Method of Electrolytic Tinning Using an Insoluble Anode

Europaisches Patentamt J European Patent Office © Publication number: 0 268 823 Office europeen des brevets A2

EUROPEAN PATENT APPLICATION

® Priority: 30.10.86 NL 8602730 © Applicant: HOOGOVENS GROEP B.V. P.O. Box 10.000 © Date of publication of application: NL-1970 CA Umuiden(NL) 01.06.88 Bulletin 88/22 @ Inventor: Bunk, Huig © Designated Contracting States: Stumphiusstraat 29 BE DE ES FR GB IT NL NL-1942 EN Beverwijk(NL) Inventor: Van Haastrecht Gijsbertus Cornells Beneluxlaan 48 NL-1946 WL Beverwijk(NL) Inventor: Mooij, Joop Nicoiaas Walstro 56 NL-1902 JP Castricum(NL)

Method of electrolytic tinning using an insoluble anode.

© A method of electrolytic tinning of metal in strip form using an insoluble anode, comprises passing ( I the strip (1) to be tinned through an electrolytic tinning bath (35) having an insoluble anode (5), the strip (1) forming the cathode, to deposit tin from the electrolyte onto the strip, and circulating the electrolyte through an electrolytic replenishing cell (6) outside the tinning bath for addition of tin ions in order to maintain the desired concentration of tin ions in the tinning bath. The replenishment cell has a If CMtin anode chamber (11) with a tin anode system 1. 1'. ^(27,28), cathode chamber *ri ' a (12) having an insoluble 1 1 jv) cathode (25), and a membrane (13) between them T (N substantially impermeable to tin ions. To make the CO replacement of the anode easier and to improve qq efficiency, the tin anode system of the replenishing tO cell comprises an insoluble tin anode (27) and a bed *^(28) of granular tin material which is fluidized by the O electrolyte. Q_ W

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METHOD OF ELECTROLYTIC TINNING USING AN INSOLUBLE ANODE

The invention relates to a method for the elec- replenishment cell, the tin anode system comprises trolytic tinning of metal in strip form using an an insoluble anode and a bed of granular tin ma- insoluble anode. The metal in strip form is known terial. The advantage of this is that the tin being as tinplate and may be steel not previously tin- dissolved can be replaced continuously or from plated or previously tinned strip. 5 time to time in the form of granular tin material. US-A-4181580 describes a tinning process in The granular tin is dissolved into the electrolyte by which the tinplate in strip form is passed as cath- contact with the insoluble anode during electrolysis. ode through a tinning bath containing an acidic Circulation of the tinning electrolyte may take liquid electrolyte including tin ions, so that tin ions place continuously or intermittently during the tin- are deposited on the strip. The electrolyte from the 70 ning process. tinning bath is transported to a source of tin ions Preferably the electrolyte is added to the tin located outside the tinning bath and is there en- anode chamber of the replenishment cell to a com- riched with tin ions. The electrolyte is then returned partment in the anode chamber which is bounded to the tinning bath, the concentration of tin ions in at its upper side by a perforated (foraminate) plate. the electrolyte in this way being kept up to a 75 This plate distributes the electrolyte through the desired level. The source of tin ions is a reactor bed of granular tin material, and supports the bed using oxygen in which tin is dissolved by a chemi- of granular tin material. The electrolyte flows cal method. A big disadvantage of the known meth- through the plate and the bed in an upwards direc- od is that the unwanted reaction Sn2+ — Sn'1* tion. As a result with a relatively simple structural occurs, so that approximately 4% of the tin is 20 arrangement of the electrolytic cell a good flow converted to sludge. This makes a separate sludge distribution of the electrolyte in the tin anode cham- removal system necessary. ber is obtained. Methods of replenishing a plastics electrolyte Preferably the electrolyte flows through the tin using electrolytic processes are disclosed in DE-A- anode chamber at such a speed that the bed of 2027793, GB-A-2041408 and FR-A-2479856. In 25 granular tin material is fluidised. An advantage of particular DE-A-2027793 describes a replenishment this is that the electrolytic cell can be constructed cell through which the electrolyte, for example a • even more compactly as the transfer of material is tinplating electrolyte, is passed through a replenish- greater in the fiuidized bed. In addition, any oxide ment cell having anode chamber containing a solu- skin on the tin grains in the fiuidized bed is broken ble anode which is dissolved to enrich the elec- 30 and/or abraded when the grains touch each other trolyte, a cathode chamber and a membrane sub- during the continuous movement of the grains with stantially impermeable to the metal ions of the respect to each other. electrolyte. Another advantage is that, although the current Electrolyte replenishment has the advantages strength required in the electrolytic cell is high, the that- 35 current density, being the current strength related 1) Process control is simpler, than in the to the large anode surface of the granular tin ma- method of US-A-4181580, since the electrical pow- terial, is low. As a result the necessary voltage and er used can be controlled much more easily than hence the energy consumption is low. the feeding of oxygen into the electrolyte. Another consequence of the low current den- 2) No or virtually no tetravalent tin ions are 40 sity is that there is still less oxidation Sn2+ — Sn4+. formed, and no or virtually no sludge is produced. A further advantage is that with a low energy con- 3) The apparatus can be much more com- sumption only a small quantity of heat is developed pact and cheaper than the oxygen reactor. in the electrolytic replenishment cell, so that the However, the known electrolytic replenishment cooling capacity required can be small. processes have some defects, particularly the need 45 A higher pressure is preferably maintained in to replace the soluble electrode from time to time. the cathode chamber of the electrolytic replenish- It is also desired to improve efficiency. ment cell during operation than in the tin anode The object of the invention is to provide a chamber. This further counteracts the transport of method of electrolytic tinning using an insoluble tin ions through the membrane. anode in the tinning bath and electrolytic replenish- so A preferred embodiment of the invention will ment of the tinning electrolyte, in which the prob- be described below by way of non-limitative exam- lem of replacement of the anode in the replenish- ple and illustrated with reference to the drawing, in ment cell is avoided or reduced and in which which efficiency is increased. According to the invention in the electrolytic 0 268 823

Figure 1 shows a process diagram of an The oxygen formed in the tinning bath 3 on embodiment of the method in accordance with the anode 5 escapes from the tinning bath 3 and also invention, and at 23 from the circulation tank 8. Figure 2 shows the electrolytic cell used in The electrolytic cell 6 is shown in more detail the process of Figure 1 . 5 in Figure 2 comprises a tank 24 with a cathode Figure 1 shows a strip 1 of tinplate which is chamber 12 containing an insoluble cathode 25, an passed as a cathode by means of reversing rollers anode chamber 11 containing a tin anode system 2 through a tinning bath 3. The tinning bath 3 26 comprising an insoluble anode 27 and the bed shown in Figure 1 is of the radial type, but can also of granular tin material 28 which is fluidised during be of a more conventional type with flat anodes. 10 operation, and a wall 13 such as a membrane The radial tinning bath 3 shown comprises a cath- which is impermeable or virtually impermeable to ode roller 38 over which the strip 1 is moved and a tin ions. The insoluble anode can be a fully inert curved insoluble anode 5 arranged with a gap 35 material such as carbon or a metal of the platinum between the strip 1 and the anode 5. At entry group or may have an electrically conductive core location 36 liquid electrolyte containing tin ions is 75 coated with a metal of the platinum group or oxides forced under pressure into the gap 35, and in this thereof. This insoluble anode can have any suitable gap under the influence of the electric field be- shape, e.g. tube, sheet, wire, rod or gauze. tween the strip 1 as cathode and the anode 5 tin The anode chamber has a compartment 29 at ions are deposited from the electrolyte onto the the bottom which is bounded at the upper side by strip 1 . The electrolyte running out of the gap 35 is 20 a perforated plate 30, on which the bed of granular collected at location 4 at the bottom of the tinning tin material 28 rests (when not fluidized). The an- bath. olyte is passed at 31 into the compartment 29 of Tin ions are continuously removed from the the anode chamber 1 1 , is passed into the bed 28 electrolyte during the tinning of the strip 1 , and are distributed evenly by perforated plate 30, divalent added to replenish the electrolyte in the electrolytic 25 tin ions being taken up in the bed, and is di- replenishing cell 6, through which the electrolyte is scharged at 32. At 33 fresh granular tin material is circulated. The electrolyte is transported through a supplied. pipe 7 from the tinning bath 3 to the circulation The catholyte is added to cathode chamber 12 bath 8 arid is transported by a pump 10 through a at 16 and discharged at 34. In the cathode cham- pipe 9 from the circulation bath 8 to the electrolytic 30 ber 12, provided that the wall 13 can withstand it, a cell 6. The electrolytic replenishing cell 6 com- higher pressure is applied than prevails in the prises a tin anode chamber 11 and a cathode anode chamber 1 1 , as a result of which the trans- chamber 12 with a wall 13 between them which is port of tin ions through the wall 13 is hindered. impermeable or virtually impermeable to tin ions. This is described in more detail below. 35 The electrolyte is passed into the anode cham- Claims ber 1 1 . The anode system is described below. Tin ions are formed electrolytically in accordance with 1. Method of electrolytic tinning of metal in the reaction. strip form using an insoluble anode, comprising the Sn — Sn2+ + 2e" 40 steps of: are added to the electrolyte. The electrolyte is then (a) contacting the strip (1) to be tinned with returned via a pipe 14 to the circulation bath 8 and an acidic liquid electrolyte including tin ions, in an from there via a pipe 15 to the tinning bath 3. The electrolytic tinning bath (35) having an insoluble tin dissolved in the tin anode chamber 11 elec- anode (5), the strip (1) forming the cathode, and trolytically is replaced continuously or intermittently 45 causing current to flow so as to deposit tin from the by a tin granulate feed device indicated by arrow electrolyte onto the strip, and 17. Suitably the granular tin has an average particle (b) circulating said electrolyte through an diamter in the range 0.1 to 10 mm, most preferably electrolytic replenishing cell (6) outside said tinning about 5 mm. bath for addition of tin ions to the electrolyte in The catholyte in the cathode chamber 12 is 50 order to maintain the desired concentration of tin circulated via a pipe 18, an overflow bath 19 and a ions in the electrolyte in the tinning bath, said pipe 20 using a pump 21 . In the cathode chamber replenishment cell comprising 12 a reaction also takes place. For example, hy- (i) a tin anode chamber (11) having a tin drogen electrolytically formed in the catholyte in anode system (27,28) accordance with the reaction 55 (ii) a cathode chamber (12) having an in- 2 H3O+ + 2e" - H2 + 2H2O soluble cathode (25), and is aspirated at 22 by a fan 37 from the overflow tank 19.

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(iii) a membrane system (1f3) between the anode chamber and the cathode chamber which is substantially impermeable to tin ions, said electrolyte being passed through the anode chamber (11) of the replenishing cell so as to contact the tin 5 anode system and there being electrolytically en- riched with tin ions, characterized in that the tin anode system of- the replenishing cell comprising an insoluble tin anode (27) and a bed (28) of granular tin material. 10 2. Method according to claim 1 wherein said bed (28) of granular tin material is supported by a perforated plate (30) in said anode chamber (11) of the replenishing cell and said electrolyte is passed into the anode chamber beneath the perforated 75 plate (30) so as to pass upwardly through the plate (30), which distributes the electrolyte, and the bed. 3. Method according to claim 1 or claim 2 wherein the said bed (28) of granular tin material is fluidized by the passage of the electrolyte through 20 it. 4. Method according to any one of claims 1 to 3 wherein in the cathode chamber (12) of the replenishing cell, a higher pressure is maintained than in the anode chamber (11). 25

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