GB2107359A

GB2107359A - Automatic replenishing of acidic solutions containing fluoride for cleaning aluminium

Info

Publication number: GB2107359A
Application number: GB08228592A
Authority: GB
Inventors: Richard Andrew Beck
Original assignee: Pyrene Chemical Services Ltd
Current assignee: Pyrene Chemical Services Ltd
Priority date: 1981-10-09
Filing date: 1982-10-06
Publication date: 1983-04-27
Also published as: GB2107359B; JPS5881980A; BR8205926A; EP0077041A1; AU8910382A; DE3237553A1; ZA827263B

Description

1 GB 2 107 359 A 1

SPECIFICATION

Process for cleaning aluminium It is known that aluminium articles, for instance cans, can be cleaned by contacting the aluminium surfaces with an acidic solution containing fluoride. For instance, Hess et al describe, at p. 417-423 of Supplement to the Journal of the American Welding Society, September 1944, processes using solutions containing, for instance, fluoride and sulphuric acid, and such a process is also described. in British Patent Specification No. 1 454974.

In practice, it is necessary to replenish the solution with acid and fluoride during use. Hess warns on page 420 there is no simple method of measuring and maintaining the activity of the bath., but the development of the fluoride sensitive electrodes described in, for instance, U.S. Patent No. 3 431 182, gave a way of controlling the fluoride content quite accurately. The use of such electrodes is indeed mentioned in British Patent Specification No. 1 454 974 as a way of measuring the fluoride content and of determining the amount of replenishment.

When a system is to be replenished with acid it 90 is customary to measure the acidity of the solution and to add acid in response to the measurement. The most common way of measuring acidity is by electrometric p11 measurement, ie with a pH electrode.7his is also 95 mentioned in British Patent Specification

No. 1 454 974.

I n practice it seems that processes such as those mentioned by Hess and in British Patent Specification No. 1 454 974 have been operated 100 commercially by maintaining the working solution temperature at a constant value by appropriate heating and/or cooling systems, and extracting samples from the working solution for analysis, and replenishing the working solution in response 105 to the analytical results. Thus the fluoride ion content is maintained constant by measuring the free fluoride ion content of the solution and automatically adding fluoride in response to the deviation of the measured content for a predetermined value and the acid content is maintained constant by measuring the pH of the solution by a pH meter and automatically adding acid in response to the deviation of the pH from a predetermined value. Such processes have been 115 widely adopted commercially, even though in practice it has proved very difficult to control them to obtain uniform results.

Of course, many other automatic control systems are known for many chemical processes, 120 and an example is the control system we described in our British Patent Specification

No. 2 050 645 and which is intended primarily for the control of a zinc phosphate solution.

This invention relates to improvements in processes of cleaning aluminium surfaces by contact with an aqueous acidic solution containing fluoride and that is replenished with acid and fluoride during use and in which replenishment is effected automatically by measuring the actual free fluoride ion content and automatically adding fluoride in response to the deviation of the actual content from a predetermined desired value of free fluoride.

It has surprisingly been discovered that in such processes replenishment of acid in response to electrometric pH measurement gives misleading results and is incapable of maintaining the solution in a stable content. In the invention therefore the use of electrometric pH measurement of acidity is avoided.

Although pH measurement gives misleading results it has surprisingly been found that measurement of acidic conductivity gives satisfactory results and so in the invention it is preferred to measure the acidic conductivity of the solution and to add acid automatically in response to the deviation of the actual acidic conductivity from a predetermined desired value.

The reason for this surprising discovery that pH measurement gives misleading results but acidic conductivity gives reliable results is not entirely clear. However it seems that acidic conductivity is independent of fluoride content whereas the recorded pH value tends to "chase" the free fluoride content, or vice versa. Thus the pH electrode is attacked by the acidic fluoride solution and records a higher pH than it should. It therefore calls for the addition of acid. This unnecessary acid addition results in the measured fluoride content being lower than it should be and so results in the fluoride sensor automatically adding more hydrogen fluoride. This increased fluoride content results in further attack on the glass electrode, and so on. Surprisingly however acidic conductivity is a reliable measurement irrespective of fluoride content.

Although very accurate control of acidity necessitates the described use of acidic conductivity to control acid replenishment, it has surprisingly been found that adequate control for most purposes can be achieved merely by adding acid in an amount that is in a predetermined fixed relationship to the amount of fluoride that is added.

In such a method replenishment may be conducted cyclically by initiating a replenishment cycle at regular intervals of time and, in each cycle, measuring the fluoride ion content and adding fluoride replenishment solution by a first pump and acid replenishment solution by a second pump, the pumps operating in each cycle at a predetermined delivery rate for a duration that is automatically controlled in response to the deviation of the measured fluoride ion content from the desired value.

The cyclic replenishment may be controlled by a control unit comprising a timer for initiating each replenishment cycle, a first pump for delivering fluoride replenishment solution at a predetermined delivery rate, a second pump for delivering acid replenishment solution at a predetermined delivery rate, means for measuring the fluoride ion content of the treatment solution 2 GB 2 107 359 A 2 and for generating a signal in response to the deviation of the measured content from a desired value and means for automatically operating the first pump in each cycle solely for a duration automatically controlled in response to the signal and for operating the second pump with the first pump. Instead of operating the second pump with the first pump, it may be operated in response to acidic conductivity measurement in an otherwise similar cyclic process.

It is known that the effectiveness of the solution is dependent on the temperature of the solution and normally considerable care is exercised to maintain the temperature as uniform as possible. In the invention it is preferred that the temperature of the solution is measured and the said predetermined desired value of free fluoride is automatically varied in response to, and to compensate for, deviation of the actual temperature from a predetermined temperature.

Because of the criticality of the temperature on the effectiveness of the solution it is important for successful operation that all the measurements are carried out in the actual working solution, and so they should not be carried out on samples 90 removed from the solution.

The rate of attack of aluminium by a given acidic fluoride solution increases with increasing temperature, the rate being approximately double for every 251C rise in temperature. It is therefore preferred in the invention that the said predetermined desired value of free fluoride is reduced when the temperature increases above a predetermined temperature (so as to compensate for the increased speed of reaction that would otherwise occur) and is increased when the temperature fails below the predetermined value.

In a simple form of process the fluoride concentration apparatus is artificially set to give a recorded concentration that is doubled for every 251C rise in temperature so that the actual concentration is reduced by half for every 250C rise. Conversely, for every 251C decrease, the recorded concentration should be reduced by an equivalent amount below the actual concentration, thereby resulting in the actual concentration being increased. In this instance the means for measuring the fluoride ion content of the solution, and for generating a signal in response to the deviation from a desired value, may include a visual indicator such as a meter, having a needle. The means for measuring the temperature and for adjusting the said desired value in response to variation of the temperature may include means for passing a signal to the 120 needle, or other indicator, to cause it to appear to record a fluoride ion concentration higher than the actual concentration, the actual concentration being the said predetermined value.

For the most accurate operation of the process 125 it is preferred to determine the precise relationship between the temperature and fluoride ion concentration, to programme this into a suitable memory device and to utilise the programmed information for automatically 130 varying the said predetermined desired value of free fluoride with variation in temperature.

The invention includes not only the described process but also apparatus comprising means for carrying out the processes. Preferably the apparatus includes a visual display device, such as a digital read-out, displaying the actual fluoride content of the solution.

When the acid replenishment is effected in a predetermined fixed relationship to the amount of fluoride that is being added it is easily possible, during normal operation, to maintain the acid value within 5% of the optimum and for many purposes this is found to be entirely satisfactory.

However it is generally desirable to monitor the acidity from time to time, even during steady state processing, and so acid may be added when necessary in response to deviations of the acidity from a desired value. Although acidity can be measured by pH it is preferred to measure it by acid conductivity measurements, as discussed above.

- Replenishment in fixed relationship to the amount of fluoride is satisfactory during steady state operation but it may give misleading results if there is little or no throughput of aluminium surfaces, for instance if the volume of solution varies abnormally, since there may be little or no utilisation of fluoride but acid may continue to be utilised since the cleaner working solution maybe pumped into a preclean tank when the line is running. Accordingly, the fluoride ion measurement will cause the pumps to be operated for, at most, a very short time and indeed may prevent the pumps operating at all.

It is then preferred to include means for overriding the normal acid replenishment control system and to permit separate replenishment of acid. In particular it is generally preferred that acid is added at time intervals not greater than a predetermined value and so the apparatus may include also an override timer for ensuring that acid is added at predetermined intervals of time, even if fluoride is not being added. Thus, if sufficient acid is being added during normal replenishment cycles to maintain the acid content adequate, the override timer will not activate the second pump to add acid replenishment solution. However, if the amount of acid replenishment solution added by normal operation of the control unit, in response to fluoride ion measurement, is inadequate to maintain a pre-selected rate of addition of acid, then the override timer causes the second pump to operate from time to time for durations sufficient to maintain the acid at an adequate level. The time for which this override timer is set can easily be determined by experiment. It will be dependent upon, primarily, the normal loss of working solution per cycle, the free acid content of the working solution, the concentration of the acid replenishment solution and the flow rate of the second pump.

Instead of, or in addition to, providing a timer override, an override may be provided that includes a system for measuring acidity, z -c 3 GB 2 107 359 A 3 preferably by acidic conductivity but possibly by pH electrode. Thus an override is provided for measuring acidity and causing automatic addition of acid replenishment, generally by operation of the said second pump, whenever the acidity varies by more than a predetermined amount from a chosen value.

The control unit used in the invention is preferably an electronic control unit that may be constructed using electronic components, a variable mark/space ratio circuit being provided as the timer which, during normal operation, determines the times at which the pump for supplying fluoride replenishment is switched on.

When acid is supplied in predetermined fixed relationship to the fluoride then the pump for supplying acid replenishment may be controlled in response to the same timer. When acid is added in response to acidic conductivity measurement the control unit will include a suitable device for measuring conductivity and for utilising the resultant signal for controlling a pump for supply of acid. The times at which this pump is switched on may also be controlled in response to the variable mark/space ratio circuit or other timing mechanism.

The control unit is preferably linked to the cleaning apparatus so that the control unit is operative whenever, and for the whole time, the cleaning apparatus is functioning. For instance, when, as is preferred, the contact of the aqueous solution with the metal surfaces is by spraying, the control unit is preferably linked to the spray pumps so that the system is operative whenever, and for the whole time, the spray pumps are operating. The mark/space ratio circuit may be set to turn on the metering pumps, acting through output relays, at regular intervals that define the replenishment cycle, and to switch off the metering pumps after a time which is dependent on the said desired value, which may be the actual fluoride ion content or a content that compensates for temperature variation.

The delivery rates of the metering pumps are preset so that if they are switched on for a predetermined proportion, preferably half, of the cycle period, sufficient of the fluoride ion replenishment and acid replenishment are added to the bath to maintain it at its desired concentrations. If the operating conditions vary, the recorded fluoride ion content will change from a predetermined reference value, generally a zero point on an indicator scale, and will cause the mark/space ratio circuit accordingly to vary the "on" time of the fluoride pump appropriately. Thus, if the throughput of aluminium surfaces decreases, the fluoride ion content of the treatment solution will tend to increase and for the next replenishment cycle the "on" time of the fluoride pump will be automatically decreased.

Typically, the period of a replenishment cycle may be four minutes and it is convenient to arrange for normal operation of the fluoride replenishment pump for about half this time.

The control unit may include means to prevent 130 the pumps being switched on and off too rapidly, for instance if abnormal conditions are approached, and may include one or more alarm means, such as warning lights, for indicating adverse conditions, for instance if the fluoride ion content has become so high or so low as to cause the mark/space ratio circuit to operate abnormally, for instance if the metering pumps are on throughout all or nearly all of the replenishment cycle.

Devices for measuring acidic conductivity and fluoride ion concentration are both well known.

An example of an electrode system for measuring fluoride ion is described in U.S. Patent No.

3 431 182. As is conventional, the system may also include a sealed reference electrode or a reference electrode in a salt bridge. Salt bridges are also encountered in other ion concentration systems, such as some pH measurement systems.

Although salt bridge systems have many advantages, they do suffer from a disadvantage that they can give faulty indications due to unobserved loss of salt from the salt bridge.

The invention also includes a modification of a salt bridge ion measurement system in such a way as to provide automatic warning if there is failure in the salt bridge.

According to this aspect of the invention, we provide a salt bridge for use in measurement of ion concentration in a solution, the salt bridge comprising a vessel for containing a salt electrolyte, a permeable member that can be immersed in the solution, and that is connected to the vessel such that the salt electrolyte can be in electrolytic contact with the solution while being substantially prevented from flowing out of the vessel, a reference electrode positioned in the vessel for use in measuring the ion concentration in the solution, and a warning electrode electrically connected to the reference electrode and positioned in the vessel such that it will generate a warning signal when the level of salt electrolyte drops below a desired level.

An ion concentration measurement system thus comprises the salt bridge and an ion concentration electrode that can be positioned in the solution and that can be electrolytically connected to the reference electrode to generate a signal in response to the concentration of desired ions in the solution.

The vessel is normally such that it contains a bath of the salt electrolyte with both the reference and warning electrodes positioned in the bath. The lowermost portion of the warning electrode is located above a position at which the reference electrode would become ineffective as a reference electrode. During normal operation, electrical contact is made between the warning electrode and reference electrode by the salt electrolyte. However, after prolonged use, or under abnormal conditions, the salt electrolyte may have been lost from the vessel, for instance as a result of leaching through the permeable member. When the level drops to such an extent that the lower-most portion of the warning 4 GB 2 107 359 A 4 electrode is no longer immersed in electrolyte, the potential difference between the reference and warning electrodes increases significantly, and this change in potential difference is utilised to generate an appropriate warning signal. This may 70 be audible, visual or may be used to operate some control mechanism, for instance relating to the process being conducted using the solution whose ion concentration is being measured.

The permeable member is generally a porous plug, usually of ceramic material, connected by a tube to the vessel.

The salt electrolyte is often aqueous potassium chloride. The warning electrode may be a stainless steel rod. The reference electrode may be chosen having regard to the ion concentration system involved but may be, for instance, a calomel electrode in glass.

The remainder of the ion concentration system, and in particular the ion concentration electrode, will be selected according to the solution and ion concentration that is to be determined. It may be, for instance, a pH measuring system in which case an electrode of the type conventional for salt bridge pH measurement systems can be used.

This aspect of the invention is, however, of particular value when applied to the measurement of free fluoride ion concentration in a solution, and in particular the acidic fluoride solution described above. The ion concentration electrode may thus be a fluoride ion concentration electrode of known construction, for instance a resilient plastic member tipped with e.g. a metal fluoride.

A preferred process and apparatus according to the invention is illustrated diagrammatically in the accompanying drawing. An acidic fluoride solution 1 is provided in a conventional reservoir and is utilised for spraying aluminium surfaces at the temperature of the working solution by means 105 not shown. The reservoir may be provided by means not shown for maintaining the temperature constant. All this is standard and so does not necessitate further description.

A fluoride ion specific electrode 2 is immersed 110 in the bath 1 and is connected to a reference electrode 3 formed of calomel in glass by conventional fluoride measuring means 4 whereby the free fluoride content of the bath can be determined. The reference electrode 3 is immersed in an aqueous potassium chloride electrolyte solution 5 contained in a salt bridge vessel 6, having a tube 7 leading to a ceramic porous plug 8 that is immersed in the working bath. A warning, stainless steel, rod electrolyte 9 dips into the salt electrolyte solution 5, with its lowermost portion being positioned at the level shown by the dashed line 22, which will be seen to be above the top of the outwardly extending reference electrode 3. Electrical connection is made between the reference and warning electrodes by warning control means 10 that include an appropriate warning device to warn when the potential difference between the electrodes 3 and 9 increases undesirably. 130 In a modification, the salt bridge and its associated components 3, 5, 6, 7, 8, 9 and 10 may be replaced by a sealed reference electrode of conventional construction.

A temperature sensor 13 is also provided in the solution 1 and generates a signal in response to the variation of the temperature from a predetermined value, generally the optimum working temperature of the solution, typically around 501C. This signal is fed to a control unit 11 that has initially been programmed with the relationship between temperature and desired fluoride content. This control unit also receives a signal from the fluoride measuring means 4 in response to the fluoride that has been measured and computes the signals to give an output signal that controls pump 14 by which hydrogen fluoride replenishment may be fed to the bath from inlet 15 through duct 16. the control unit 11 also generates signals to display the actual fluoride content on the digital display device 12.

A conventional acidic conductivity measurement device 17 is also immersed in the bath and is connected to the acid control unit 18.

This generates a signal in response to variations of the acidic conductivity from a predetermined value and this signal operates the pump 19 whenever acidity drops below the predetermined value. Pump 19 feeds acid replenishment from inlet 20 and through discharge duct 21 into the bath.

Effective operation of the control units 11, and 18, and the sensors 2, 13 and 17 is controlled by a timer 23 that includes a mark/space ratio circuit. This timer starts a replenishment cycle every four minutes and in each cycle the pump 14 supplying fluoride solution replenishment is operated at a predetermined rate for a time that is controlled by the control unit 11 and that depends upon a fluoride and temperature values, so as to maintain the fluoride ion concentration at the desired, temperature compensated, constant value. The pump 19 supplies sulphuric acid or other acid replenishment solution also at a predetermined rate for a time that is controlled in response to the signal from the control unit 18. Alternatively the control unit 18 and the conductivity sensor 17 may be omitted and the pump 19 may be operated in fixed relationship to the time of operation of the pump 14. The timer 22 typically is designed to start each replenishment cycle every four minutes and during each cycle the duration of operation of the pumps is dependent upon the amount of replenishment that is required. The fluoride concentration is displayed on the display unit 12, and the temperature may also be displayed on this.

The bath is generally initially formulated to have a temperature of between 30 and 701C, a pH of below 3, often from 1 to 2.5 and most preferably about 1.5 and a free fluoride content of 0.00005 to 0. 1 g/1 preferably 0.000 1 to 0.005 g/1, and most preferably about 0.0008 9/1 free fluoride.

Such a bath can easily be formulated using GB 2 107 359 A 5 appropriate amounts of hydrofluoric acid, or other fluoride-providing material, and sulphuric acid, and the bath may also include anionic surfactant. The bath is used for spray cleaning of aluminium cans in conventional manner.

Claims

1. A process of cleaning aluminium surfaces by 65 contact with an aqueous acidic solution containing fluoride that is replenished during use and in which replenishment of fluoride is effected automatically by measuring the active free ion content and automatically adding fluoride in response to the deviation of the actual content from a predetermined desired value of free fluoride, and in which replenishment of acid is effected by (a) measuring the acid conductivity of the solution and automatically adding acid in response to the deviation of the actual acid conductivity from a predetermined desired value, or (b) by adding acid in an amount that is in a predetermined fixed relationship to the amount of fluoride that is added.

2. A process of cleaning aluminium surfaces by contact with an aqueous acidic solution containing fluoride that is replenished during use and in which replenishment of fluoride is effected automatically by measuring the active free fluoride ion content and automatically adding fluoride in response to the deviation of the actual content from a predetermined desired value of free fluoride and in which the temperature of the solution is measured and the said predetermined desired value of free fluoride is automatically varied in response to, and to compensate for, the deviation of the actual temperature from a predetermined temperature.

3. A process according to claim 2 in which 95 replenishment of acid is effected by (a) measuring the acid conductivity of the solution and automatically adding acid in response to the deviation of the actual acid conductivity from a predetermined desired value, or (b) by adding acid 100 in an amount that is in a predetermined fixed relationship to the amount of fluoride that is added.

4. A process according to claim 2 or claim 3 in which the actual free fluoride ion content is 105 displayed visually.

5. A process according to claim 1 or claim 3 in which replenishment of acid is by adding acid in an amount that is in a predetermined fixed relationship to the amount of fluoride that is 110 added.

6. A process according to claim 5 in which acid is added at time intervals not greater than a predetermined value.

7. A process according to claim 5 in which 115 acidity is measured and acid is added in response to deviations of acidity from a desired value.

8. A process according to claim 7 in which acid conductivity is measured and acid is added in response to deviations of the acid conductivity from the desired value.

9. Apparatus for use in the process of claim 1 comprising means for measuring free fluoride ion content, means for adding fluoride in response to the deviation of the actual content from a predetermined desired value of free fluoride, and means for effecting acid replenishment comprising (a) means for measuring the acid conductivity of the solution and means for automatically adding acid in response to the deviation of the actual acid conductivity from a predetermined desired value or (b) means for adding acid in an amount that is in a predetermined fixed relationship to the amount of fluoride that is added.

10. Apparatus for use in the process of claim 2 comprising means for measuring free fluoride, means for adding fluoride in response to the deviation of the actual content from a predetermined desired value of free fluoride, and means for measuring the temperature of the solution and for automatically varying the said predetermined desired value of free fluoride in response to, and to compensate for, the deviation of the actual temperature from a predetermined temperature.

11. Apparatus for measuring the ion concentration in a solution, the apparatus comprising a salt bridge comprising a vessel for containing a salt electrolyte, a permeable member that can be immersed in the solution and that is connected to the vessel such that the salt electrolyte can be in electrolytic contact with the solution while being substantially prevented from flowing out of the vessel, a reference electrode positioned in the vessel for use in measuring the ion concentration in the solution, and a warning electrode electrically connected to the reference electrode and positioned in the vessel such that it will generate a warning signal when the level of salt electrolyte in the vessel drops below a desired level.

12. Apparatus according to claim 11 also including an ion concentration electrode that can be positioned in the solution and that can be electrolytically connected to the reference electrode to generate a signal in response to the concentration of desired ions in the solution, the ion concentration electrode being a free fluoride ion electrode.

13. Apparatus according to claim 9 or claim 10 in which the means for measuring free fluoride ion is apparatus according to claim 12.

Printed for Her Majesty's Stationery Office by the Courier Press, Leamington Spa, 1983. Published by the Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained