EP3385409B1

EP3385409B1 - Electrolytic sulphuric acid bath and method for tin electrorefining

Info

Publication number: EP3385409B1
Application number: EP18382212.1A
Authority: EP
Inventors: Raúl FIGUEROA MARTÍNEZ; Xosé Ramón NÓVOA RODRÍGUEZ
Original assignee: Estanos Y Soldaduras Senra SLU
Current assignee: Estanos Y Soldaduras Senra SLU
Priority date: 2017-04-04
Filing date: 2018-03-27
Publication date: 2019-11-27
Anticipated expiration: 2038-03-27
Also published as: EP3385409A1

Description

FIELD OF THE INVENTION

The present invention relates to the field of hydrometallurgical processes. More particularly, the present invention relates to the field of tin electrorefining.

BACKGROUND

The amount of waste from electronic products is increasing due to the rapid development of IT technology and faster life cycles of small electronic devices. For this reason, much attention has been raised on retrieving metal resources from wasted electronic devices which contain valuable metals. In particular, recovery of tin metal has been widely studied using various recycling methods since, after more than 50 years of using tin-lead (SnPb) solder as the primary means of interconnection between electronic devices, the electronics industry has been forced to adopt solders and termination finishes free of lead due to legal restrictions in hazardous substances (RoHS directive, February 2003).
Generally, two processes are used to recover tin metal from electronic waste: pyrometallurgical and hydrometallurgical treatments. The pyrometallurgical treatment consists on melting the tin-containing scrap and suffers from the main disadvantages of low efficiency and the high prime costs due to high amounts of energy needed to keep high temperature. On the other hand, hydrometallurgical treatment consists of the use of aqueous chemistry for the recovery of metals from the residual materials by electrodeposition and includes electrowining and electrorefining processes. Electrowinning is the primary extraction of a metal from a residual material that has been put in solution via a process commonly referred to as leaching. Electrorefining is the subsequent refining of tin to high purity. Both operations are accomplished in an electrolytic cell which comprises two electrodes immersed in an ionically conducting liquid (aqueous electrolyte) containing tin metal dissolved as positive ions. At the negative charged cathode, the tin cations are reduced and deposit as neutral tin atoms.
Tin can be electrodeposited from various electrolytes. In particular, acid solutions are usually preferred since alkaline solutions require higher temperatures and double specific charge. In the past ten years, the sulfonic acid was favoured as electrolyte solution because of the low corrosivity and nice ability to dissolve metallic impurities that are insoluble in other organic or mineral acids, such as lead. Indeed, most of the tin electrodeposition plants in the world used the FERROSTAN® process, which is based in phenolsulfonic acid as electrolyte, and the RONASTAN® process which is based in methanesulfonic acid as electrolyte. However, compared with sulfonic acid, sulphuric acid has many advantages, including its environmental friendliness and relatively low cost. Therefore, sulphuric acid-based electrolytes for tin electrodeposition have attracted growing interest.
However, tin electrodeposition using sulphuric acid suffer from deposition of tin species in the shape of needles, whiskers and dendrites on the surface of the anode, which is commonly known as "dendritic growth" (see Figure 1(a), Figure 2 and Figure 4(c) and 4(d)), and which lead to decrease of tin electrodeposition efficiency. Consequently, organic additives are necessary if smooth, shiny and dense films of tin metal are desired.
Additive examples may include surfactants to promote the electrode reaction, oxidation inhibitors to reduce the formation rate of stannic ions (Xiao et al. Inter. J. Minerals, Metallurgy and Materials, 2013, 20(5), 472), grain refiners such as tartrate (Rockwell et al. Thin Solid Films, 2008, 516 (21), 7361) or formaldehyde and polyoxyethylene octylphenol ether (Xiao et al. Mater. Prot., 2011, 44(1), 1) to avoid dendritic growth and brighteners such as pyridine and quinoline compounds ( US 4,000,047 A1 ) to obtain matte or bright deposits.
However, due to the increased demand in electronics for tin of the highest purity (≥ 99.9%), there is still a need for developing new electrolytic baths which provide enhanced performance, among other benefits.

SUMMARY OF THE INVENTION

The object of the present invention is the provision of an electrolytic sulphuric acid bath which provide enhanced tin electrorefining performance. In particular, the sulphuric acid bath of the present invention is characterized by a combination of additives comprising gelatine and a compound of general formula (I) such as bisphenol sulfone, di-tolyl sulfoxide, or a derivate thereof. The authors of the present invention have observed that when only gelatine is used as additive in an electrolytic sulphuric acid bath, the electrorefined tin metal is mostly porous (see Figure 1(b) and Figure 2(b)), which results in a violent oxidation with atmospheric air, which in turn decreases the final yield of tin electrorefining. In addition, when bisphenol sulfone is used alone as additive, since its solubility in aqueous medium is very low, vigorous agitation is required. Moreover, suspended additive particles get trapped at the cathode which contaminates the tin electrodeposition decreasing the tin electrorefining performance (see Figure 2(c)). However, the authors of the present invention have surprisingly found that when small amounts of gelatine and a compound of general formula (I) are combined as additives in an electrolytic sulphuric acid bath, a tin deposit of very good characteristics (morphology and purity) is obtained (see Figure 1(c), Figure 3 and Figure 4(a) and (b)).
Accordingly, in a first aspect, the present invention relates to an electrolytic sulphuric acid bath for tin electrorefining which comprises:

a sulphuric acid solution,
a source of Sn(II) ions,
an anode comprising the target tin to be refined,
a cathode,
gelatine, and
a compound of general formula (I)
wherein
- n is 1 or 2,
- each R independently stands for halogen, alkyl, cycloalkyl, aryl, aralkyl, hydroxyl, alkoxy, allyloxy, carboxyl, or carboalkoxy group; and
- wherein a and b are independently selected from an integer of from 0 to 3, and adjacent Rs may be combined to form a ring.

In a second aspect, the present invention is directed to a method for tin electrorefining comprising the application of a current to the electrolytic sulphuric acid bath as defined above.
In a further aspect, the present invention is directed to the use of gelatine and a compound of general formula (I) as a combination of additives for an electrolytic sulphuric acid bath for tin electrorefining.
The present invention allows a good electrical efficiency of tin deposition, keeping impurities in the tin deposit in the ppm range and thus, obtaining an electrorefined tin having high purity reaching or being greater than 99.9%, which fulfils the requirements for electronic applications. Besides the resulting deposited tin shows a lower porosity than that obtained without additives, which limits its oxidation in the atmosphere.
These aspects and preferred embodiments thereof are additionally also defined hereinafter in the detailed description and in the claims.

DESCRIPTION OF THE DRAWINGS

To better understand the invention, its objects and advantages, the following figures are attached to the specification in which the following is depicted:

Figure 1 : Morphologies of the electroplated tin from 0.1M SnSO₄ solution at different magnifications: a) without additives; b) bath with 1g/L of gelatine as additive; and c) bath with 0.2 g/L gelatine and 0.4 g/L bisphenol sulfone as additives.
Figure 2 : Macrographs of electroplated tin showing dendrite growth: a) without additives; b) bath with 1g/L of gelatine as additive; and c) bath with 3 g/L of bisphenol sulfone as additive.
Figure 3 : Macrograph of electroplated tin showing good deposition with big grains using a bath with 0.2 g/L of gelatine and 0.4 g/L of bisphenol sulfone as additives.
Figure 4 : Morphologies of the electroplated tin from 0.1M SnSO₄ solution at different magnifications: a) and b) electroplated tin showing good deposition using a bath with with 0.2 g/L gelatine and 0.15 g/L di-p-tolyl sulfoxide; c) and d) tin electroplated without additives showing dendrite growth.

DETAILED DESCRIPTION OF THE INVENTION

Unless defined otherwise, all technical and scientific terms and expressions used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs.
A first aspect of the invention is directed to an electrolytic sulphuric acid bath suitable for tin electrorefining which comprises:

a sulphuric acid solution,
a source of Sn(II) ions,
an anode comprising the target tin to be refined,
a cathode,
gelatine, and
a compound of general formula (I) as defined above.

The term "electrolytic bath" refers to a chamber (for example, a cell) comprising an electrically conducting solution (electrolyte) that generally contains ions, atoms or molecules that have lost or gained electrons when they are dissolved in a polar solvent, such as water, and two electrodes (anode and cathode) immersed in the conducting solution. In the present invention, the electrically conducting solution is an aqueous solution of sulphuric acid.
The electrolytic sulphuric acid bath of the present invention is suitable for tin electrorefining. The term "electrorefining" or "electrolytic refining" refers to refining of a metal (tin, in the context of the present invention) by electrolysis where the target material containing the metal is used as the anode going into an electrically conducting solution and the refined metal being deposited upon the cathode.
As defined above, the electrolytic sulphuric acid bath of the present invention comprises a sulphuric acid solution which comprises an aqueous solution of sulphuric acid to produce a sufficiently high acidity to avoid Sn²⁺ hydrolysis to Sn⁴⁺ which, otherwise precipitates, lowering the process yield. The presence of sulphate ions in the electrolytic sulphuric acid bath helps the removal of impurities, such as Pb and Sb, by precipitation at the anode or at the bottom of the bath, as insoluble salts. As a consequence, an excess of the acid improves the elimination of certain impurities.
In a preferred embodiment, the sulphuric acid solution has a pH below or equal to 1.
A non-limitative example of a sulphuric acid solution suitable for the electrolytic bath of the present invention can be obtained by dissolving between about 30 g and about 200 g of 96% H₂SO₄ in 1 L of water, preferably about 100 g in 1 L.
In addition, an initial concentration of stannous ions (Sn²⁺) in the electrolytic sulphuric acid bath of the present invention is necessary in order to start (induce) the tin deposition. Thus, the electrolytic sulphuric acid bath of the present invention further comprises a source of Sn(II) ions.
In a preferred embodiment, the Sn(II) ions are in a concentration between about 0.05 M and about 0.1 M.
A non-limitative example of a source of Sn(II) ions suitable for the electrolytic sulphuric acid bath of the present invention is a tin salt, such as tin sulphate, directly dissolved in the sulphuric acid solution to produce Sn²⁺ positive ions.
In a preferred embodiment, the source of Sn(II) ions is tin sulphate (SnSO₄), since it contributes simultaneously to increase the concentration of Sn(II) ions as well as of sulphate ions, thus favouring the precipitation of impurities such as Pb.
The electrolytic sulphuric acid bath of the present invention further comprises an anode comprising the target tin to be refined.
Non-limitative examples of anodes suitable in the electrolytic sulphuric acid bath of the present invention include anodes made of tin-based alloys like antifriction alloys (also known as white metal or Babbitt metal), pewter alloys, soft solder alloys or combinations thereof.
In the context of the present invention, the term "tin-based alloy" refers to alloys containing high percentages of Sn as well as minor percentages of Fe, Ni, Cd, Bi, Zn, As, Ge and In, among others.
In particular, the term "antifriction alloy" or "white metal" or "Babbitt metal" refers to alloys employed for machine parts which operate with low or sliding friction and preferably contains more than about 80% of tin.
In the context of the present invention, the term "pewter alloy" refers to alloys preferably containing between about 90% and about 95% of tin, as well as between about 1% and about 3% Cu with Sb as balance.
The term "soft solder alloy" refers to alloys preferably containing more than 50% of tin in the form of binary or ternary alloys such as Sn-Pb, Sn-Cu, Sn-Ag, Sn-Bi or Sn-Ag-Cu.
The electrolytic sulphuric acid bath of the present invention further comprises, gelatine and a compound according to the general formula (I) as defined above as additives. These additives may be added to the bath either previously premixed or not.
The term "gelatine", as the skilled person knows, refers to a mixture of peptides and proteins produced by partial hydrolysis of collagen extracted from the skin, bones, and connective tissues of animals such as domesticated cattle, chicken, pigs, and fish.
A non-limitative example of commercial gelatine suitable in the electrolytic sulphuric acid bath of the present invention is gelatine with high-protein polymers of amino acids linked by peptide chains (--CO--NH--), and having molecular weights in the range of 10,000 to 300,000. In a preferred embodiment, animal glue is used as gelatine additive since it is relatively inexpensive, commercially available and convenient to handle.
As mentioned above, the electrolytic sulphuric acid bath of the present invention further comprises compounds represented by the general formula (I)
wherein

n is 1 or 2,
each R independently stands for halogen, alkyl, cycloalkyl, aryl, aralkyl, hydroxyl, alkoxy, allyloxy, carboxyl, or carboalkoxy group; and
wherein a and b are independently selected from an integer of from 0 to 3, and adjacent Rs may be combined to form a ring.

As the skilled person readily appreciates, when a or b is more than 1, each R group must be bound to a different carbon atom of the benzene ring.
In the compound of formula (I) the groups can be selected in accordance with the following guidance:
The term "halogen" refers to bromine, chlorine, iodine or fluorine.
The term "alkyl" refers to a linear or branched alkane derivative containing from 1 to 12, preferably from 1 to 6, carbon atoms and which is bound to the rest of the molecule through a single bond. Illustrative examples of alkyl groups include methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, t-butyl, pentyl, hexyl, etc.
The term "cycloalkyl" refers to a radical derived from cycloalkane containing from 3 to 7, preferably from 3 to 6 carbon atoms. Illustrative examples of cycloalkyl groups include cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, etc.
The term "aryl" refers to an aromatic group having between 6 and 10, preferably 6 or 10 carbon atoms, comprising 1 or 2 aromatic nuclei fused to one another. Illustrative examples of aryl groups include phenyl, naphthyl, indenyl, phenanthryl, etc. Preferably, it is phenyl.
The term "aralkyl" refers to an alkyl group as defined above substituted with an aryl group as defined above. Examples of such groups include benzyl, phenylethyl, phenylpropyl, naphthylmethyl, etc. Preferably, it is benzyl.
The term "alkoxy" refers to a radical of the formula -O-alkyl, where alkyl has been previously defined, e. g., methoxy, ethoxy, propoxy, etc.
The term "allyloxy" refers to a radical having the formula: -O-CH₂-CH=CH₂.
The term "carboxyl" refers to a radical having the formula -C(=O)OH.
The term "carboalkoxy" refers to a radical having the formula -C(=O)O-alkyl, wherein alkyl is as defined above. Suitable carboalkoxy groups include groups such as methyl carboxylate, ethyl carboxylate, propyl carboxylate, butyl carboxylate, etc.
As understood in this technical area, there may be a certain degree of substitution in the aforementioned radicals. Therefore, the previous groups can be substituted in one or more available positions with one or more substituents.
General formula (I) comprises both symmetric and non-symmetric sulfones and sulfoxides. Preferably, the compound of general formula (I) is symmetric.
In a preferred embodiment, the compound of general formula (I) is selected from the group consisting of:

Bisphenyl sulfone (diphenyl sulfone);
Bisalkylphenyl sulfones including but not limited to 2,2'-dialkyldiphenyl sulfones (such as di-o-tolyl sulfone) and 4,4'-dialkylphenyl sulfones (such as di-p-tolyl sulfone);
Bishalogenphenyl sulfones including but not limited to 2,2'-dihalogendiphenyl sulfones (such as bis(2-chlorophenyl) sulfone and bis(2-fluorophenyl) sulfone) and 4,4'-dihalogendiphenyl sulfones (such as bis(4-chlorophenyl) sulfone and bis(4-fluorophenyl) sulfone);
Bisphenol sulfones including but not limited to 2,2'-diphenol sulfone and 4,4'-diphenol sulfone;
Bisphenol sulfones derivatives i.e. having additional substitutions at the benzene rings (i.e. additional Rs) including but not limited to 2,2'-dihydroxydialkyldiphenyl sulfones (such as 2,2'-dihydroxy-3,3'-dialkyldiphenyl sulfones, 2,2'-dihydroxy-4,4'-dialkyldiphenyl sulfones, 2,2'-dihydroxy-5,5'-dialkyldiphenyl sulfones, 2,2'-dihydroxy-6,6'-dialkyldiphenyl sulfones), 2,2'-dihydroxydihalogendiphenyl sulfones (such as 2,2'-dihydroxy-3,3'-dihalogendiphenyl sulfones, 2,2'-dihydroxy-4,4'-dihalogendiphenyl sulfones, 2,2'-dihydroxy-5,5'-dihalogendiphenyl sulfones, 2,2'-dihydroxy-6,6'-dihalogendiphenyl sulfones), 2,2'-dihydroxydihydroxydiphenyl sulfones (such as 2,2'-bis(3-hydroxyphenol) sulfone, 2,2'-bis(4-hydroxyphenol) sulfone, 2,2'-bis(5-hydroxyphenol) sulfone, 2,2'-bis(6-hydroxyphenol) sulfone), 4,4'-dihydroxydialkyldiphenyl sulfones (such as 4,4'-dihydroxy-2,2'-dialkyldiphenyl sulfones, 4,4'-dihydroxy-3,3'-dialkyldiphenyl sulfones, 4,4'-dihydroxy-5,5'-dialkyldiphenyl sulfones, 4,4'-dihydroxy-6,6'-dialkyldiphenyl sulfones), 4,4'-dihydroxydihalogendiphenyl sulfones (such as 4,4'-dihydroxy-2,2'-dihalogendiphenyl sulfones, 4,4'-dihydroxy-3,3'-dihalogendiphenyl sulfones, 4,4'-dihydroxy-5,5'-dihalogendiphenyl sulfones, 4,4'-dihydroxy-6,6'-dihalogendiphenyl sulfones), 4,4'-dihydroxydihydroxydiphenyl sulfones (such as 4,4'-bis(2-hydroxyphenol) sulfone, 4,4'-bis(3-hydroxyphenol) sulfone, 4,4'-bis(5-hydroxyphenol) sulfone, 4,4'-bis(6-hydroxyphenol) sulfone).

Non-limitative examples of bisphenol sulfones of general formula (I) suitable for the electrolytic sulphuric acid bath of the present invention also include:

2,2'-bisphenol sulfones and derivatives thereof such as 2,2'-diphenol sulfone, 2,2'-bis(4-methylphenol) sulfone, 2,2'-bis(6-methylphenol)sulfone, 2,2'-bis(4-iso-propylphenol) sulfone, 2,2'-bis(4-n-butylphenol) sulfone, 2,2'-bis(4-sec-butylphenol) sulfone, 2,2'-bis(4-tertbutylphenol) sulfone, 2,2'-bis(6-tert-butylphenol) sulfone, 2,2'-bis(4-tert-amylphenol) sulfone, 2,2'-bis(4-tert-octylphenol) sulfone, 2,2'-bis(4-nonylphenol) sulfone, 2,2'-bis(4-tert-butyl-6-methylphenol) sulfone, 2,2'-bis(4-methyl-6-tertbutylphenol) sulfone, 2,2'-bis(4,6-dimethylphenol) sulfone, 2,2'-bis(4,6-ditert-butylphenol) sulfone, 2,2'-bis(4,5-dimethylphenol) sulfone, 2,2'-bis(4-cyclohexylphenol) sulfone, 2,2'-bis(4-cyclohexyl-6-methylphenol) sulfone, 2,2'-bis(4,6-dicyclohexyl-phenol) sulfone, 2,2'-bis(4-α,α'-dimethyl-benzylphenol) sulfone, 2,2'-bis(4-benzylphenol) sulfone, 2,2'-bis(4,6-dibenzylphenol) sulfone, 2,2'-bis(4-phenylphenol) sulfone, 2,2'-bis(4-phenyl-6-methylphenol) sulfone, 2,2'-bis(4-α,α'-dimethylbenzyl-6-phenylphenol) sulfone, 2,2'-bis(4-chlorophenol) sulfone, 2,2'-bis(4,6-dichlorophenol) sulfone, 2,2'-bis(4,5,6-trichlorophenol) sulfone, 2,2'-bis(4-bromophenol) sulfone, 2,2'-bis(4,6-dibromophenol) sulfone, 2,2'-bis(4-hydroxyphenol) sulfone, 2,2'-bis(4,6-dimethoxyphenol) sulfone, 2,2'-bis(4-carboxyphenol) sulfone, 2,2'-bis(4-carbomethoxyphenol) sulfone, 2,2'-bis(4-carbobutoxyphenol) sulfone, 1,1'-bis(2-naphthol) sulfone, 2,2'-bis(1-naphthol) sulfone, 2,2'-dihydroxy-5,5'-dimethyldiphenyl sulfone, 2,2'-dihydroxy-5,5'-diethyldiphenyl sulfone, 2,2'-dihydroxy-5,5'-dipropyldiphenyl sulfone, 2,2'-dihydroxy-5,5'-dibutyldiphenyl sulfone, 2,2'-dihydroxy-5,5'-dipentyldiphenyl sulfone, 2,2'-dibydroxy-5,5'-dihexyldiphenyl sulfone, 2,2'-dihydroxy-5,5'-diheptyldiphenyl sulfone, 2,2'-dihydroxy-5,5'-dioctyldiphenyl sulfone, 2,2'-dihdrox-5,5'-dinonyldiphenyl sulfone, 2,2'-dihydroxy-5,5'-didecyldiphenyl sulfone, 2,2'-dihydroxy-5,5'-diunndecyldiphenyl sulfone, 2,2'-dihydroxy-5,5'-didodecyldiphenyl sulfone and the like; and
4,4'-bisphenol sulfones and derivatives thereof such as 4,4'-diphenol sulfone, 4,4'-bis(2-chlorophenol) sulfone, 4,4'-bis(3-chlorophenol) sulfone, 4,4'-bis(2-methylphenol) sulfone, 4,4'-bis(3-methylphenol) sulfone, 4,4'-bis(2,5-dimethylphenol) sulfone, 4,4'-bis(2-isopropyl-5-methylphenol) sulfone, 4,4'-bis(2-methyl-6-tertbutylphenol) sulfone, 4,4'-bis(2,6-tert-butylphenol) sulfone, 4,4'-bis(2-hydroxyphenol) sulfone, 4,4'-bis(2-tert-butylphenol) sulfone, 4,4'-bis(2-cyclohexylphenol) sulfone, 4,4'-bis(2-tert-butyl-5-methylphenol) sulfone, 4,4'-bis(2-benzylphenol) sulfone, 4,4'-bis(2-methoxyphenol) sulfone, 4,4'-bis(2-phenoxyphenol) sulfone, 4,4'-bis(2-carboxyphenol) sulfone, 4,4'-bis(2-carbomethoxyphenol) sulfone and the like.

Non-limitative examples of compounds of general formula (I) suitable for the electrolytic sulphuric acid bath of the present invention also include the sulfoxide homologues of the compounds defined above such as bisphenyl sulfoxide, di-o-tolyl sulfoxide, di-p-tolyl sulfoxide, bis(2-chlorophenyl) sulfoxide, bis(2-fluorophenyl) sulfoxide), bis(4-chlorophenyl) sulfoxide, bis(4-fluorophenyl) sulfoxide, 2,2'-diphenol sulfoxide and 4,4'-diphenol sulfoxide.
In a preferred embodiment, in the compound of general formula (I) a and b are independently selected from 0, 1 or 2. In a more preferred embodiment, in the compound of general formula (I) a and b are 0, 1 or 2 wherein each R independently stands for halogen, alkyl or hydroxyl.
In another more preferred embodiment, the compound of general formula (I) is selected from the group consisting of bisphenyl sulfone (diphenyl sulfone); dialkyldiphenyl sulfones including but not limited to 2,2'- and 4,4'-dialkyldiphenyl sulfones (such as di-o-tolyl sulfone and di-p-tolyl sulfone); dihalogendiphenyl sulfones including but not limited to 2,2'- and 4,4'-dihalogendiphenyl sulfones (such as bis(2-chlorophenyl) sulfone, bis(2-fluorophenyl) sulfone, bis(4-chlorophenyl) sulfone and bis(4-fluorophenyl) sulfone); bisphenol sulfones including but not limited to 2,2'- and 4,4'-bisphenol sulfones (such as 2,2'-diphenol sulfone and 4,4'-diphenol sulfone) and derivatives thereof selected from alkyl-substituted bisphenol sulfones (such as 2,2'-dihydroxy-3,3'-dialkyldiphenyl sulfones, 2,2'-dihydroxy-4,4'-dialkyldiphenyl sulfones, 2,2'-dihydroxy-5,5'-dialkyldiphenyl sulfones, 2,2'-dihydroxy-6,6'-dialkyldiphenyl sulfones, 4,4'-dihydroxy-2,2'-dialkyldiphenyl sulfones, 4,4'-dihydroxy-3,3'-dialkyldiphenyl sulfones, 4,4'-dihydroxy-5,5'-dialkyldiphenyl sulfones, 4,4'-dihydroxy-6,6'-dialkyldiphenyl sulfones),
halogen-substituted bisphenol sulfones (such as 2,2'-dihydroxy-3,3'-dihalogendiphenyl sulfones, 2,2'-dihydroxy-4,4'-dihalogendiphenyl sulfones, 2,2'-dihydroxy-5,5'-dihalogendiphenyl sulfones, 2,2'-dihydroxy-6,6'-dihalogendiphenyl sulfones, 4,4'-dihydroxy-2,2'-dihalogendiphenyl sulfones, 4,4'-dihydroxy-3,3'-dihalogendiphenyl sulfones, 4,4'-dihydroxy-5,5'-dihalogendiphenyl sulfones, 4,4'-dihydroxy-6,6'-dihalogendiphenyl sulfones),
hydroxy-substituted bisphenol sulfones (such as 2,2'-bis(3-hydroxyphenol) sulfone, 2,2'-bis(4-hydroxyphenol) sulfone, 2,2'-bis(5-hydroxyphenol) sulfone, 2,2'-bis(6-hydroxyphenol) sulfone, 4,4'-bis(2-hydroxyphenol) sulfone, 4,4'-bis(3-hydroxyphenol) sulfone, 4,4'-bis(5-hydroxyphenol) sulfone, 4,4'-bis(6-hydroxyphenol) sulfone).
In the aforementioned lists of compounds are also included their sulfoxide homologues.
In the aforementioned lists of compounds alkyl is preferably selected from methyl, ethyl, propyl and butyl and halogen is preferably selected from Br, Cl and F.
In an even more preferred embodiment, the compound of general formula (I) is di-p-tolyl sulfoxide or 4,4'-diphenol sulfone (also known as 4,4'-sulfonylbisphenol, bisphenol S or BPS). In the present application, this latter compound is sometimes simply referred to as bisphenol sulfone.
The authors of the present invention have surprisingly observed that the addition of small amounts of gelatine and a compound of general formula (I) in the sulphuric acid solution thereof causes a positive synergistic effect on the tin deposition process. Compounds of general formula (I) are not soluble in aqueous medium and therefore a good efficiency of this additive is not possible. Without wishing to be bound by any particular theory, it is believed that the use of a small amount of gelatine improves the solubility of compounds of general formula (I) and, as a result, the dendritic growth is completely blocked. In addition, a grain refinement respect to the deposit obtained with gelatine alone is achieved. As a consequence, less porous tin is obtained, which avoids the violent oxidation explained above. Thus, the synergistic combination of both additives produces a tin deposit of very good characteristics (purity and morphology).
In a preferred embodiment, the mixture of additives of the electrolytic sulphuric acid bath of the present invention comprises gelatine in a concentration between about 0.05 g/L and about 3 g/L and a compound of general formula (I) in a concentration between about 0.05 g/L and about 1 g/L, preferably it comprises gelatine in a concentration between about 0.1 g/L and about 3 g/L and a compound of general formula (I) in a concentration between about 0.1 g/L and about 1 g/L, more preferably it comprises gelatine in a concentration between about 0.1 g/L and about 1 g/L and a compound of general formula (I) in a concentration between about 0.2 g/L and about 0.4 g/L, even more preferably it comprises gelatine in a concentration between about 0.1 g/L to about 0.2 g/L and a compound of general formula (I) in a concentration between about 0.2 g/L and about 0.4 g/L.
As defined above, the electrolytic sulphuric acid bath of the present invention further comprises a cathode where the deposition of tin takes place.
Non-limitative examples of cathodes suitable in the electrolytic sulphuric acid bath of the present invention include cathodes made of raw tin or, in order to reduce costs, a material as copper or stainless steel that resists the aggressiveness of the electrolytic acid medium. For example, it has been observed that a sheet of commercial stainless steel "AISI 316" produces good results without being attacked by the sulphuric acid solution.
Another aspect of the present invention is directed to a method for electrorefining of tin comprising the application of a current to the electrolytic sulphuric acid bath as defined above.
In the method of the present invention, the operating current must be adjusted to carry out the selective oxidation of tin at the anode and its deposition at the cathode and without contamination of the tin deposit by other elements present in the anode. This is due to the fact that common metallic impurities present in tin-containing waste are those with standard reduction potentials very close to Sn (-0.14 V) such as Pb (-0.13 V), In (-0.14 V) and Sb (0.15 V).
In addition, the current to be applied depends on the geometry of the electrolysis cell. The term "current density" refers to the electric current per unit area of a cross section. In SI units, the electric current density is measured in amperes per square metre.
In a preferred embodiment, the current applied in the method of the present invention has a density between about 100 A/m² and about 150 A/m². Nevertheless, the selection of the most appropriate current in each individual case may be determined by one of ordinary skill in the art using routine experimentation.
As a result of the method as define above, the tin at the anode is selectively oxidized and deposited onto the surface of the cathode as a tin layer. The resulting tin layer can be typically peeled off easily from that surface.
Thus, the combined use of gelatine and a compound of general formula (I) as additives in an electrolytic sulphuric acid bath for tin electrorefining according to the present invention provides a synergistic effect allowing a good electrical efficiency of tin deposition, keeping impurities in the tin deposit in the ppm range and thus, obtaining an electrorefined tin having a purity equal or greater than 99.9%, even 99.99% or even 99.999%. This purity fulfils requirements for electronic applications.
As used herein, the term "about" means a slight variation of the value specified, preferably within 10 percent of the value specified. Nevertheless, the term "about" can mean a higher tolerance of variation depending on for instance the experimental technique used. Said variations of a specified value are understood by the skilled person and are within the context of the present invention. Further, to provide a more concise description, some of the quantitative expressions given herein are not qualified with the term "about". It is understood that, whether the term "about" is used explicitly or not, every quantity given herein is meant to refer to the actual given value, and it is also meant to refer to the approximation to such given value that would reasonably be inferred based on the ordinary skill in the art, including equivalents and approximations due to the experimental and/or measurement conditions for such given value.

EXAMPLES

The present invention will now be described by way of examples which serve to illustrate the construction and testing of illustrative embodiments. However, it is understood that the present invention is not limited in any way to the examples below.

Example 1. Tin electrorefining with sulphuric acid electrolytic solutions comprising gelatine and 4,4'-sulfonylbisphenol (BPS).

An experiment was performed to evaluate electrolytic solutions including sulphuric acid and a mixture of gelatin and BPS as additives. The electrolytic solution included 5.4% volume percent sulphuric acid in deionized water. The tin sulfate was dissolved into the sulphuric acid electrolyte to a concentration of 20 g/L which acts as starting electrodeposition bath. The two additives, gelatine and BPS, were added to this electrolytic solution. The gelatine had a concentration of 0.2 g/L in the electrolytic solution. BPS had a concentration of 0.4 g/L in the electrolytic solution.
The electrolysis system included a 1000 L polypropylene tank equipped with a vertical pump for solution agitation and filtration. The system also included a stainless steel cathode, and two anodes of tin alloy placed alternatively, and a DC power supply connected to the cathode and anodes. Electrolysis was performed at room temperature.
The electrorefining parameters were adjusted in this experiment as follows: sulphate ions (SO₄ ^2-) concentration in the electrolytic solution (107 g/L); Sn(II) ions concentration in the electrolytic solution (12 g/L); current density 150 A/m².
The composition of the tin alloy used as anode was a Babbitt alloy and it was analysed using SPECTROMAXx Arc Spark Optical Emission Spectrometry (Spark OES) is shown in Table 1. Table 1: Spark OES analysis of the tin alloy used as anode (Babbitt alloy)

Element Percentage (%)

Sn 80.29

Sb 11.19

Cu 5.92

Pb 2.16

Ag 0.127

Fe 0.083

Ni 0.06

Cd 0.016

Bi 0.022

Zn 0.002

As 0.128

In 0.002

The electrolysis was performed for 7 days. After electrorefining, the refined tin was harvested from the cathodes and cast to produce refined tin samples. The refined tin samples were analyzed after casting using Spark OES and for trace elements using Varian's Vista Pro ICP-AES. The results are presented in Table 2 below.

Table 2: Analysis of electrodeposited tin in the cathode

Element	Percentage (%)*
Sn	99.9991
Ag	< 0.0002
Al	<0.0001
As	<0.0007
Au	< 0.0001
Bi	0.0002
Cd	< 0.0002
Co	<0.0003
Cu	0.0005
Fe	0.0002
Ge	0.0002
In	< 0.0001
Ni	0.0003
P	<0.0005
Pb	<0.0003
Pd	0.0001
Sb	0.0001
Zn	<0.0001

*The values that fall below the limit of detection are preceded by "<" symbol.

Therefore, by adding the mixture of gelatine and BPS to the sulphuric acid electrolyte, the present inventors were able to achieve impurities removal and good morphology as shown in Figure 1(c) and Figure 3. The electrorefined tin has high purity and can be melted open to air without significant oxidation.

Example 2. Tin electrorefining with sulphuric acid electrolytic solutions comprising gelatine and di-p-tolyl sulfoxide.

An experiment was performed to evaluate electrolytic solutions including sulphuric acid and a mixture of gelatin and di-p-tolyl sulfoxide as additives. The electrolytic solution employed was 5.4% v/v H₂SO₄ in deionized water. The starting electrodeposition bath was obtained dissolving tin sulfate in the sulphuric acid electrolyte to 20 g/L concentration. The two additives, gelatine and di-p-tolyl sulfoxide, were added to this electrolytic solution. 0.1 g/L being the gelatine concentration and 0.2 g/L the di-p-tolyl sulfoxide concentration.
The electrolysis system included a 1000 L polypropylene tank equipped with a vertical pump for solution agitation and filtration, a stainless steel cathode, and two tin anodes for homogeneous current distribution and tin(II) supply. A DC power supply provided the required current. Electrolysis was performed at room temperature.
The electrorefining parameters were adjusted in this experiment as follows: [SO₄ ^2-] = 107 g/L); [Sn²⁺] =12 g/L; I= 150 A/m².
A Babbitt alloy was used as anode. The alloy was analysed using SPECTROMAXx Arc Spark Optical Emission Spectrometry (Spark OES). The results are shown in Table 3. Table 3: Spark OES analysis of the tin alloy used as anode (Babbitt alloy)

Element Percentage (%)

Sn 80.47

Sb 9.1

Cu 6.75

Pb 3.58

Ag 0.02

Fe 0.05

Ni 0.002

Cd 0.003

Bi 0.019

Zn <0.002

As 0.081

In 0.002

The deposited tin, mechanically harvested from the cathode, was cast to produce tin chips for Spark OES analysis using a Varian's Vista Pro ICP-AES. The results are summarised in Table 4 below.

Table 4: Analysis of electrodeposited tin in the cathode

Element	Percentage (%)*
Sn	99.9985
Ag	< 0.0002
Al	<0.0001
As	<0.0007
Au	< 0.0001
Bi	0.0003
Cd	< 0.0002
Co	<0.0003
Cu	0.0005
Fe	0.0002
Ge	0.0002
In	< 0.0001
Ni	0.0001
P	<0.0005
Pb	<0.0003
Pd	0.0001
Sb	0.0001
Zn	<0.0001

*The values that fall below the limit of detection are preceded by "<" symbol

Therefore, by adding the mixture of gelatine and di-p-tolyl sulfoxide to the sulphuric acid electrolyte, the present inventors were able to achieve impurities removal. Adequate morphology was also obtained, as shown in Fig. 4a and Fig.4b. The macrographs of electroplated tin show dendrite growth when no additives were added, as shown in Fig. 4c and Fig. 4d.
The tin deposits obtained using a mixture of gelatine and di-p-tolyl sulfoxide can be melt open to air without significant oxidation.

Theoretical considerations on the electrodeposition and dendritic growth supporting generalization to compounds of formula (I).

For any electrodeposition to take place, a current has to flow through an electrochemical cell. This may be limited by any of three factors:

A. Conductivity of the bulk solution.
B. Rate of electron transfer and/or chemical reactions coupled to the electron transfer step.
C. Transport of reactants to the electrode or products away from the electrode.

When a net current flows through the cell, the cell is not at equilibrium. The cathodic potential can be expressed by Eq. 1 $E = E_{e} + η$
where η is the overpotential and E_e is the equilibrium potential at which no current flows through the cell or there is no electrodeposition. The overpotential η is the extra potential necessary for current circulation through the electrochemical cell. The value of η is influenced by solution and interfacial processes that are involved in the three parameters outlined above. The conductivity of the solution, λ, represents the Ohm's type opposition to current circulation according to Eq. 2, where η_R represents the resistance overpotential and I the current circulating through the electrochemical cell. $η_{R} = l / λ$
The current distribution according to Eq. 2 and Laplace's equation, when the deposition process is controlled by step (A) above, produces current maxima (and thus higher deposit thickness) at the cathode edges in what is called primary current distribution¹.
The current at edges however cannot be infinite because of the reduction process at the cathode that follows the Tafel logarithmic law (Eq. 3) that corresponds to step (B) above. $η_{A} = a - b Log |l|$
In Eq. 3 η_A represents the part of the overpotential associated to the charge transfer process, which tends to smooth the deposit at edges, limiting the effect of the primary current distribution in what is called secondary current distribution.
Finally, step (C) above represents the concentration polarisation due to Fick's law controlled transport of reactant, according to Eq. 4. $η_{C} = \frac{RT}{zF} Ln (1 - \frac{I}{I_{L}})$
In EQ. 4 R,T and F have the usual meaning, and z represents the electron number involved in the redox cathodic process. I_L holds for the limiting current given by Fick's law. η_C represents the overpotential due to transport.
The conductivity of the bulk solution is important because it is a measure of the rate of charge transport through the solution under the influence of an electric potential gradient. Typically, this parameter is controllable and one can obtain sufficient conductivity of the solution by adding electrolytes that contain ions for electrolytic conductance. Under normal circumstances, step (C), η_C, is usually the rate-limiting step. The electron transfer rates (step B) are usually much faster than step (C).
In electrodeposition, it is generally not desirable to have step (C) be the rate-limiting step (i.e. η≈ η_C. Dendritic deposits will result from the lack of metal ions transported to the electrode interface to support further cathodic reduction. Under such circumstances, the incipient fine crystals recrystallize and grow larger to lower the total energy of the system. At the electrode interface, the dimension that has more degrees of freedom is the direction perpendicular to the electrode surface. This is normally how dendrites are formed.
In order to avoid dendritic growth, it is necessary to make the electron transfer process (step B) the rate-limiting step² (i.e. η≈ η_A). This can be achieved either by lowering current density, (i.e. smaller η), or using organic molecules to inhibit the electron transfer process by blocking the high-energy sites of the electrode surface, thus increasing the activation energy for step B, which increases η_A.
Organic molecules adsorb at the metallic surface (thus blocking active sites) depending on their functional group. It is thus expected that molecules of the same family behave similarly in terms of providing η≈ η_A.

References:

[1] Corrosion and surface chemistry of metals. D. Landolt. EPFL Press. 2003. pp. 575-577.
[2] Modern Electroplating, 5th Edition. M. Schlesinger, M. Paunovic (Eds). Wiley. 2010. pp. 159-160.

Claims

An electrolytic sulphuric acid bath comprising:
- a sulphuric acid solution,

- a source of Sn(II) ions,

- an anode comprising the target tin to be refined,

- a cathode,

- gelatine, and

- a compound of general formula (I)
wherein
n is 1 or 2,

each R independently stands for halogen, alkyl, cycloalkyl, aryl, aralkyl, hydroxyl, alkoxy, allyloxy, carboxyl, or carboalkoxy group; and

wherein a and b are independently selected from an integer of from 0 to 3, and adjacent Rs may be combined to form a ring.
The electrolytic sulphuric acid bath according to claim 1, wherein the sulphuric acid solution has a pH below or equal to 1.
The electrolytic sulphuric acid bath according to any of claims 1 to 2, wherein the concentration of Sn(II) ions is between 0.05 and 0.1 M.
The electrolytic sulphuric acid bath according to any of claims 1 to 3, wherein the source of Sn(II) ions is tin sulphate (SnSO₄).
The electrolytic sulphuric acid bath according to any of claims 1 to 4, wherein the anode comprising the target tin to be refined is an anode made of an antifriction alloy.
The electrolytic sulphuric acid bath according to any of claims 1 to 5, comprising gelatine in a concentration between 0.05 and 3 g/L and a compound of general formula (I) in a concentration between 0.05 and 1 g/L.
The electrolytic sulphuric acid bath according to any of claims 1 to 6, wherein in the compound of general formula (I) a and b are 0, 1, 2, or 3 and each R independently stands for halogen, alkyl or hydroxyl.
The electrolytic sulphuric acid bath according to any of claims 1 to 7, wherein the compound of general formula (I) is bisphenyl sulfone, a dialkyldiphenyl sulfone, a dihalogendiphenyl sulfone, a bisphenol sulfone, an alkyl-substituted bisphenol sulfone, an halogen-substituted bisphenol sulfone, an hydroxy-substituted bisphenol sulfone or the corresponding sulfoxide homologues of these compounds.
The electrolytic sulphuric acid bath according to any of claims 1 to 8, wherein the compound of general formula (I) is bisphenyl sulfone, di-o-tolyl sulfone, di-p-tolyl sulfone, bis(2-chlorophenyl) sulfone, bis(2-fluorophenyl) sulfone, bis(4-chlorophenyl) sulfone, bis(4-fluorophenyl) sulfone, 2,2'-diphenol sulfone, or 4,4'-diphenol sulfone or the corresponding sulfoxide homologues of these compounds.
The electrolytic sulphuric acid bath according to any of claims 1 to 9, wherein the compound of general formula (I) is di-p-tolyl sulfoxide or 4,4'-sulfonylbisphenol.
The electrolytic sulphuric acid bath according to any of claims 1 to 10, wherein the cathode is made of pure tin, stainless steel or copper.
A method for tin electrorefining comprising the application of a current to the electrolytic sulphuric acid bath according to any of claims 1 to 11.
The method according to claim 12, wherein the current applied has a density between 100 and 150 A/m².
Use of gelatine and a compound of general formula (I) as a combination of additives for an electrolytic sulphuric acid bath for tin electrorefining.