GB2080832A

GB2080832A - Process for cleaning a device

Info

Publication number: GB2080832A
Application number: GB8123513A
Authority: GB
Original assignee: Interatom Internationale Atomreaktorbau GmbH
Current assignee: Interatom Internationale Atomreaktorbau GmbH
Priority date: 1980-08-01
Filing date: 1981-07-31
Publication date: 1982-02-10
Also published as: DE3029166C2; GB2080832B; DE3029166A1; FR2487860A1; FR2487860B1

Abstract

A device is cleaned by contacting it with a reagent. The device can comprise a soldered aluminium heat exchanger, and the reagent can be used to clean it after soldering. The reagent is passed in contact with the device for a time which is such that the reactivity increase of a unit volume of reagent, resulting from a temperature increase due to an exothermic reaction between the reagent and that device, is substantially balanced by a reactivity decrease caused by exhaustion of the reagent. Such a behaviour is useful when cleaning the interior of small components, since, with known reagents, the inlet gets excessively cleaned and the reagent is exhausted by the time it reaches the interior.

Description

SPECIFICATION Process for cleaning a device This invention relates to a process for cleaning a device. This invention more particularly, but not exclusively, relates to a cleaning process for soldered hollow aluminium components with a very small ratio of interior volume to interior interior surface, particularly for those having thin walls, moreover, the present invention relates to a cleaning process for such aluminium components, the solder of which contains elemental silicon and which are later to come into contact with gases which contain Uranium hexafluoride (UF6). Aluminium-plate heat exchangers which are soldered for example in hot salt baths are produced from very thin sheets and contain numerous channels which with only have a small clear sectional area.

After soldering, soluble and insoluble impurities and those in the form of dust remain, which can cause disturbances in the heat exchangers or in a unit containing the heat exchanger. In the presence of UF6, the elemental silicon which occurs in solder leads to a further disturbance by the formation of solid and gaseous reaction products.

Normally, aluminium components are dipped for a short time in a quick-acting reagent of a corrosive type, e.g. a solution of nitric acid and hydrofluoric acid. This process is not suitable however for components which have thin walls and small interior areas and therefore are not easily accessible to the reagent. When the reagent comes into contact with the metal, heat is produced which increases the reactivity of the utilised reagent, thereby accelerating the chemical process and leading to an undesirable strong removal of the metallic surface near the inlet of a hollow device, as the amount of the fluid filling such as interior is only small and is heated up quickly. The heat produced can only be conducted away with difficulty because of the shape of the component.On the other hand, the effectiveness of the reagent is quickly reduced so that the parts which only come into contact with it later are not cleaned sufficiently. The furthermore desirable selective removal of the elemental silicon from the soldered zones has not been possible with known reagents.

What is desirable is a cleaning process for soldered aluminium components which have thin walls and small hollow areas, particularly for heat exchangers made of thin aluminium sheets. Also, if the solder contains elemental silicon and is later to come into contact with gases containing UF6, then this silicon should be removed.

According to the present invention there is provided a process for cleaning a device with a liquid cleaning reagent, wherein each unit volume of the reagent is passed in contact with the device to clean that device for a time which is such that the reactivity increase of said unit volume of reagent, resulting from a temperature increase due to an exothermic reaction between the reagent and that device, is substantially balanced by a reactivity decrease of said unit volume of reagent caused by exhaustion of the reagent, whereby the reactivity of the reagent remains substantially constant whilst the said unit volume of reagent is in contact with said device.

Preferably the reagent comprises an acid and an inhibitor. The process can be applied to soldered devices, for example soldered aluminium devices, and to hollow devices.

The reagent is conveniently passed in contact with the device a plurality of times, with the reactivity of each unit volume of the reagent remaining substantially constant during each time that it is in contact with the device. The reagent can be passed in contact with the device by forced circulation. Preferably the reagent is passed in contact with the device, until the reactivity of the reagent is one tenth of its initial reactivity.

The reagent can comprise nitric acid and hydrofluoric acid, together with the inhibitor.

By the circulation of a reagent of a deliberately reduced initial reactivity, the rate of change of the reactivity, as a result of the exhaustion of the reagent on the one hand and the increase in temperature arising from the reagent reacting on the other hand, is negligible. These reactions which oppose each other even out in the main (i.e. with a deviation of not more than about i 10%) so that all parts react evenly with the reagent and therefore neither damage due to too high reactivity or inadequate cleaning due to too little reactivity can occur.This process is preferably applied by circulation of the reagent until the process is complete, whereby the final reactivity of the reagent should not be less than one tenth of its initial reactivity in order that the length of the process, which is increased anyway in relation to the process in use up to now, is not allowed to rise too sharply.

Aluminium nitrate can be used as an inhibitor, and is a substance which is formed by the reaction of aluminium with a reagent comprising nitric acid and hydrofluoric acid and it reduces the effectiveness of the reagent. Its intentional addition before the beginning of the process reduces the reactivity of the reagent during the first highly-reactive phase of the reagent and converts this initial phase immediately into a state which would otherwise only be reached after a certain reaction time.

The reagent is preferably passed through a device from a lowest point of that device to a highest point thereof, in order to bring about an evening out of the reaction process, the reagent flowing for example upwards through a plate heat exchanger in a vertical direction.

Thus the speed of through-put is able to be controlled better and the temperature increase limited. If filling with the reagent took place from above, the reagent would spread out unevenly and in individual partial flows, such that the reaction time and the cleaning result would vary.

A device is preferably additionally cleaned by contact with a second reagent comprising an alkali and a second inhibitor, for removing elemental silicon present in solder of the soldered device. The alkali provides for selective removal of the elemental silicon from the surface of the soldered zones. Sodium silicate can be added as an inhibitor to cause the considerable removal of aluminium otherwise ensuing, to be reduced sufficiently to bring about a desired concentration and the reaction time of the second reagent necessary for removing the silicon. The further inclusion of sodium aluminate improves the surface quality of the finished device.

For a better understanding of the present invention, a description will now be given, by way of example, of a particular process in accordance with the present invention.

The process was carried out on a heat exchanger, which had the following dimensions: Ratio of interior volume to interior surface 0. 3-3 1 /m2, preferably 1 1 /m2; Height 1-6m, preferably 2.50m; Cross-sectional area 0.04-4m2, preferably 1 m2; Thickness of the heat exchanger sheets 0.1-0.8mm, preferably 0.2mm; Clear distance between the heat exchanger plates 1-10 mm, preferably 2mm.

The cleaning process was carried out by passing various reagents, and rinsing fluids, upwards through the heat exchanger. By passing the fluid upwards through the heat exchanger, the flow rate of the various reagents and the time spent passing through separate passageways within the heat exchanger corre spond to one another and to the geometrical shapes of the passageways and the total surface, interior volume and height of the heat exchanger. In the following steps carried out during the process, the percentage figures refer to percentages by weight.

The individual steps of the process are: a) Rinsing the heat exchanger with de-salted tap water at room temperature and with a chloride content of less than 10 ppm.

b) Pre-washing the heat exchanger with a dilute tenside solution at 50-80'C, preferably with a pH value of 1.5.

c) Rinsing the heat exchanger with de-salted tap water at room temperature and with a chloride content of less than 1 Oppm.

d) Cleaning the heat exchanger with a first reagent which initially is at a temperature in the range 70-95'C and which comprises: Nitric acid 2-1 0%, preferably 5%; Hydrofluoric acid 0.05-0.05%, preferably 0.2%; Aluminium nitrate 0.5-5%, preferably 2.1%.

The ratio of the volume of first reagent to the interior surface area contacted by the first reagent is preferably 5 1 /m2. The time the reagent remains in contact with the heat exchanger during each pass through the heat exchanger is 1.5 mins, and the total reaction time is 30 mins.

e) Rinsing the heat exchanger with completely desalted water, having a conductivity of less than 1 ,uS.

f) In order to remove silicon, cleaning the heat exchanger with a second reagent, which is at a temperature in the range 70-90'C, and which comprises: Sodium hydroxide 0.3-2%, preferably 0.4%; Sodium aluminate 1-0.5%, preferably 0.002% (as aluminium powder); Sodium silicate 0.5-5%, preferably 2.5%.

The ratio of the volume of the second reagent to the interior surface area contacted by the second reagent is preferably 3 l/m2. The total reaction time is 120 mins. Other alkalis with othersuitable negative ions can also be used.

g) Rinsing the heat exchanger with completely de-salted water, having a conductivity of less than 1 ,uS.

h) Cleaning the heat exchanger with a third reagent which comprises: Nitric acid 2-10% preferably 3%; Hydrofluoric acid 0.05-0.5% preferably 0.1%; Aluminium nitrate 0.5-5% preferably 1%; The ratio of the volume of the third reagent to the interior surface area contacted by the third reagent is preferably 5 I/rn2. The time the third reagent remains in contact with the heat exchanger during each pass through the heat exchanger is 1.5 mins, and the total reaction time 5 mins.

i) Rinsing the heat exchanger with completely de-salted water having a conductivity of less than 1 yS.

j) Rinsing the heat exchanger with pure, technically residuefree methanol or ethanol of at least 99% purity at room temperature (this alcohol should contain no dissolved organic substances, e.g. traces of laquer from transport containers).

k) Evacuating the heat exchanger to a pres sure of 10 - mbar under an oil-free vacuum at room temperature.

i) Filling the heat exchanger with dry inert gas (nitrogen or Argon) and then pmviding ah alf tight closure.

When carrying out the above described process, in addition to the heat exchanger or other part to be cleaned, a mechanical filter should be provided in order to prevent a soiling of the various reagents and rinsing fluids by impurities already removed in previous circuits.

In the above described process, for rinsing a device, de-ionised water can be used instead of de-salted water.

Claims

1. A process for cleaning a device with a liquid cleaning reagent, wherein each unit volume of the reagent is passed in contact with the device to clean that device for a time which is such that the reactivity increase of said unit volume of reagent, resulting from a temperature increase due to an exothermic reaction between the reagent and that device, is substantially balanced by a reactivity decrease of said unit volume of reagent caused by exhaustion of the reagent, whereby the reactivity of the reagent remains substantially constant whilst the said unit volume of the reagent is in contact with said device.

2. A process as claimed in claim 1, in which the cleaning reagent comprises an acid and an inhibitor.

3. A process as claimed in claim 1 or 2, when applied to soldered devices.

4. A process as claimed in claim 1, 2 or 3, when applied to hollow devices which are open at opposite ends thereof, wherein the reagent is passed through and/or around a hollow device.

5. A process as claimed in any preceding claim, wherein the reagent is passed in contact with the device a plurality of times, with the reactivity of each unit volume of the reagent remaining substantially constant during each time that it is in contact with the device.

6. A process as claimed in claim 5, wherein the reagent is passed in contact with the device by forced circulation.

7. A process as claimed in claim 5 or 6, wherein the reagent is passed in contact with the device until the reactivity of the reagent is one tenth of its initial reactivity.

8. A process as claimed in claim 2 or any one of claims 3 to 7 when appendant to claim 2, in which the reagent comprises nitric acid and hydrofluoric acid, together with said inhibitor.

9. A process as claimed in claim 3 or any one of claims 4 to 8 when appendant to claim 3 when applied to soldered aluminium devices.

10. A process as claimed in claims 3 and 4 or any one of claims 5 to 9 when appendant to claims 3 and 4, when applied to hollow soldered devices which have a small ratio of internal volume to internal surface area.

11. A process as claimed in claim 2 or any one of claims 3 to 10 when appendant to claim 2, in which the inhibitor comprises aluminium nitrate.

12. A process as claimed in claim 4 or any one of claims 5 to 11 when appendant to claim 4, in which the reagent is passed through a device from a lowest point of that device to a highest point thereof.

13. A process as claimed in any preceding claim, in which a device is given a first rinse in de-salted water at room temperature with a chloride content of less than 1 Oppm, before the reagent is passed in contact with the device.

14. A process as claimed in claim 13, in which after the first rinse has been effected but before the reagent is passed in contact with the device, the device is given a prewash in a dilute tenside solution at 50-80 C followed by a second rinse in de-salted water at room temperature with a chloride content of less than 1 Oppm.

15. A process as claimed in claim 14, in which the dilute tenside solution has a pH of 1.5.

16. A process as claimed in claim 11 when appendant to claim 8, or claim 12, 13, 14 or 15 when appendant to claims 8 to 11, wherein the reagent comprises a solution which includes 2-10% nitric acid, 0.05-0.5% hydrofluoric acid, and 0.05-5% aluminium nitrate.

17. A process as claimed in claim 16, wherein the solution comprises 5% nitric acid, 0.2% hydrofluoric acid and 2.1% aluminium nitrate.

18. A process as claimed in any preceding claim, wherein after the reagent has been passed in contact with the device, the device is given a third rinse with de-salted water having a conductivity of less than 1 ,uS.

19. A process as claimed in claim 3 or any one of claims 4 to 18 when appendant to claim 3, wherein after the first-mentioned reagent has been passed in contact with the soldered device, or when a third rinse is given after this third rinse has been given, a second reagent comprising an alkali and a second inhibitor is passed in contact with the soldered device to remove any elemental silicon present in solder of the soldered device.

20. A process as claimed in claim 19, wherein the second reagent comprises sodium hydroxide, sodium silicate and the second inhibitor.

21. A process as claimed in claim 19 or 20, wherein the second inhibitor comprises sodium aluminate.

22. A process as claimed in claim 21 when appendant to claim 20, wherein the second reagent comprises a solution including 0.3-2% sodium hydroxide, 1-0.5% sodium aluminate, and 0.5-5% sodium silicate.

23. A process as claimed in claim 22, wherein the solution of the second reagent comprises 0.4% sodium hydroxide, 0.002% sodium aluminate, and 2.5% sodium silicate.

24. A process as claimed in claim 19, 20, 21, 22 or 23, wherein after a second reagent has been passed in contact with a soldered device, the soldered device is given a fourth rinse of de-salted water, with a conductivity of less than 1 ,uS.

25. A process as claimed in claim 24, wherein, after the soldered device has been given a fourth rinse, a third reagent is passed in contact with the soldered device.

26. A process as claimed in claim 25, wherein the third reagent has the same composition as the first-mentioned reagent.

27. A process as claimed in claim 25 or 26, wherein, after the third reagent has been passed in contact with the soldered device, the soldered device is given a fifth rinse with de-salted water with a conductivity of less than 1 juS.

28. A process as claimed in claim 27, wherein after the fifth rinse has been given the soldered device is given a sixth rinse with rnsiduefree methanol or ethanol at room temperature having at least 99% purity.

29. A process as claimed in claim 28, when appendant to claim 4, wherein, after the sixth rinse, the hollow soldered device is evacuated to a pressure of 10-1m bar by an oilfree vacuum and is then filled with dry inert gas which is sealed in.

30. A process, for cleaning a device, substantially as hereinbefore described.

31. A device when cleaned in accordance with a process as claimed in any preceding claim.