Two-chamber electrodialysis cell with anion and cation exchange membrane for use as an anode in alkaline zinc electrolytes and zinc alloy electrolytes for the purpose of deposition of metal in electroplating systems

Abstract

The invention relates to an anode for use in electroplating applications for highly alkaline electroplating electrolytes based on sodium hydroxide for depositing zinc and zinc alloys onto steel substrates and die-cast zinc substrates.

Claims

1. A two-chamber anodic system which is suitable for utilizing in a galvanic system comprising the two-chamber anode system and a cathode, the two-chamber anodic system containing an anodic substrate in combination with an alkaline zinc- or zinc alloy electrolyte for the deposition of zinc- or zinc alloy, wherein: the anodic substrate is separated from the alkaline zinc- or zinc alloy electrolyte by a cation- and an anion exchange membrane; the cation and anion exchange membranes form two separate anolyte chambers which are inner anolyte chamber and outer anolyte chamber; the inner anolyte chamber, where the anodic substrate is located, is flowable through by an anolyte; wherein the inner anolyte chamber has an inflow device through which the anolyte flow is directed to a foot of the anodic substrate; and the outer anolyte chamber has openings with inflow- and outflow devices to be filled with the anolyte; wherein the outer anolyte chamber surrounds the inner anolyte chamber; wherein the inner anolyte chamber and the outer anolyte chamber each has an inflow device and an outflow device; and wherein the inner anolyte chamber has an outlet flow device through which the ascending anolyte flow is conveyed into an outlet line, which opens into an anolyte reservoir, in a collecting line.

2. The two-chamber anodic system according to claim 1, wherein the cation exchange membrane is placed towards the anodic substrate and the anion exchange membrane is placed towards the cathode.

3. The two-chamber anodic system according to claim 1, wherein the inner anolyte is sodium hydroxide or potassium hydroxide.

4. The two-chamber anodic system according to claim 1, wherein outer anolyte is sodium hydroxide or potassium hydroxide.

5. The two-chamber anodic system according to claim 1, wherein the anodic substrate is steel, stainless steel, nickel or nickel-plated steel.

6. The two-chamber anodic system according to claim 1, which is constructed in different geometric shapes.

7. The two-chamber anodic system according to claim 6, which is constructed in a box-type design.

8. The two-chamber anodic system according to claim 1, wherein the outer anolyte chamber is retrofittable with an anion exchange membrane.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) An advantageous embodiment of the invention is illustrated in FIGS. 1, 2 and 3, and is taken as a basis for the further description. Other designs, described herein and graphically illustrated in FIG. 5, are technically possible.

(2) It shows:

(3) FIG. 1 the principle structure of the electro-dialysis cell with the function-relevant components,

(4) FIG. 2 the detailed illustration of the structure of a cylindrical electro-dialysis cell,

(5) FIG. 3 the arrangement of the ion exchange membranes and the anode tube in a cross-section,

(6) FIG. 4 the technical construction diagram of a galvanic bath with the electro-dialysis cells and the necessary technical peripherals,

(7) FIG. 5 the graphical illustration of a possible box construction design.

DETAILED DESCRIPTION OF THE INVENTION

(8) As described herein, instead of an ion exchange membrane, two ion exchange membranes are used to form a solid electro-dialysis module, as shown in FIGS. 1 and 2, so that the two anolyte chambers (5) and (6) are built.

(9) The electro-dialysis module is composed of two structural components screwed together:

(10) a) anode (7) with a screw cap (8), FIGS. 1 and 2

(11) b) plastic body

(12) The anode may be made of a stainless steel tube (7), of which diameter and length can be different depending on the application and which is tapered on one side, and a circular stainless steel plate (14), which is firmly connected to the anode tube (e.g., welded). A common tube diameter for the application would be e.g. 2 inches. Two holes of different diameters in the plate serve for screwing or welding the inlet- and outlet valve (1) and (2) for the anolyte sodium hydroxide (concentration approx. 160 g/l), referred to as anolyte 1 in the further description. The inlet- and outlet valve can be hose nozzles of different diameters, wherein the smaller diameter is to be used for the inlet, in order to prevent additional hydrostatic pressure inside the electro-dialysis cell during the flowing of the anolyte 1. Furthermore, the suspension device (18) is firmly connected to the plate, which simultaneously serves for the current transmission from the anode to the electro-dialysis cell.

(13) The plastic body consists of a plastic-foot cap, e.g. PVC (16), into which a plastic grid tube piece, e.g. polypropylene, of a defined length, e.g. 700 mm and a defined diameter, e.g. 80 mm with thereon lying cation exchange membrane (4) as well as a second grid tube piece of a defined length, e.g. 640 mm and diameter, e.g. 100 mm with thereon lying anion exchange membrane (3) are hermetically sealed, e.g. pouring in resin. The upper part of the two-chamber cylinder is also hermetically sealed in a plastic collar (17), so that both chambers have no connection to one another. The tubular plastic collar (17) has an external thread at the upper end, e.g. 2. The anode (7) is inserted into the plastic body. A flat gasket ring (15) is located under the plate. With a plastic screw cap (8), which has an opening at the top of which diameter must be approximately 10 mm smaller than the diameter of the plate (14) and has an internal thread, e.g. 2, the anode is screwed with the plastic body.

(14) In the plastic collar, there are two opposite thread holes, the passage to the outer anolyte chamber. These are used for the screwing of two valves (9) and (10), e.g. angled thread valves with hose nozzles. Through one of these two valves, the outer anolyte chamber (5) is filled with sodium hydroxide (concentration, e.g., 160 g/l), referred to as anolyte 2 in the further description, while venting takes place via the other valve. Thereafter, one of the two nozzles is provided with a cover cap (12), in order to prevent the later penetration of zinc-/zinc alloy electrolytes into the anolyte 2 during the production process. The outlet valve (10) for the overflowing anolyte 2 in the working condition of the electro-dialysis cell is provided with a hose (11) or a plastic tube arch (13) with an opening pointing downwards for the same reason.

(15) With the invention, it is achieved, when the current is flowing in the galvanic bath, the positively charged sodium ions released at the anode pass from the inner anolyte chamber (6) through the cation exchange membrane (4) into the outer anolyte chamber (5), and there a further transport into the zinc- or zinc alloy electrolytes are blocked by the anion exchange membrane (3). In return, equivalent amount of charges of negatively charged hydroxide ions migrate from the zinc- or zinc alloy electrolyte in the direction of the anode (7), and pass the anion exchange membrane (3) into the outer anolyte chamber (5) of the electro-dialysis cell. Here, they are prevented from being further transported to the anode through the cation exchange membrane (4).

(16) As a result of the electrochemical metal deposition process, the sodium hydroxide concentration continuously rises in the outer anolyte chamber (5), and osmosis is set to counteract the increase of the concentration gradient between the outer anolyte chamber and the zinc/zinc alloy electrolytes. Thereby, water is drawn from the zinc-/zinc alloy electrolytes through the anion exchange membrane (3) and reaches the outer anolyte chamber (5). The volume of the anolyte 2 in the outer anolyte chamber thus increases continuously. The volume surplus is removed from the electro-dialysis cell via the outlet device (10). In practical application, the excess amount of sodium hydroxide solution (anolyte 2) should be recycled to 50% each in the zinc-/zinc alloy electrolyte and the anolyte 1, in order to maintain the concentration and volume ratios of zinc-/zinc alloy electrolyte and anolyte 1 approximately constant, because the charge carriers, sodium ions and hydroxide ions reach into the anolyte 2 chamber (5) from the anolyte 1 and the zinc-/zinc alloy electrolytes in an equivalent quantity.

(17) The supply of the anolyte 1 that is required for electrochemical oxidation at the anode with a recommended concentration of approx. 160 g/l of sodium hydroxide takes place, as shown in FIG. 4, in the circulation system by means of a circulating pump (22) from a storage reservoir (23) via stop-valves (20) and flow measuring meter (21) to each individual electro-dialysis cell.

(18) The discharge of the anolyte 1 from the electro-dialysis cells must be carried out without an additional counter pressure in the anolyte 1 reservoirs (23), in order not to over-extend the ion exchange membranes, which can result in microcracks and leakages. A practical way of realization is the connection of the return hoses of anolyte 1 in the free outlet according to FIG. 4, (24) into a central return line, FIG. 4, (25), of suitably large capacity and slight slope to the anolyte 1 supply reservoir.

(19) In FIG. 2, the anolyte flow of anolyte 1 through the electro-dialysis cell is illustrated for better understanding with arrows.

(20) The outlet of the surplus volume of anolyte 2 into the zinc-/Zinc alloy electrolytes is very easily carried out by passing it freely through the valve with a nozzle (10) and tube arch (13), (see FIG. 2), in the half of the number of electro-dialysis cells located in the galvanic system. The outlet of the overflowing anolyte 2 volume into the anolyte 1 storage reservoir occurs, by the other half of the number of the electro-dialysis cells located in the galvanic system, through the valve with a nozzle (10), in which a plastic tube (11) is inserted, which opens into a central return line to the anolyte 1 reservoir, see FIG. 4, (19).

(21) To ensure a reliable function of the described invention, the following basic chemical requirements must be met, which is to be ensured by regular analysis:

(22) The concentration of sodium hydroxide of the anolyte 1 should always be approx. 30 g/l greater than the sodium hydroxide concentration of the zinc-/zinc alloy electrolytes. However, it must be smaller than the sodium hydroxide concentration of the anolyte 2. Only then, it is possible that the osmotic water is pushed mainly from the zinc-/zinc alloy electrolytes into the anolyte 2 chamber of the electro-dialysis cell by osmotic pressure through the anion exchanger membrane.

(23) The initial concentrations of sodium hydroxide of the anolyte 1 and 2 can be the same before the start-up of the electro-dialysis cells, as shown in the reference list below (5), (6), because the concentration of anolyte 2 increases after the application of the galvanic current with the running of operation time and the concentration of anolyte 1 decreases.

(24) With the application of the described invention, the following effects can be achieved: 1. Saving of process chemicals, because an oxidative conversion, in particular, of organic additions such as brightening additive solutions and complexing agents at the anode, is prevented. 2. Significantly less sodium carbonate formation in zinc-/zinc alloy electrolytes. 3. An increase of cathodic current yield. 4. An increase of the throughput in the galvanic system. 5. Saving of the electrical energy per square meter of galvanized surface. 6. Regeneration of old electrolytes, since no new degradation products are formed by anodic oxidation, and the existing ones are gradually removed with the coated product. 7. Saving of additional equipment for the evaporation of volume-surplus, e.g. Vacuum evaporator. 8. Saving of disposal costs for volume-surplus of zinc-/zinc alloy electrolytes.

LIST OF REFERENCE NUMBER

(25) FIGS. 1, 2, 3, 4, 5 1 Inlet valve for anolyte 1 2 Outlet valve for anolyte 1 3 Grid tube/plastic grid with anion exchange membrane 4 Grid tube/plastic grid with cation exchange membrane 5 Outer anolyte chamber with anolyte 2 (sodium hydroxide, initial concentration 160 g/l) 6 Inner anolyte chamber with anolyte 1 (sodium hydroxide, initial concentration 160 g/l) 7 Anode (tube) 8 Plastic screw cap with internal threads 9 Valve with inlet-/outlet nozzles for anolyte 2 (sodium hydroxide, initial concentration 160 g/l) 10 Valve with inlet-/outlet nozzles for anolyte 2 (sodium hydroxide, initial concentration 160 g/l) 11 Outlet hose for anolyte 2 to the central return line to the anolyte 1 storage reservoir 12 Cap for outlet nozzle of anolyte 2 13 Tube arch for outlet nozzle of anolyte 2 14 Welded plate on anode tube with inlet and outlet nozzle for anolyte 1 15 Gasket 16 plastic foot cap 17 plastic collar 18 suspension- and power supply 19 Central return line of anolyte 2 to the storage reservoir of anolyte 1 20 Stop valve in the inlet line of anolyte 1 21 flow measuring meter 22 Circulating pump for anolyte 1 23 Reservoir for anolyte 1 24 Return line of anolyte 1 from the electro-dialysis cell to a central return line 25 Central return line into the anolyte 1 storage reservoir