Method for continuously operating acid or alkaline zinc or zinc alloy baths
View Patent ↗The invention provides a process for depositing metallic layers from acidic or alkaline zinc or zinc alloy baths containing organic additives selected from brighteners, surfactants and complexing agents, a soluble zinc salt and optionally further metal salts selected from Fe, Ni, Co, Sn salts, wherein the bath can be purified continuously so that the process can be operated without interruption, as well as apparatus for carrying out this process.
1. A process for the deposition of functional layers from acidic or alkaline zinc or zinc alloy baths containing organic additives selected from brighteners, surfactants and complexing agents; a soluble zinc salt and optionally further metal salts selected from Fe, Ni, Co, Sn salts, which process comprises the following steps:
(i) providing a zinc or zinc alloy bath containing the aforementioned components,
(ii) depositing a zinc or zinc alloy layer on the workpiece to be coated according to processes which are known as such,
(iii) withdrawing a part of the zinc or zinc alloy bath and transferring the withdrawn part to a device for phase separation,
(iv) adding an acid or base to the withdrawn acidic or alkaline part, wherein, when adding acid to the withdrawn acidic part, sufficient acid is added to reduce the pH of the withdrawn acidic part to a pH <1,
(v) adjusting the temperature for acceleration of the phase separation,
(vi) separating the organic phase and, if present, the solid phases,
(vii) recycling the aqueous phase to the zinc or zinc alloy bath in such a way that the pH or hydroxide content of the zinc or zinc alloy bath remains within its operating range so that the bath can be operated without interruption, and
(viii) replenishing spent components of the zinc or zinc alloy bath.
2. Process according to claim 1 wherein the withdrawal of the part of the zinc bath and the recycling are carried out continuously or discontinuously.
3. Process according to claim 1 wherein the addition of the acid and the phase separation are carried out at a temperature within the range of 5-90° C.
4. Process according to claim 1 wherein the addition of the base and the phase separation are carried out at a temperature within the range of −5-30° C. and, after separation of the organic phase, the bath is cooled to a temperature within the range of −5-20° C. in order to obtain a solid inorganic phase which is separated before recycling the aqueous phase to the zinc or zinc alloy bath.
5. Process according to claim 4 wherein the addition of the base and the phase separation are carried out at a temperature within the range of 0-8° C.
6. Process according to claim 4 wherein, after separation of the organic phase, the bath is cooled to a temperature within the range of 0-8° C.
7. Process according to claim 1 wherein the acid used is a mineral acid, and the base used is an alkali or alkaline earth hydroxide.
8. Process according to claim 7 wherein the mineral acid is hydrochloric acid and the alkali or alkaline earth hydroxide is sodium hydroxide.
9. Process according to claim 1 wherein the regeneration rate is 0.1-20% of the bath volume per day.
10. Process according to claim 1 wherein the recycling of the aqueous phase is carried out in such a way that the pH or the hydroxide ion concentration of the zinc or zinc alloy bath remains constant.
11. Process according to claim 1 wherein the formation of the organic phase in the container is detected by a sensor, the sensor initiating the removal of the organic phase from the container.
12. Process according to claim 1 wherein the addition of the acid and the phase separation are carried out at a temperature within the range of 20-50° C.