl of a 16% b.w. aqueous hydrochloric acid, the sample was homogenized by stirring for 5 minutes prior to titration with 0.1 NaOH. Automatic titration was conducted with a Methrom Ti-Touch 916 equipped with a pH electrode (Mettler Toledo DG115-SC Combined glass pH electrode).
Depending on the sample composition, two to three equivalence points can be observed.

The first equivalence point (EP1) occurring at a voltage of approx. +130 to +210 mV describes the end of the strong mineral acid buffer zone (e.g. HCl/NaCl) and the beginning of the carboxylic acid/carboxylate buffer zone (-COOH/-COONa). The second equivalence point (EP2) occurring at a voltage of 0 to -70 mV describes the end of the carboxylic acid/carboxylate buffer zone. Therefore, the difference between equivalence points two and one can be used for calculating the carboxylate concentration of the sample. In case of samples that have been subjected to a partial hydrolysis of the amide groups, a third equivalence point can be observed that can be addressed to the presence of ammonia/ammonium formed in this step.

4.2 Determination of the total charge of the copolymer
The total net charge of the polymer was determined by a colloidal charge titration. For this, the streaming potential was detected, while the polymer solution was titrated with 0.001 N-polyelectrolyte titrant of the opposite charge (cationic titrant), here PolyDadmac, to the point of zero charge.

The setup consists of a Mutek PCD-02 particle charge detector equipped with an automatic dosing/titration unit (Mettler DL 21).

All polymer samples were previously adjusted to pH 7.5 to 8.0, and 0.1% solution was prepared. 1 ml of the respective solution was placed into the PTFE measuring cell and completed to 10 ml with DI water to ensure complete covering of the instruments gold electrodes. The solutions were titrated with a 0.001 N Poly-Dadmac solution, until the endpoint (0 mV of streaming potential) was reached.

III. Commercial Copolymers
Commercial copolymers, which are useful as conventional anti-scaling agents:
Polymer V1: 43% b.w. aqueous solution of a copolymer of acrylic acid- and AMPS having a molecular weight Mw of 4500 g in its free acid form (Acumer (Trademark) 2000)
Polymer V2: 45% b.w. aqueous solution of a terpolymer of acrylic acid, AMPS and N-tert.-butyl-acrylamide having a molecular weight M_W of 5000 to 7000 g/mol (Acumer (Trademark) 5000)
Polymer V3: A 31% b.w. aqueous solution of poly(sodium acrylate) homopolymer having pH 8 (Mw of 800000-100000 g/mol as sodium salt) (Flosperse (Trademark) AZ-10)

Commercial copolymers suitable in the method and use of the present invention:
Polymer P1: 20% b.w. aqueous dispersion of an anionic acrylamide copolymer: Terpolymer of approx. 70 mol-% of AM and a total of approx. 30 mol-% of anionic monomers (Sum of Acrylic Acid and AMPS) having a molecular weight M_W of >10⁶g/mol and an anionic charge density of about 3.0 mol/kg (Ferrocryl 8704)
Polymer P2: A copolymer (solid powder form) of 92 mol-% of AM and 8 mol-% of acrylic acid having a molecular weight M_W of >10⁶ g/mol and an anionic charge density of about 0.99 mol/kg (Ferrocryl 8720)
Polymer P3: 21% b.w. aqueous solution of a copolymer of 87 mol-% of AM and 13 mol-% of AMPS having a molecular weight M_W of >10⁶ g/mol and an anionic charge density of about 1.33 mol/kg (Kurifloc DA 4020)

IV. Preparation examples:
The following polymerization reactions were carried out in a 1000 ml double jacket glass reactor equipped with a stainless steel stirrer, a condenser connected to a bubble counter outlet and ports for tube fittings. The reactor was connected to a heating bath circulation thermostat. All polymerizations were carried out under nitrogen atmosphere. Except for the outlet of the condenser, the reactor is sealed gas tight.

Preparation Example 1: Copolymer of MAM and AMPS in a molar ratio of 85:15
The initiator mixtures were prepared, charged into disposable PP syringes equipped with PTFE tubes (1 mm inner diameter) and mounted to syringe pumps:
1. Syringe 1: 6.31 g of a 39.1% b.w. aqueous solution of sodium bisulfite.
2. Syringe 2: An aqueous solution of 0.32 g sodium persulfate in 20 ml of water.

A solution of 108.45 g of MAM and 51.55 g of AMPS-Na in 593.6 g of water, containing 300 ppm MEHQ was charged into a glass cylinder connected to a double piston pump.

A mixture of water (180.0 g) and 10% b.w. aqueous sulfuric acid (0.3 g) was added to the reactor and heated to 70°C with stirring (stirrer speed: 200 rpm). Once the internal temperature of the reactor reached the target temperature of 70°C, about 0.76 g of the aqueous sodium bisulfite solution was charged to the reactor. The reactor temperature was set to automatic control with a target temperature of 70°C. Feeds of monomer and initiator solutions (Syringes 1 and 2) were started simultaneously within 1 minute from this point at the respective feed rates. The feeds were added through separate dozing nozzles.

The feed/dosing times of the separate solutions were
・ 40% b.w. aqueous solution of sodium bisulfite: 180 minutes
・ sodium persulfate in water: 220 minutes
・ monomer solution: 180 minutes

10 minutes after the end of the last feed (sodium persulfate solution), the pH of the reaction mixture was adjusted to 10 by slow addition (10 minutes) of 0.65 g of 50% b.w. aqueous NaOH. Stirring was continued for three hours at 70°C. After cooling to below 40°C, 0.6 of a 30% b.w. aqueous solution of hydrogen peroxide was added to remove any remaining SO₂. Then, the pH was adjusted to 6.6 by addition of 9.8 g of 70% b.w. sulfuric acid. For pH stabilization, 25 ml of a 1 M disodium citrate buffer were added. Water was added to complete the batch to a total mass of 1000 g.

The resulting polymer solution had the following properties:
pH: 5.3 (20°C)
Viscosity (Brookfield): 15 mPas
Solids content: 14.5% b.w. (15% b.w. copolymer)
M_W: 30000 [g/mol]
M_N: 7800 [g/mol]

The copolymer had an anionic charge density of 1.52 mol/kg based on the solid content.

Preparation Example 2: Copolymer of MAM and AMPS in a molar ratio of 85:15
The copolymer of preparation example 2 was prepared by analogy to the protocol of preparation example 1. Initiator amounts were increased in comparison to example 1. The amount of sodium persulfate was 0.63 g, and the total amount of sodium bisulfite solution 40% was 13.8 g. In this case, 1.8 g of sodium hydroxide 50% were added within 10 minutes to adjust to pH 10, and stirring was continued for 3 h at 70°C. After cooling to below 40°C, 1.09 g of a 30% b.w. aqueous solution of hydrogen peroxide was added to remove any remaining SO₂. Then, pH was adjusted to 7.0 by addition of 7.3 g of 70% b.w. sulfuric acid. Water was added to complete the batch to a total mass of 1000 g.

The resulting polymer solution had the following properties:
pH: 7.0 (20°C)
Viscosity (Brookfield): 11 mPas
Solids content: 14.9% b.w.
M_W: 17100 [g/mol]
M_N: 2460 [g/mol]

The copolymer had an anionic charge density of 1.4 mol/kg based on the solid content.

Preparation Example 3: Copolymer of MAM and AMPS in a molar ratio of 85:15, partially hydrolyzed
The copolymer of preparation example 3 was prepared by analogy to the protocol of preparation example 1. The amount of sodium persulfate was 0.16 g, and the total amount of 40% b.w. sodium bisulfite solution was 3.45 g. In this case, the sample was cooled to a temperature of below 40°C after the end of the NaPS feed. Then, 1.8 g of a 5% b.w. aqueous solution of an isothiazolinone preservative were added followed by 0.61 g of aqueous 50% b.w. solution of sodium hydroxide. 42 g of water were added to complete the batch to a total mass of 1000 g. Directly after synthesis, the product solution had a pH of 9.2. The sample was stored at 30°C for 3 months prior to testing.

The resulting polymer solution after storage had the following properties:
pH: 9.8. (20°C)
Viscosity (Brookfield): 30 mPas
Solids content: 15.7% b.w.
M_W: 86400 [g/mol]
M_N: 25000 [g/mol]

The copolymer had an anionic charge density of 2.4 mol/kg based on the solid content.

The amount of carboxylic groups was 0.165 mol/kg in the product solution (1.06 mol/kg based on the solid matter).

Preparation Example 4: Copolymer of AM and AMPS in a molar ratio of 85:15, partially hydrolyzed
The copolymer of preparation example 4 was prepared by analogy to the protocol of preparation example 3. The monomer feed solution consisted of 409 g of a freshly prepared 50% b.w. aqueous solution acrylamide and 233 g of a 50% b.w. aqueous solution of AMPS-Na. The amount of sodium persulfate was 0.35 g, and the amount of the 40% b.w. aqueous sodium bisulfite feed solution was 7.1 g. 0.73 g of the 40% b.w. aqueous sodium bisulfite feed solution were placed in the reactor before the monomer feed was started. The reaction mixture was cooled to a temperature of below 40°C. After the end of the NaPS feed, 0.61 g of aqueous 50% b.w. solution of sodium hydroxide were added followed by 1.8 g of a 5% b.w. aqueous solution of an isothiazolinone preservative. Directly after polymerization, the product solution had a pH of 9.7. Water was added to complete the batch to a total mass of 1000 g. The sample was stored at 30°C for 3 months prior to testing.

The resulting polymer solution had the following properties:
pH: 10.2 (20°C)
Viscosity (Brookfield): 580 mPas
Solids content: 33.5% b.w.
M_W: 46700 [g/mol]
M_N: 10700 [g/mol]

The copolymer had an anionic charge density of 2.04 mol/kg based on the solid content.

The amount of carboxylic groups was 0.16 mol/kg in the product solution (0.47 mol/kg based on the solid content).

Preparation Example 5: Copolymer of MAM and AMPS-Na in a molar ratio of 70:30
The copolymer of preparation example 5 was prepared by analogy to the protocol of preparation example 1. The amounts of monomers and initiators are summarized in the following table 1. After the last feed had been completed (NaPS), the pH of the polymer solution was directly adjusted to approx. pH 6 by addition of 0.36 g of a 50% b.w. aqueous solution of NaOH, before it was cooled to below 40°C. Then, 1.63 g of a 30% b.w. aqueous solution of hydrogen peroxide were added to remove remaining SO₂. Then, 1.8 g of a 5% b.w. aqueous solution of an isothiazolinone preservative were added followed by 0.61 g of aqueous 50% b.w. solution of sodium hydroxide to readjust pH to approx. 6. Water was added to complete the batch to a total mass of 1000 g.

The resulting polymer solution had the following properties:
pH: 6.22 (20°C)
Viscosity (Brookfield): 12.5 mPas
Solids content: 16.0% b.w.
M_W: 32000 [g/mol]
M_N: 6800 [g/mol]

The copolymer had an anionic charge density of 2.44 mol/kg based on the solid content.

The amount of carboxylic groups was 0.003 mol/kg in the product solution (0.019 mol/kg based on the solid content).

Preparation Example 6: Copolymer of MAM and AMPS-Na and AA in a molar ratio of 70:15:15
The copolymer of preparation example 6 was prepared by analogy to the protocol of preparation example 1. In this example, an additional monomer feed of acrylic acid (16.5 g) was added simultaneously and in parallel to the aqueous solution of methacrylamide and AMPS with a syringe pump. The total amounts of monomers and initiators are summarized in table 1. After the last feed was completed, the product solution was cooled to below 40°C, pH followed by immediately adjusting the pH to approx. pH 7 to 8 by addition 18.7 g of a 50% b.w. aqueous solution of NaOH. Then, 1.22 g of a 30% b.w. aqueous solution of hydrogen peroxide were added to remove remaining SO₂. Finally, 1.8 g of a 5% b.w. aqueous solution of an isothiazolinone preservative were added followed by 3.4 g of 10% b.w. of aqueous sulfuric acid to readjust pH to 6.7. Water was added to complete the batch to a total mass of 1000 g.

The resulting polymer solution had the following properties:
pH: 6.73 (20°C)
Viscosity (Brookfield): 22.0 mPas
Solids content: 16.3% b.w
M_W: 53000 [g/mol]
M_N: 16000 [g/mol]

The copolymer had an anionic charge density of 2.65 mol/kg based on the solid content.

Preparation Example 7: Copolymer of MAM and AA in a molar ratio of 70:30
The copolymer of preparation example 7 was prepared by analogy to the protocol of preparation example 6. In this example, the monomer feed of acrylic acid (41.26 g) was added simultaneously and in parallel to the aqueous methacrylamide solution, which did not contain AMPS-Na. The total amounts of monomers and initiators are summarized in table 1. After the last feed was completed, the product solution was cooled to below 40°C, pH followed by immediately adjusting the pH to approx. pH 7 to 8 by addition 46 g of a 50% b.w. aqueous solution of NaOH. Then, 1.05 of a 30% b.w. aqueous solution of hydrogen peroxide were added to remove remaining SO₂. Finally, 1.8 g of a 5% b.w. aqueous solution of an isothiazolinone preservative were added followed by 11.2 g of 10% b.w. of aqueous sulfuric acid to readjust pH to 6.8. Water was added to complete the batch to a total mass of 1000 g.

The resulting polymer solution had the following properties:
pH: 6.84 (20°C)
Viscosity (Brookfield): 39.0 mPas
Solids content: 16.6% b.w
M_W: 63000 [g/mol]
M_N: 17700 [g/mol]

The copolymer had an anionic charge density of 3.81 mol/kg based on the solid content.

The amount of carboxylic groups was 0.63 mol/kg in the product solution (3.80 mol/kg based on the solid content).

Preparation Example 8: Copolymer of MAM and AMPS-Na and MAA in a molar ratio of 73:15:12
The copolymer of preparation example 8 was prepared by analogy to the protocol of preparation example 1. In this example, an additional monomer feed of methacrylic acid was added simultaneously and in parallel to the aqueous methacrylamide and AMPS solution with a syringe pump (180 minutes of feed time). The total amounts of monomers and initiators are summarized in table 1. After the last feed was completed, the product solution was cooled to below 40°C, pH followed by immediately adjusting the pH to approx. pH 6 by addition 12.9 g of a 50% b.w. aqueous solution of NaOH. Then, 2.04 g of a 30% b.w. aqueous solution of hydrogen peroxide were added to remove remaining SO₂. Finally, 1.8 g of a 5% b.w. aqueous solution of an isothiazolinone preservative were added. Water was added to complete the batch to a total mass of 1000 g.

The resulting polymer solution had the following properties:
pH: 6.1 (20°C)
Viscosity (Brookfield): 15.0 mPas
Solids content: 14.2% b.w
M_W: 51400 [g/mol]
M_N: 14200 [g/mol]

The copolymer had an anionic charge density of 2.64 mol/kg based on the solid content.
The amount of carboxylic groups was 0.185 mol/kg in the product solution (1.30 mol/kg based on the solid content).

Preparation Example 9: Copoylmer of MAM and AMPS in a molar ratio of 85:15 partially hydrolyzed.

The copolymer of preparation example 9 was prepared by analogy to the protocol of preparation example 1. The amount of sodium persulfate was 0.32 g and the total amount of 40 % b.w. sodium bisulfite solution was 5.45 g. In this case, the sample was cooled to a temperature of below 40°C after the end of the NaPS feed. Then, 1.8 g of a 5 % b.w. aqueous solution of an isothiazolinone preservative were added followed by 0.81 g of aqueous 50 % b.w. solution of sodium hydroxide. Water was added to complete the batch to a total mass of 1000 g. Directly after synthesis, the product solution had a pH of 9.6. The sample was stored at 30°C for 40 days prior to testing.

The resulting polymer solution had the following properties:
pH: 9.7 (20°C)
Viscosity (Brookfield): 24 mPas
Solids content: 15.7 % b.w.
M_W: 73500 [g/mol]
M_N: 17000 [g/mol]

The copolymer had an anionic charge density of 2.02 mol/kg based on the solid content.

The amount of carboxylic groups was 0.102 mol/kg in the product solution (0.65 mol/kg based on solid matter)

Preparation Example 10: Copolymer of MAM and AMPS in a molar ratio of 75:25.
The copolymer of preparation example 10 was prepared by analogy to the protocol of preparation example 1. The total amounts of monomers and initiators are summarized in table 1. This monomer solution was split in a ratio of 60:40 parts. In this case, the amount of water used for the reaction mixture was decreased to 60.0 g, while the amount of 10% b.w. aqueous sulfuric acid was increased to 0.7 g. This reaction mixture was then preheated to 80 °C under stirring with the same stirrer speed of the example 1 and completed with the 60 parts of the above-mentioned monomer solution (532.9 g). After reaching the target temperature of 92 °C, reduced amount of the aqueous sodium bisulfite solution (about 0.51 g) was charged to the reactor. The reactor temperature of this reaction was set to 95 °C.

The feed/dosing times of the separate solutions were varied in comparison to example 1 as follows:
・ aqueous solution of sodium bisulfite: 220 minutes
・ sodium persulfate in water: 300 minutes
・ monomer solution-40 parts fraction (355.3 g): 100 minutes

In this case, the reaction mixture was cooled to 50 °C 10 minutes after the end of the last feed. Afterwards, 1 g of citric acid monohydrate was added and the reaction mixture was further cooled to 40 °C. The pH of the reaction mixture was adjusted to 6.1 by slow addition (10 minutes) of 3.3 g of 20% b.w. aqueous NaOH. Being distinct from the example 1, 0.78 of a 30% b.w. aqueous solution of hydrogen peroxide was added to remove any remaining SO2 after cooling to below 35 °C. Then, the pH was adjusted to 6.1 by addition of 1.0 g of 20% b.w. aqueous NaOH. 0.7 g of water were added to complete the batch to a total mass of 1000 g.

The resulting polymer solution had the following properties:
pH: 6.1 (20°C)
Viscosity (Brookfield): 68 mPas
Solids content: 20.8 % b.w.
M_W: 52000 [g/mol]
M_N: 9500 [g/mol]

The copolymers charge density has not been determined but would correspond to a theoretical value of 2.0 mol/kg.

Preparation Comparative Example C1: Low molecular weight copolymer of MAM and AMPS in a molar ratio of 85:15

The polymer of comparative example C1 was prepared by analogy to the protocol of preparation example 1. Initiator amounts were increased in comparison to example 1 and 2. The total amounts of monomers and initiators are summarized in table 1. The amount of sodium persulfate was 1.26 g, and the total amount of 40% b.w. aqueous sodium bisulfite solution was 28.13 g. 4.26 g of a 50% b.w. aqueous solution of sodium hydroxide were added within 10 minutes to adjust to pH 10, and stirring was continued for 3 h at 70°C. After cooling below 40°C, 1.71 g of a 30% b.w. aqueous solution of hydrogen peroxide was added to remove any remaining SO₂. Then, pH was adjusted to 7.0 by addition of 9.41 g of 70% b.w. sulfuric acid.

The resulting polymer solution had the following properties:
pH: 7.1 (20°C)
Viscosity (Brookfield): 7 mPas
Solids content: 16.7% b.w.
M_W: 5100 [g/mol]
M_N: 900 [g/mol]

The polymer had an anionic charge density of 1.59 mol/kg based on the solid content.

Preparation Comparative Example C2 (high charge): Copolymer of MAA, MAM and AMPS in a molar ratio of 50:35:15 having high charge density
The polymer of comparative example C2 was prepared by analogy to the protocol of preparation example 1. In this example, an additional monomer feed of methacrylic acid was added simultaneously and in parallel to the aqueous methacrylamide and AMPS solution with a syringe pump (180 minutes of feed time). 5 minutes after the start of the methacrylic acid feed, 126.8 g of 20% b.w. aqueous sodium hydroxide solution were added continuously with a syringe pump within 175 minutes. The total amounts of monomers and initiators are summarized in table 1. After cooling below 40°C, 0.3 g of a 30% hydrogen peroxide solution were added to remove remaining SO₂. Finally, 1.8 g of a 5% b.w. aqueous solution of an isothiazolinone preservative were added followed by 19.8 g of 20% b.w. aqueous sodium hydroxide solution to adjust pH to 6.6. Water was added to complete the batch to a total mass of 1000 g.

The resulting polymer solution had the following properties:
pH: 6.6 (20°C)
Viscosity (Brookfield): 22.5 mPas
Solids content: 16.8% b.w
M_W: 111300 [g/mol]
M_N: 30000 [g/mol]

The polymer had an anionic charge density of 6.1 mol/kg based on the solid content.

Preparation Comparative Example C3 (high charge): Copolymer of MAA and AMPS in a molar ratio of 85:15 having high charge density
The polymer of comparative example C3 was prepared by analogy to the protocol of preparation of comparative example C2. The total amounts of monomers and initiators are summarized in table 1. In this example, no sodium hydroxide was added during the monomer feed. After cooling below 40°C, 2.35 of a 30% hydrogen peroxide solution were added to remove remaining SO₂. Then,1.8 g of a 5% b.w. aqueous solution of an isothiazolinone preservative were added. The pH of the solution was pH 1.9 and adjusted to approx. pH 6 by addition of 90.0 g of a 50% b.w. aqueous sodium hydroxide solution. Water was added to complete the batch to a total mass of 1000 g.

The resulting polymer solution had the following properties:
pH: 6.2 (20°C)
Viscosity (Brookfield): 29.1 mPas
Solids content: 17.1% b.w
M_W: 270000 [g/mol]
M_N: 38000 [g/mol]

The polymer had an anionic charge density of 7.73 mol/kg based on the solid content.

Water was added to complete the batch to a total mass of 1000 g.

V. Tests for stabilization of stibnite in aqueous solutions
The respective copolymer solutions obtained in the above preparation examples and the commercial polymer solutions were diluted with deionized water to a concentration of 1.5 g/L of the copolymer, and the pH was adjusted to pH 7.5 by addition of either caustic soda or sulfuric acid. These solutions are used as stabilizer stock solutions.

A test water solution was prepared by dissolving a proper amount of magnesiumsulfate heptahydrate (MgSO₄ * 7 H₂O), calcium chloride dihydrate (CaCl₂ * 2 H₂O) and sodium hydrogencarbonate (NaHCO₃) in deionized water.

For carrying out the stabilization tests, the test water was supplemented with 20 ppm of antimony(III) sulfide.

For this, 28.7 mL of a commercial 5% b.w. aqueous solution of antimony(III) chloride was added to 10 L of the test water followed by adjusting the pH to 6.3 (± 0.1). This solution was used as a test stock solution. To 1 L of the test stock solution the stabilizer stock solution was added with stirring to achieve the desired stabilizer concentration. Then, 275 μl commercial 5% b.w. aqueous solution of sodium sulfide were added with stirring, such that the desired concentration of 20 ppm of antimony(III) sulfide was achieved. Stirring was stopped 15 min. after the addition of sodium sulfide.

The thus obtained solutions were observed visually at 1 h, 3 h, 4 h and 24 h after the addition of the aqueous solution of sodium sulfide. During observation, the test solutions were not stirred. A strong yellow colour indicated the formation of a stable soluble complex of antimony sulfide and the inhibitor, while the formation of precipitate indicated the formation of solid antimony (III) sulfide. The quality of the stabilizer was ranked for each stabilizer concentration according to the following grades (table 3).

The results after 24 h are summarized in the following table 4:

The copolymers were also tested in deionized water adjusted to pH 7.5 and supplemented with 175 ppm antimony sulfide by analogy to the protocol described above. Similar results were observed.

Claims

A method for reducing sulfide-containing scaling in a water bearing system, which comprises the addition of an anionic copolymer to the water in the water bearing system, where the copolymer comprises
e. polymerized units of a monomer M1, which is a primary amide of a monoethylenically unsaturated monocarboxylic acid having 3 to 6 C atoms and
f. polymerized units of a monoethylenically unsaturated monomer M2, which is selected from monoethylenically unsaturated monomer having a sulfonate group and or a sulfate group and monoethylenically unsaturated monomer having a carboxyl group and mixtures thereof;
where the copolymer has an anionic charge density in the range from 0.5 to 6.0 mol/kg,
and where the copolymer in the form of its sodium salt has a weight average molecular weight Mw of at least 10000 g/mol, as determined by gel permeation chromatography.
The method of claim 1, where the copolymer in the form of its sodium salt has a weight average molecular weight in the range from 15000 to 10⁶ g/mol, as determined by gel permeation chromatography.
The method of any one of the preceding claims, where the copolymer has an anionic charge density in the range from 0.8 to 5.0 mol/kg.
The method of any one of the preceding claims, where the amount of polymerized units of the monomer M1 is in the range from 55 to 95 mol-%, based on the total molar amount of polymerized units in the copolymer.
The method of any one of the preceding claims, where the amount of polymerized units of the monomer M2 is in the range from 5 to 45 mol-%, based on the total molar amount of polymerized units in the copolymer.
The method of any one of the preceding claims, where the total amount of polymerized units of the monomers M1 and M2 is at least 95 mol-% based on the total molar amount of polymerized units in the copolymer.
The method of any one of the preceding claims, where the monomer M1 is selected from acrylamide and methacrylamide.
The method of any one of the preceding claims, where the monomer M2 comprises a monoethylenically unsaturated monomer having a carboxyl group.
The method of any one of the preceding claims, where the monomer M2 is a combination of a monoethylenically unsaturated monomer having a sulfonate group and a monoethylenically unsaturated monomer having a carboxyl group.
The method of any one of the preceding claims, where the monoethylenically unsaturated monomer having a carboxyl group is selected from acrylic acid, methacrylic acid and mixtures thereof.
The method of any one of the preceding claims, where the monoethylenically unsaturated monomer having a sulfonate group is selected from N-acrylamido-C₂-C₄-alkylsulfonic acids, N-methacrylamido-C₂-C₄-alkylsulfonic acids, acryloxy-C₂-C₄-alkylsulfonic acids and methacryloxy-C₂-C₄-alkylsulfonic acids and mixtures thereof.
The method of any one of the preceding claims, where the copolymer is a statistical copolymer.
The method of any one of the preceding claims, where the copolymer is obtainable by a process, which comprises a free-radical aqueous solution copolymerization of at least one monomer M1 with at least one monomer M2 or by a process, which comprises a free-radical aqueous solution polymerization of at least one monomer M1 and optionally at least one monomer M2 and a subsequent partial hydrolysis of the polymerized units of the monomer M1.
The method of any of the preceding claims, where the water bearing system is a water loop of geothermal power plant.
The method of any of the preceding claims, where the copolymer is added to the water, such that a concentration of the copolymer in the water is in the range from 1 to 100 g/m³.
The method of any of the preceding claims, where the water contains antimony salts and/or arsenic salts and sulfide.
The method of any of the preceding claims, where the water in the water bearing system has a pH level in the range of pH 4 to 9, in particular in the range of pH 5 to 8.5, as determined at 22°C.
The use of a copolymer as defined in any one of claims 1 to 14 for reducing sulfide containing scale formation in water-bearing systems, in particular in the brine loops of geothermal power plants.
The use of claim 18, where sulfide containing scale formation includes in particular the formation of stibnite-containing scale.
An anionic copolymer, which comprises
polymerized units of a monomer M1, which is a primary amide of a monoethylenically unsaturated monocarboxylic acid having 3 to 6 C atoms and
polymerized units of a monoethylenically unsaturated monomer M2, which is selected from monoethylenically unsaturated monomer having a sulfonate group and monoethylenically unsaturated monomer having a carboxyl group and mixtures thereof;
where the copolymer has an anionic charge density in the range from 0.5 to 5.0 mol/kg,
and where the copolymer in the form of its sodium salt has a weight average molecular weight Mw in the range from 15000 to 500000 g/mol, as determined by gel permeation chromatography.
The anionic copolymer of claim 20, where the amount of polymerized units of the monomer M1 is in the range from 60 to 95 mol-% and the amount of polymerized units of the monomer M2 is in the range from 5 to 40 mol-%, based on the total molar amount of polymerized units in the copolymer.
The anionic copolymer of claim 20 or 21, where the copolymer has an anionic charge density in the range from 0.8 to 4.0 mol/kg, and in particular in the range from 0.8 to 3.0 mol/kg.
The anionic copolymer of any one of claims 20 to 22, which has at least one of the features of claims 6 to 13.