GB2058846A

GB2058846A - Apparatus and process for phosphating metal surfaces

Info

Publication number: GB2058846A
Application number: GB8009614A
Authority: GB
Original assignee: Pyrene Chemical Services Ltd
Current assignee: Pyrene Chemical Services Ltd
Priority date: 1979-04-07
Filing date: 1980-03-21
Publication date: 1981-04-15

Abstract

A metal surface is phosphated in a system comprising a final pre-rinse (R2) and optionally one or more earlier pre-rinses (R1), a phosphating stage (B), and a first post-rinse (R3) and optionally one or more subsequent post-rinses (R4) and the water in each rinse is replaced during the process and water removed from the first post-rinse is purified (T,S) and recycled to a pre-rinse or to a subsequent post-rinse. <IMAGE>

Description

SPECIFICATION Apparatus and process for phosphating metal surfaces A typical metal finishing process involves cleaning the metal surface by contact with a cleaner, rinsing it, treating it with a phosphating solution and then rinsing it again. In particular the conventional process used commercially involves, after any cleaner stage, sequentially rinsing the surface with water in a pre-rinse, phosphating it, and then rinsing it with water in first and second post-rinses. Thus in such a process there are three rinse stages and each of these requires high throughput of water, the water being contaminated after use and thus normally being run to effluent and causing a serious effluent problem.

Various processes have been proposed for recycling the water in an attempt at reducing or eliminating the effluent problem.

In British Patent Specification No. 1,195,528 used water from the first post-rinse is purified by adding a precipitant for contaminating ions and separating the resultant precipitate, and the purified water is recycled to the same post-rinse.

Accordingly this process results in a reduction of about one third in the total effluent, since any prerinse and second post-rinse apparently are conducted in the traditional manner using fresh water that is run to waste.

In British Patent Specifications Nos. 1,504,475 and 1,540,308 processes are described in which the rinse water supplied to the second post-rinse is passed to the first post-rinse and then to the pre-rinse, i.e., in counter flow to the direction of passage of the surfaces being treated, and the used water from the pre-rinse is then purified by adding a precipitant for contaminating ions and separating the resultant precipitate, and the purified water is recycled to the second post-rinse.

This is an entirely closed system and so generates little or no effluent and reduces the water requirement for the process by 90% or more.

Both these processes suffer from the disadvantage that the precipitant cannot precipitate ions that are introduced with some of the traditional oxidising accelerators that would otherwise be desired in the phosphating solution.

As a result it is necessary to avoid using such accelerators and this means that, for instance, sodium chlorate cannot be used satisfactorily.

Instead, the phosphating treatment has to be accelerated using accelerators such as hydrogen peroxide but processes using such accelerators can have disadvantages, including the difficulty of controlling the process conditions sufficiently accurately to obtain optimum results.

In the present invention a metal surface is phosphated in a system comprising a final prerinse and optionally one or more earlier pre-rinses, a phosphating stage and a first post-rinse and optionally one or more subsequent post-rinses and in this process the water in each rinse is replacedduring the process and the water removed from the first post-rinse is passed through a purifying apparatus in which it is purified by addition of precipitant for contaminating ions and separation of the resultant precipitate and the purified water from the purifying apparatus is recycled to a prerinse or to a subsequent post-rinse.

By adopting these particular systems of recycling, and especially the preferred systems described in more detail below it is possible to achieve adequate or very high economy of water consumption and very satisfactory rinsing. For instance the degree of contamination of the rinse water at the various rinsing stages in the processes of the invention tends to be such that the rinsing achieved in the invention is considerably between than the rinsing achieved in the processes of, for instance, British Patent Specification No. 1,504,475. Also it is possible to operate some of the processes of the present invention using accelerators such as chlorate.

Passivation before phosphating is avoided.

It is generally preferred that there should be at least one subsequent post-rinse and water removed from a subsequent post-rinse is recycled to the final pre-rinse and the water from the final pre-rinse is passed to the first post-rinse.

In such a process the purified water from the separating apparatus may be recycled to a subsequent post-rinse, this post-rinse and the post-rinse from which water is taken to the final pre-rinse generally being the second post-rinse.

This process can operate as a totally closed system thereby achieving at least 90% reduction in water consumption (compared to the conventional process in which each rinse stage is supplied with fresh water and water is run to waste from that rinse stage). When the system is closed it is generally necessary to use an accelerator such as hydrogen peroxide so as to avoid the risk of unsatisfactory build up of sodium or other contaminated ions from the accelerator in the rinse waters.

Even in a closed system it is necessary to add water during the process, e.g. to counteract evaporation losses, and addition is generally made at the second post-rinse. If the amount added is greater than the amount lost due to evaporation and drag out excess water must be run out of the system, and it is no longer necessary to use accelerators such as peroxide.

The excess water may be run out of the system immediately after the purifying apparatus or may be recycled to an earlier pre-rinse and run out of the system from that.

Instead of aiming to operate the process wholly or mainly as a closed system water may be added to the subsequent post-rinse as described above but all the purified water from the purifying apparatus may then be recycled to an earlier prerinse and then run out of the system, generally from that pre-rinse. Such a process can easily achieve 70% saving in water.

Instead of having a recycle between the second post-rinse and that final pre-rinse and the final pre-rinse and the first post-rinse the process may be operated as a simple sequential process with water being added into a post-rinse, being taken from the first post-rinse to the purifying apparatus and then recycled from the purifying apparatus to a pre-rinse and run out of the system from a prerinse.

This type of system can be used in a process where water is added both the the first post-rinse and to the final pre-rinse and water from both these rinses is taken to the purifying apparatus and the purified water from that is recycled to an earlier pre-rinse and run out of the system from that. This, and the processes in which there is a cycle between the second post-rinse, the final prerinse and the first post-rinse are preferably operated as spray processes.

A particularly simple process which is therefore very suitable for use when rinsing is by dipping is one in which all the water to be purified is taken from the first post-rinse and recycled to the final pre-rinse. There may be two or more pre-rinses in which event the water from the final pre-rinse may be passed to an earlier pre-rinse and then run out of the system from that. There may be two or more post-rinses in which event the water may be added to one post-rinse and passed from that to the first post rinse.

The invention includes not only the described processes but also apparatus suitable for carrying out these processes. Such apparatus will broadly comprise means for subjecting metal surfaces to a final pre-rinse and optionally to one or more earlier pre-rinses, means for subjecting the metal to phosphating, means for subjecting the metal to a first post-rinse and optionally to one or more subsequent post-rinses, water purifying apparatus comprising means for adding precipitant for contaminating ions and for separating the resultant precipitate from the purified water and means for recycling the purified water to a prerinse or to a subsequent post-rinse.

The invention is illustrated in the accompanying drawings each of which is a flow diagram. In each drawing R2 represents the final pre-rinse, B the bonding stage at which the metal surfaces are contacted with the phosphating liquid and R2 represents the first post-rinse. R1 is an optional earlier pre-rinse and R4 is an optional subsequent post-rinse. T and S are the purifying apparatus, T being the treatment apparatus in which precipitant is added and S being separating apparatus, such as described in British Patent Specification No. 1,504,475. Various letters F represent points at which fresh water is introduced into the system and the letters W indicate points at which water is run out of the system. The metal pieces being treated are passed from left to right. The water passes in the direction shown in the arrows.

In Figure 1 valves 1 and 2 may be permanently closed or the lines carrying them may be omitted.

In this event Figure 1 operates as a totally closed system with fresh water being introduced at R4, being passed from there to R2, from R2 to R3 and from R3 toT, Sand back to R4. Such a process can give at least 90% water saving and should be operated using a peroxide accelerator. The prerinse R1 supplied with water at F2 which is run to waste at W2 is optional.

In a modification valve 1 may be opened and some purified water run to waste at W1, excess water being added at F1 to make up for the loss.

Without increasing water consumption very much this permits the use of accelerators other than peroxide. A better way of operating the process with accelerators other than peroxide is to have valve 1 closed and to open instead valve 2 and to bleed off some of the purified water from the separator S and to pass it to R1, so that purified water is being recycled both to R4 and R1.

In Figure 2 purified water from S is recycled to R1 but none is recycled to R4 and so this system always operates as an open system and although it has a slightly higher water consumption than any of the systems of Figure 1 it gives very satisfactory purity and can be operated with accelerators other than peroxide. Water is run out of the system atW3 in amounts substantially equivalent (after allowing for evaporation and drag out) to the amounts added at F,.

The systems of Figures 1 and 2 are intended primarily for spray systems. The system of Figure 3 is preferably a spray system but can be operated as a dip system. In Figure 3 R4, and its separate water supply F, and run off system Ws, are optional. Water is supplied into the system of Figure 3 both at R2 and R3 and the water is taken from that to the treatment apparatus and recycled to the pre-rinse R, from which it is run to waste at W3.

The system in Figure 4 is intended primarily as a dip system. Water is introduced as F, at R4 and is taken from that to R3 to the purifying apparatus to R2 and to R, from which it is run to waste at W3. R4 can be omitted, in which event water is run direct into R3. R1 can be omitted in which event water is run out of the system from R2.

In each of the illustrated systems the water supply F1 may be water from the mains but often is is water that has been used in a final post-rinse that is not illustrated. In this final post-rinse deionised or other water is contacted with the rinsed metal surfaces and the water that runs off is then used as the supply for F1.

Claims

1. A process in which a metal surface is phosphated in a system comprising a final pre rinse and optionally one or more earlier pre-rinses, a phosphating stage and a first post-rinse and optionally one or more subsequent post-rinses and in which the water in each rinse is replaced during the process and in which the water removed from the first process is passed to purifying apparatus in which it purified by addition of precipitant for contaminating ions and separating the resultant precipitate and the purified water from the purifying apparatus is recycled to a pre-rinse or to a subsequent post-rinse.

2. A process according to claim 1 in which there is at least one subsequent post-rinse and water removed from a subsequent post-rinse is recycled to the final pre-rinse and the water from the final pre-rinse is passed to the first post-rinse.

3. A process according to claim 2 in which water is recycled from the purifying apparatus to a subsequent post-rinse.

4. A process according to claim 3 in which the subsequent post-rinse to which water is recycled from the purifying apparatus and from which water is passed to the final pre-rinse is the second post-rinse.

5. A process according to claim 4 in which water is added into the second post-rinse.

6. A process according to claim 5 in which excess water is added into the second post-rinse and water is run out of the system directly after the purifying apparatus.

7. A process according to claim 5 in which excess water is added into the second post-rinse and water is passed from the purifying apparatus to an earlier pre-rinse and is run out of the system from that.

8. A process according to claim 2 in which water is added into the subsequent post-rinse, the water recycled from the purifying apparatus is passed to an earlier pre-rinse and is then run out of the system.

9. A process according to claim 8 in which water is recycled from the purifying apparatus to the pre-rinse preceding the final rinse and is run out of the system from that.

10. A process according to claim 1 in which water is added into a post-rinse and the purified water is recycled to a pre-rinse and water is run out of the system from a pre-rinse.

1 A process according to claim 10 in which water is added both into the final pre-rinse and the first post-rinse and water from both the final prerinse and the first post-rinse is passed to the purifying apparatus and the purified water is recycled to an earlier pre-rinse and is run out of the system from that.

12. A process according to claim 10 in which the purified water is recycled to the final pre-rinse.

13. A process according to claim 12 in which there are two post-rinses and the water is added to the second post-rinse and the water removed from that is passed to the first post-rinse.

14. A process according to claim 12 or claim 13 in which there are two pre-rinses and the water removed from the final pre-rinse is passed to the preceding rinse and run out of the system from that.

1 5. A process according to any of claims 12 to 14 operated as a dip process.

16. A process according to any of claims 2 to 11 operated as a spray process.

17. A process according to claim 1 substantially as herein described with reference to any of the drawings.

18. Apparatus suitable for use in a process according to any preceding claim comprising means for subjecting the metal to a final pre-rinse and optionally to one or more earlier pre-rinses, means for subjecting the metal to phosphating, means for subjecting the metal to a first post-rinse and optionally one or more subsequent post rinses, water purifying apparatus comprising means for adding a precipitant for contaminating ions and for separating the resultant precipitant from the purified water and means for recycling purified water to the means for subjecting the metal to a pre-rinse or to the means for subjecting the metal to a subsequent post-rinse.