Liquid purification or separation – Processes – Including controlling process in response to a sensed condition
Patent
1985-10-02
1987-07-07
Hruskoci, Peter
Liquid purification or separation
Processes
Including controlling process in response to a sensed condition
55 19, 55 85, 210752, 210764, 422 3, 422 13, C02F 166
Patent
active
046785868
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to the treatment of wash water in paint spray booths by the addition of an alkaline solution to the water.
In paint spray booths it is common practice to use a water curtain falling over the back wall of the booth and to collect sprayed paint which is not deposited on the article to be painted by means of wash water. Air is in some cases extracted from the booth through the water curtain and/or through water sprays and the waste paint is usually accumulated in a water tank situated below the wash section.
The paint tends to coagulate and form a thick layer floating on the surface of the water. To facilitate breakdown of the paint and eventual separation of the paint from the wash water it is known to use chemical additives in the wash water. Specialized additives have been developed for treating particular types of paint but a common type of additives is alkaline and contains a high percentage of sodium hydroxide. Such additives are corrosive and hazardous to handle, especially if used in powder form, requiring the use of protective clothing and proper ventilation. Operators of paint spray booths do not, in general, have much experience of handling hazardous chemicals and cannot be expected to exercise continuous control over the handling of the additive and the rate at which it is added to the water. Small operators at present use powder additives which are tipped into the water from time to time in a very erratic and wasteful manner.
Larger operators use liquid additives and attempt to exercise some control over their introduction. Automatic dosing is at present impractical in most cases because of the high cost and technical complexity of the equipment. In particular attempts to measure the pH of the water in the tank continuously by means of a probe positioned in the water have been unsuccessful because the probe is rapidly coated with paint and becomes inaccurate.
The suggested use of a pH probe to control the introduction of additive is in any case based on an incorrect view of the requirements and it is this, in addition to the difficulties outlined above, which has prevented the introduction of a more satisfactory method of treatment. Firstly it has been thought that best results would be obtained by maintaining the same condition of the water at all times, at least while handling a particular type of paint. Apart from the fact that this is more difficult to achieve because it requires continuous monitoring and control of the introduction of small quantities of additive, it has now been established that stable uniform conditions encourage the growth of anaerobic bacteria in the tank. Secondly it has been thought that the amount of additives required would be proportional to the amount of paint to be treated and this has often led to over-use of the additive, which may have the effect of making conditions worse rather than better.
In accordance with the present invention there is provided a method of treating wash water in paint spray booths by addition of an alkaline solution to the water characterised in that the addition is effected according to a predetermined program in successive treatment cycles each lasting for a number of days, the amount added in each cycle being sufficient to maintain the water at a pH value not substantially below 9 while the concentration of hydroxide ion in the water does not exceed 0.01M, and in each treatment cycle over half the amount to be added is introduced as a single discrete dose and the remainder in smaller doses at intervals.
In this method the pH value is chosen to minimize corrosion of the metal tank by the wash water. There is no direct control of the amount of alkaline solution added. All that is necessary is to check the pH with a test paper from time to time and adjust the amount added in the course of a treatment cycle if the pH value falls too low. The required quantity of alkali in the water is very much lower than that resulting from most conventional methods of treatment. While there is not direct control of the addit
REFERENCES:
patent: 4550011 (1985-10-01), McCollum
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