Process for removing metal impurities from wet process phosphoric acid and compositions thereof

Abstract

The present invention process and compositions remove heavy metal ions, such as cadmium, copper, lead, nickel, arsenic, manganese and mercury ions from wet-process phosphoric acid by providing a simple, single-step process that uses relatively small amounts of reagent. The process involves treating either the crude acid prior to gypsum filtration or the filtered acid with an organic precipitating agent composition, precipitating metals such as copper, cadmium, nickel, mercury, zinc, and separating the precipitate by either filtration or flotation, to produce phosphoric acid with reduced levels of the metals. The compositions of the present invention include a diorgano dithiophosphinic acid (or alkali metal or ammonia salts thereof), a first dithiophosphoric acid (or alkali metal or ammonia salts thereof) with alkyl or alkylaryl or aralkyl moieties, and optionally a second diaryl dithiophosphoric acid (or alkali metal or ammonia salts thereof).

Description

FIELD OF THE INVENTION

[0001] This invention relates to a process and compositions for removing heavy metal ions, such as copper, cadmium, nickel, mercury, zinc, arsenic, manganese and combinations thereof from wet-process acidic carrier solutions and more particularly to a process and compositions for removing these heavy metal ions from wet process phosphoric acid.

BACKGROUND OF THE INVENTION

[0002] The production of phosphoric acid is very well known in the art. Typically, phosphoric acid is produced by treating phosphate rock or ore containing minerals such as apatite, with sulfuric acid at high temperatures, for example, 60-80° C., in a reactor. The resulting slurry of crude phosphoric acid and gypsum solids is then filtered to remove gypsum. The filtered acid is further concentrated successively and then either sold as acid for industrial applications or converted into fertilizers. During the production of the acid, certain metal impurities that can include, heavy metal ions, such as cadmium, copper, lead and mercury are present as minerals in the phosphate rock and are dissolved into the phosphoric acid. Some of these metal impurities are considered unacceptable above a certain level, depending on the application, because of their toxicity and they thus have to be either completely removed or their levels have to be reduced significantly. Many processes have been developed over the years for their removal. Additionally, where feasible, these metal ions are also recovered from phosphoric acid for their economic value.

[0003] The art known processes for the partial removal of these heavy metals from phosphoric acid include solvent extraction, ion flotation, metal ion precipitation by cementation, adsorption, precipitation by means of hydrogen sulfide, and ion exchange.

[0004] Japanese Patent Specification JP-PS No. 72 575 115 and European Patent Application No. 0 023 195 each teach a process for removing the heavy metal cadmium by means of precipitation using hydrogen sulfide gas. Sophisticated equipment and conditions, such as high pressure or increased pH must be used since the precipitated cadmium sulfide is soluble in phosphoric acid and the acid must be successively freed from gas, thereby making the process very expensive and difficult to practice.

[0005] Solvent extraction has also been suggested for cadmium removal from phosphoric acid. A typical example of this method is illustrated by DE-A-3,218,599. According to this method, the solvent consists of an amino halide that extracts most of the cadmium present in the acid. This process suffers however, from several disadvantages such as difficulty in the recovery of the solvent, limited applicability to phosphoric acid of high concentration (above 45% P2O5) and high costs of extractant and operation.

[0006] Bierman, et al (U.S. Pat. No. 4,511,541) disclose another process for the selective recovery of cadmium, molybdenum, zinc, nickel and other metal values from wet process phosphoric acid and other acidic to slightly basic carrier solutions. In the Bierman, et al. process the metal-bearing solution is contacted with an organophosphine extractant to precipitate the metal values for subsequent separation from the solution. Difficulties associated with this process result from the fact that the metal ions are already contained in complex form in the wet processed phosphoric acid. Also, the extracting reagent is dissolved in an organic solvent from which it has to be separated after the extraction step. This solvent extraction method of removal of the precipitated metals makes the overall process very expensive and is restricted to use only for filtered or pre-purified phosphoric acid. Phase separation and crude formation are two more major problems.

[0007] Ion exchange processes have also been evaluated for removal of cadmium and other metals from phosphoric acid, but have only been found to remove about 50% of the cadmium present therein. Also these types of processes are quite expensive considering the capital investment and reagent costs.

[0008] U.S. Pat. No. 4,452,768 (Gradl, et al.) teaches the use of dialkyl dithiophosphoric acid ester to precipitate cadmium from wet process phosphoric acid followed by the adsorption of the resulting cadmium containing precipitate on a solid adsorbent such as active carbon or an aluminosilicate. This process suffers from two main disadvantages. (1) It requires very high dosages of the dithiophosphoric acid ester (see, the Examples in Gradl, et al. wherein the required dosages of reagents are in the range of 2-4 kg per 1000 kg of treated phosphoric acid); and (2) The adsorbent has to be regenerated by stripping, using for example, concentrated hydrochloric acid which makes it expensive and unattractive.

[0009] Another process for the liquid-liquid extraction of heavy metals from phosphoric acid has been described in U.S. Pat. No. 4,503,016 to Schimmel, et al. that provides for the heavy metals to be extracted from the phosphoric acid by means of a solution of a dialkyl dithiophosphoric acid ester. A disadvantage encountered with this process resides in the need to use phosphoric acid that has been freed from emulsion-forming organic contaminants, i.e. pre-purified. The liquid-liquid extraction is additionally rendered problematic by the fact that phase separation occurs very reluctantly only and that emulsified or dissolved dithiophosphoric acid ester is liable to be removed together with separated phosphoric acid so that it is generally necessary for the latter to be subjected to complementary after-treatment, i.e. to stripping or other methods (as described e.g. in European Patent Application No. 0016264).

[0010] European patent application 0333 489 B1 describes a process of simultaneously using dialkyl dithiophosphinate, a reducing agent and an adsorbent (either inherently present in or added to the solution) as a precipitant to remove cadmium from phosphoric acid. This process also suffers from its disadvantages such as high reagent dosage requirements, the necessity to use a reducing agent (such as iron powder, aluminum powder, hydrazine, red phosphorous, etc.), and an adsorbent (such as calcium sulfate, active carbon and aluminosilicates) all of which further add to the cost. Furthermore, the efficiency of the cadmium removal is not commercially satisfactory.

[0011] It is clear that the prior art processes suffer from many disadvantages, including the necessity of using high dosages of precipitating agents and thus high treatment costs; the use of capital intensive and sophisticated equipment; difficult operation in practice; lack of applicability of certain processes to all grades of phosphoric acid; difficulty in phase separation; and the use of additional reagents such as absorbents or reducing agents which add significantly to the overall treatment cost and which may have detrimental effects in downstream operations. Accordingly, there has been a need for an improved approach to removing heavy metals from phosphoric acid and other mildly acidic carrier solutions that overcomes the aforementioned disadvantages.

SUMMARY OF THE INVENTION

[0012] The present invention process and compositions for removing heavy metal ions, such as cadmium, copper, lead, nickel, arsenic, manganese and mercury ions from wet-process phosphoric acid overcomes the disadvantages associated with the prior art by providing a simple, single-step process that uses relatively small amounts of reagent. The process involves treating either the crude acid prior to gypsum filtration or the filtered acid with an organic precipitating agent composition, precipitating metals such as copper, cadmium, nickel, mercury, zinc, and separating the precipitate by either filtration or flotation, to produce phosphoric acid with reduced levels of said metals. The compositions of the present invention are comprised of a diorgano dithiophosphinic acid (or alkali metal or ammonia salts thereof), a first dithiophosphoric acid (or alkali metal or ammonia salts thereof) with alkyl or alkylaryl or aralkyl moieties, preferably derived from a secondary alcohol, and optionally a second diaryl dithiophosphoric acid (or alkali metal or ammonia salts thereof).

[0013] Compositions and the process according to the present invention provide an unexpected synergistic performance efficiency of metals removal when compared to the individual components used by themselves. Since the dosages used for the compositions and in the process according to the present invention are much lower than those required for individual components when used alone, the compositions are much safer to use and they have significantly reduced downstream effects in the phosphoric acid plant. Additionally, the compositions of the present invention perform well over a wide temperature range and remain liquid in the phosphoric acid phase, thereby permitting efficient dispersion in acid and efficient metal capture, the precipitated metal-reagent compounds can be very efficiently filtered, no metal compounds are released back into filtered phosphoric acid during washing of gypsum filter cake (stable precipitates), the precipitates formed are easy to filter even in the absence of any gypsum, and no additional reagents such as a reducing agent or absorbent agent is required for efficient metal removal. The present composition is water-soluble and, therefore, can be used with great ease and flexibility as water-solutions of any desired strength for high efficiency of metal removal.

DETAILED DESCRIPTION OF THE INVENTION

[0014] The process of the present invention provides for the removal or recovery of metal impurities from wet process phosphoric acid by using the metal-precipitating compositions according to the present invention. The present process comprises treating either the crude acid prior to gypsum filtration or the filtered acid with an amount of from about 0.1 to about 5.0 kgs per ton of phosphoric acid, of an organic precipitating agent composition comprising a diorgano dithiophosphinic acid (or salts thereof of alkali metals or ammonia) in an amount of from about 20 to about 70% and preferably from about 30 to about 40% by weight of the total and a first diorgano dithiophosphoric acid (or salts thereof of alkali metals or ammonia) in an amount of from about 30 to about 80% and preferably from about 50 to about 60% by weight of the total. The present composition optionally comprises a second diorgano dithiophosphoric acid (or salts thereof of alkali metals or ammonia), the amount being the remainder of the total of 100% after subtracting the amounts for the diorgano dithiophosphinic acid and the first diorgano dithiophosphoric acid. The metals present in the phosphoric acid, such as for example copper, cadmium, nickel, mercury, zinc, arsenic and manganese, are allowed to precipitate, and the precipitate is separated from the phosphoric acid by any known separation method in the art. Preferred separation methods include, but are not limited to filtration or flotation. Phosphoric acid with reduced levels of heavy metals is thereby produced.

[0015] A preferred diorgano dithiophosphinic acid (or salts thereof of alkali metals or ammonia) for use in the present invention is represented by the structure I 1

[0016] wherein R=linear or branched hydrocarbon group such as alkyl, aryl, alkylaryl, or aralkyl, containing 3-20 carbon atoms and M=H or an alkali metal or ammonia. Preferred examples of the hydrocarbon groups in the diorgano dithiophosphinic acid (or alkali metal or ammonia salts thereof) include, but are not limited to, linear or branched alkyl, cycloalkyl, alkylaryl, aralkyl having from 3-20 carbon atoms. More preferably, suitable hydrocarbon groups include, but are not limited to, cyclohexyl, isopropyl, isobutyl, n-propyl, octyl, hexyl, phenylethyl, and 2,4,4-trimethyl pentyl. In a preferred embodiment, the diorgano dithiophosphinic acid (or salt thereof) used in the present invention is diisobutyl dithiophosphinate.

[0017] A preferred first diorgano dithiophosphoric acid for use in the present invention is represented by the structure II 2

[0018] wherein R=a linear or branched hydrocarbon group such as alkyl, alkylaryl or aralkyl containing 5-20 carbon atoms and M=H or an alkali metal or ammonia. The first dithiophosphoric acid is preferably derived from a secondary alcohol. Examples of suitable hydrocarbon groups in the first diorgano dithiophosphoric acid (or salts thereof) include, but are not limited to, linear or branched alkyl, cycloalkyl, alkylaryl, aralkyl having 5-20 carbon atoms. More preferably, the hydrocarbon groups include, but are not limited to 4-methyl-2-pentyl and 3-methyl-2-pentyl. In a preferred embodiment, the first diorgano dithiophosphoric acid (or salt thereof) used in the present invention is di(4-methyl-2-pentyl) dithiophosphate.

[0019] A preferred second diorgano dithiophosphoric acid for use in the present invention is represented by the structure III 3

[0020] wherein R=a linear or branched hydrocarbon group such as alkylaryl or aralkyl, containing 6-20 carbon atoms and M=H or an alkali metal or ammonia. In a preferred embodiment the second dithiophosphoric acid is derived from primary alcohol and contains a benzene ring. Examples of the hydrocarbon groups in the second diorgano dithiophosphoric acid (or salts thereof) include, but are not limited to, linear or branched alkylaryl or aralkyl having 6-20 carbon atoms. In a preferred embodiment suitable hydrocarbon groups include, but are not limited to dicresyl, dinonylphenyl and diphenylethyl. In a preferred embodiment, the second diorgano dithiophosphoric acid (or salt thereof) used in the present invention is dicresyl dithiophosphate.

[0021] The metal precipitating compositions described herein can optionally be used in conjunction with other reagents known in the art, such as including, but not limited to a reducing agent and/or an absorbent. Reducing agents suitable for use include, but are not limited to, iron powder, aluminum powder, hydrazine and other reducing agents known in the art. Similarly, any suitable absorbent can be used optionally with the present invention, suitable absorbents include, but are not limited to, aluminosilicates (for example zeolites), gypsum, activated carbon and other absorbents known in the art.

[0022] The metal-precipitating agent compositions and the process of the present invention can be used over a wide temperature range, for example anywhere in the range of from about 10 to about 85° C. and preferably in the range of from about 50 to about 80° C. Treatment times for contacting the compositions with the phosphoric acid should preferably be from about 5 seconds to about 60 minutes; however, in those instances where the agents precipitate the metals very rapidly, the preferred treatment times are from about 5 seconds to about 5 minutes. In embodiments of the present invention, the treatment times are from about 10 seconds to about 60 seconds.

[0023] The dosage of the compositions of the present invention and the removal efficiency for the various metals will depend on the amount of metal impurities present in the phosphoric acid. Generally the greater the number of metals present and the higher their concentrations, the greater will be the overall dosage of the composition. Those skilled in the art will be able to readily establish the optimum dosage required. Generally the dosages may be in the range of 1-5 molar concentration based on the individual metal ions to be removed.

[0024] The phosphoric acid used in the present invention can be the crude acid after digestion containing gypsum solids, or filtered phosphoric acid of any concentration, or weak acid generated during washing of the gypsum filter cake. The concentration of the crude acid is typically 25-32% P2O5, the weak acid is 3-15% and the filtered acid 28-52%.

[0025] The compositions of the present invention can be added to the phosphoric acid all in one stage or added in several stages depending on the situation and metal removal efficiency that can be achieved. Since the compositions are completely soluble in water, they can be conveniently added to the phosphoric acid as water solutions of any strength to facilitate dispersion of the reagent in the phosphoric acid or slurry. Typically the solution strengths will be from about 1 to about 100%, preferably from about 2 to about 50%, and more preferably from about 5 to about 20%. In a preferred embodiment of the present invention, the composition is fed as a dilute solution for example a 5-10% solution. The dilute solution may disperse better in the phosphoric acid or slurry, thereby enhancing the capture of metals.

[0026] The precipitated metals can be removed from the phosphoric acid or slurry by any method(s) described in the art. These methods include, but are not limited to, filtration, precipitate flotation, froth flotation, liquid-liquid extraction and solvent extraction.

[0027] The following examples illustrate the invention that is naturally not limited thereto.

EXAMPLES

[0028] The general procedure used in all the examples is as follows. An amount of 500-1000 grams of crude phosphoric acid slurry containing 21 ppm cadmium is treated, while stirring in a reactor vessel, with the compositions described herein at different dosages and solution strengths, at a temperature in the range of 60-80° C. for various time intervals (5 seconds to 2 hours). Samples of the treated acid are taken at different time intervals, filtered and analyzed for residual metal content. The percent removal of cadmium is calculated based on the metal analysis of the filtered acid.

Examples A-F and 1-15

[0029] The general procedure outlined above is followed. The reagents are used as 5 or 10% solutions. The results for percent cadmium removal during the initial 30 seconds of treatment time for compositions of the present invention are compared in Table 1 with those of individual components and of prior art compositions. No other reagents, such as a reducing agent or an adsorbent, is added.

TABLE I
			Dosage,
		Dosage,	kg/ton	% Cd
Examples	Composition	mg/l P2O4	P2O4	Removal
Invention,
A	Mixture of di(4-methyl-2-pentyl) dithiophosphate	123	0.875	65
B	and diisobutyl dithiophosphinate, ratio 59/41	140	1	65
C		175	1.25	80
Invention,
D	Mixture of di(4-methyl-2-pentyl)	118	0.84	90
E	dithiophosphate, diisobutyl dithiophosphinate,	147	1.05	85
F	and dicresyl dithiophosphate, ratio 60/35/5	235	1.68	90
Control 1	Di(4-methyl-2-pentyl) Dithiophosphate	245	1.75	65
Control 2	Dicresyl Dithiophosphate	350	2.5	45
Control 3	Diisobutyl Dithiophosphinate	175	1.25	50
		245	1.75	55
Control 4	Di(2-ethylhexyl) dithiophosphate	350	2.5	50
Control 5	Diisobutyl Dithiophosphate	350	2.5	10
Control 6	D(sec-butyl) dithiophosphate	350	2.5	55
Control 7	Dinonylphenyl Dithiophosphate	350	2.5	40
Control 8	Mixture of dicresyl dithiophosphate and	175	1.25	55
	diisobutyl dithiophosphinate, ratio 39/61
Control 9	Mixture of di(2-ethylhexyl) dithiophosphate and	175	1.25	55
	diisobutyl dithiophosphinate, ratio 50/50
Control 10	Mixture of dinonylphenyl dithiophosphate and	175	1.25	45
	diisobutyl dithiophosphinate, ratio 59/41
Control 11	Mixture of di(sec-butyl) dithiophosphate and	175	1.25	50
	diisobutyl dithiophosphinate, ratio 50/50
Control 12	Mixture of diisobutyl dithiophosphate and	245	1.25	50
	diisobutyl dithiophosphinate, ratio 70/30
Control 13	Mixture of diisobutyl dithiophosphate and	245	1.75	55
	diisobutyl dithiophosphinate, ratio 50/50
Control 14	Mixture of diisobutyl dithiophosphate and	175	1.25	55
	diisobutyl dithiophosphinate, ratio 40/60
Control 15	Mixture of diisobutyl dithiophosphate and	245	1.75	60
	diisobutyl dithiophosphinate, ratio 30/70

[0030] The results in Table I demonstrate that with the compositions of the present invention cadmium removal is significantly increased at reduced dosages when compared to the required dosages for individual components. For example, in tests A-C, the mixture of di(4-methyl-2-pentyl) dithiophosphate and diisobutyl dithiophosphinate, in the ratio of 59/41, give cadmium removal in the amount of 65-80% at dosages in the range of 123-175 mg/liter of phosphoric acid slurry and 80% cadmium removal is achieved at dosages of 175 mg/l. Even at the lowest dosage of 123 mg/l, cadmium removal is 65%, showing poor performance at the same or even increased dosages. For example, di(4-methyl-2-pentyl) dithiophosphate alone removes 65% of cadmium at a dosage of 245 mg/l that is twice the dosage used for Example A at the same cadmium removal efficiency and diisobutyl dithiophosphinate removes only 50-55% cadmium at dosages of 175 and 245 mg/l.

[0031] Similarly, in examples D-F the mixture of di(4-methyl-2-pentyl) dithiophosphate, diisobutyl dithiophosphinate, and dicresyl dithiophosphate, in the ratio of 60/35/5 removes 85-90% of cadmium in the dosage range of 118-235 mg/l. Even at the lowest dosage of 118 mg/l, cadmium removal is 90%. Not only is the dosage requirement low for this ternary mixture, but cadmium removal efficiency is observed to be the highest. Dicresyl dithiophosphate alone removes 45% of cadmium at a dosage of 350 mg/l which is three times the dosage used in Example D.

[0032] A number of other individual precipitating agents and their mixtures with dithiophosphinate are tested (see Table I), but none of them give cadmium removal efficiency that was better than 65% at even the high dosage of 350 mg/l. The removal efficiencies for other metals such as for example copper and mercury are similar to, or better than, those reported for cadmium, depending on the type of metal.

Claims

1. A process for removing heavy metal ions from wet-process phosphoric acid, comprising the steps of: a) bringing crude wet-process phosphoric acid into contact with from about 0.1 to about 5.0 kgs per ton of phosphoric acid of a precipitating agent composition, said precipitating agent composition comprised of: 1) from about 20 to about 70 weight % of the composition of a diorgano dithiophosphinic acid or the alkali metal or ammonia salt thereof represented by structure I 4wherein R=linear or branched alkyl, aryl, alkylaryl, or aralkyl hydrocarbon group containing 3-20 carbon atoms and M=H, an alkali metal or ammonia, and 2) from 30 to about 80 weight % of the composition of a first diorgano dithiophosphoric acid or the alkali metal or ammonia salt thereof represented by structure II 5wherein R=a linear or branched alkyl, alkylaryl or aralkyl hydrocarbon group containing 520 carbon atoms and M=H, an alkali metal or ammonia; b) allowing heavy metals present in the wet process phosphoric acid to precipitate; and c) separating precipitated heavy metals from the phosphoric acid by filtration or flotation to produce phosphoric acid with reduced levels of said heavy metals.

2. The process according to claim 1, wherein said composition further comprises a second diorgano dithiophosphoric acid or the alkali metal or ammonia salt thereof in an amount that is the remainder of 100% after subtracting the amount of structures I and II present in the composition, said second diorgano dithiophosphoric acid represented by the structure III

3. The process according to claim 1, wherein R in Structure I is selected from the group consisting of cyclohexyl, isopropyl, iso-butyl, n-propyl, octyl, hexyl, phenylethyl, and 2,4,4-trimethyl pentyl.

4. The process according to claim 1, wherein R in Structure II is selected from the group consisting of 4-methyl-2-pentyl and 3-methyl-2-pentyl.

5. The process according to claim 2, wherein R in Structure III is selected from the group consisting of dicresyl, dinonylphenyl and diphenylethyl.

6. The process according to claim 1, wherein the first dithiophosphoric acid is derived from a secondary alcohol.

7. The process according to claim 1, wherein the heavy metal ions are selected from the group consisting of copper, cadmium, nickel, mercury, zinc, arsenic, manganese and combinations thereof.

8. The process according to claim 1, further comprising the step of filtering the wet process phosphoric acid before contacting it with the precipitating agent composition.

9. The process according to claim 2, wherein the dithiophosphoric acid is derived from a primary alcohol and contains a benzene ring.

10. The process according to claim 1, wherein the crude wet-process phosphoric acid is brought into contact with the precipitating agent composition for a time period of from about 5 seconds to about 60 minutes.

11. The process according to claim 10, wherein the time period is about 5 seconds to about 5 minutes.

12. The process according to claim 11, wherein the time period is from about 10 seconds to about 60 seconds.

13. The process according to claim 1, carried out at a temperature that is from about 10 to about 85° C.

14. The process according to claim 13, carried out at a temperature that is from about 50 to about 80° C.

15. A composition for removing heavy metal ions from wet-process phosphoric acid comprising: from about 20 to about 70 weight % of a diorgano dithiophosphinic acid or the alkali metal or ammonia salt thereof represented by structure I 7wherein R=linear or branched alkyl, aryl, alkylaryl, or aralkyl hydrocarbon group containing 3-20 carbon atoms and M=H, an alkali metal or ammonia, and from about 30 to about 80 weight % of a first diorgano dithiophosphoric acid or the alkali metal or ammonia salt thereof represented by structure II 8wherein R=a linear or branched alkyl, alkylaryl or aralkyl hydrocarbon group containing 5-20 carbon atoms and M=H, an alkali metal or ammonia.

16. The composition according to claim 15, further comprising a second diorgano dithiophosphoric acid or the alkali metal or ammonia salt thereof in an amount that is the remainder of 100 weight % after subtracting the amount of structures I and II present in the composition, said second diorgano dithiophosphoric acid represented by the structure III

17. The composition according to claim 1, wherein R in Structure I is selected from the group consisting of cyclohexyl, isopropyl, iso-butyl, n-propyl, octyl, hexyl, phenylethyl, and 2,4,4-trimethyl pentyl.

18. The composition according to claim 17, wherein Structure I is diisobutyl dithiophosphinate.

19. The composition according to claim 15, wherein R in Structure II is selected from the group consisting of 4-methyl-2-pentyl and 3-methyl-2-pentyl.

20. The composition according to claim 19, wherein Structure II is di(4-methyl-2-pentyl) dithiophosphate.

21. The composition according to claim 16, wherein R in Structure III is selected from the group consisting of dicresyl, dinonylphenyl and diphenylethyl.

22. The composition according to claim 21, wherein Structure III is dicresyl diothiophosphate.

23. The composition according to claim 15, wherein the diorgano dithiophosphinic acid (or alkali metals or ammonia salts thereof) is present in an amount that is from about 30 to about 40% by weight of the composition.

24. The composition according to claim 15, wherein the first diorgano dithiophosphoric acid (or alkali metals or ammonia salts thereof) is present in an amount that is from about 50 to about 60% by weight of the composition.

25. The composition according to claim 15, wherein the first dithiophosphoric acid is derived from a secondary alcohol.

26. The composition according to claim 15, wherein the second dithiophosphoric acid is derived from a primary alcohol and contains a benzene ring.