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Waterbased cleaning of mixing vessels

Appendix B

TRANSLATION of

1 Aim of the project

The mapping of waste flows from the lacquer and paint industry which was carried out by the Danish Paintmakers’ Association for lacquer and paint manufacturers for the Danish National Environmental Protection Agency in 1996, shows that the VOC-emission is in an area where it is possible to reduce the environmental impact.

Based on the above an assessment of the VOC-emission has been carried out in each department of the reference company. The VOC-emission from cleaning the mobile mixing vessels is estimated to 25-30% of the total VOC-emission from the production of solvent-borne paint.

The total VOC-emission from the Danish lacquer and paint production is estimated to approx. 450 tons per year. In other words it will be possible to abate the emission of VOC with approx. 110 tons per year by changing to a water-based cleaning of the mixing vessels. The reduction will correspond to approx. 10% of the total VOC-emission from the chemical industry in Denmark.

Another considerable argument for transition to water-based cleaning of the mixing vessels is a significant improvement of the working environment in the vessel cleaning area.

2 Thinner-based cleaning

Today, the cleaning of the mixing vessels takes place in a heavily loaded working environment. The premises are equipped with powerful ventilation, but nevertheless it is necessary to wear personal respiratory protective device.

First the mixing vessel is cleaned manually with organic solvents in order to remove the major part of the paint residues. Subsequently the vessel is placed on the cleaning machine. The machine cleans the vessel with rotating brushes in presence of organic solvents.

Organic solvents and paint sludge are collected by means of a grating in the floor and are lead to distillation. The solvents distilled off are reused for cleaning of mixing vessels. Due to this process it is impossible to determine the exact composition and with that the dangerousness of this mixture of organic solvents.

The distillation residues are collected and sent to destruction at the Intermunicipal Chemical Waste Treatment Plant, "Kommunekemi".

Hot alkaline bath

Removal of paint can be carried out by alkaline cleaning (pH > 13), at a temperature of 50-95 degr. C.

The chemical paint binders are hydrolysed due to the high alkalinity. The penetration of the lacquer film and the migration during this can be increased by addition of surface-active agents.

Strong hydrolysed and aggressive leaching solutions can be used to remove paint from steel whereas strong organic bases are used for other metals.

Alkaline cleaning is very economical and is capable of removing most kinds of paints /2/, /3/.

Acid paint remover

These are based on strong inorganic acid. They are effective by degenerating the paint binder in the lacquer film. Sulphuric acid (pH < 1) is often used.

The acidic paint removers are used to remove epoxy and polyamide paint from hot sensitive items. By choice of acid, the acid resistance of the material must be considered. The temperature in the cleaning vessel is usually from 20-50 degr. C /2/, /3/.

Neutral paint removers

Neutral paint removers are based on glycol, glycol ether, N-Methyl-pyrrolidone or mixtures from this. The temperature of the cleaning vessel is 20-40 degr. C.

The use of neutral paint removers is confined to the physically drying lacquers /2/, /3/.

Mechanical removal of paint

By means of airless cleaning with water paint can be removed from hard surfaces (700 - 1000 bar).

Blasting with sand, plastic or other suitable materials may also be used /2/, /5/.

Thermal removal of paint

The methods are based on combustion and incineration of the paint at temperatures of 300-500 degr. C for 20-60 min.

The incineration may also take place in salt baths increasing the speed to 30-120 sec.

These pyrolytic methods are very energy-demanding and the construction and operating costs are considerable /2/, /4/.

Low temperature paint removers

Due to the fact that the paint shrinks by cooling with liquid nitrogen (-196 degr. C) this is also a suitable method for removal of paint (the cooling

process last 1-3 min.) /2/.

Airless cleaning with water

Airless cleaning with water and subsequent filtration must be considered the most pollution free cleaning method. This type of plant will approx. amount to DKK 250,000 (everything included).

WORMA in Düisburg, Germany is represented by O & J Højtryk in Esbjerg /5/.

The method is only reported used on soft lacquers, however it is considered suitable for the purpose.

CIP-cleaning with leaching solution/acid

These methods are well-known from among others the pharmaceutical industry and the food processing industry. The suppliers of the plant say/declare concurrently that approx. 2.5% ALKAREN 45 or a similar product is used. Based on this information the preliminary tests with this concentration are carried out.

The temperature in the cleaning vessels fluctuates from 70-80 degr. C. Søren Hauritz from Akzo Nobel is of the opinion that alkaline cleaning is the best solution. The reasons adduced for this opinion is the bad experience with alkaline CIP-cleaning of mixing vessels. Akzo Nobel use a 30% sodium hydroxide (corresponding to approx. 15% NaOH) with a bath temperature of approx. 70 degr. C.

However, it should be investigated if a combined product like ALKAREN 45 may be used in a lower concentration (possible 2.5% as stated by the suppliers of the plant).

Neutral paint removers

Glycols, glycol ethers and N-pyrrolidone are organic compounds acting as organic solvents. Due to their high boiling point and low evaporative pressure the substances are not defined as organic solvents. These substances will contribute to a strong increase of COD in the wastewater.

Preliminary laboratory tests are made to examine the efficiency of the substances.

Thermal methods

The pyrolytic methods will cause considerably initial costs and besides, these methods demand a lot of energy. Obviously, it is estimated that these methods are not applicable for this purpose.

Cooling with liquid nitrogen must be considered as a pure academic method, being possibly applicable within the laboratory area.

Nothing further will be done in relation to these methods.

5 Laboratory tests

The investigations are instituted with a view to prove that it is possible to remove paint with acidic or alkaline CIP-cleaning materials. Further the efficiency of the Neutral paint remover, especially developed for this project, is investigated.

Apply paint on the sheets and air-dry for one hour at ambient temperature. After this the sheets are dipped in the cleaning solvent while stirring carefully. The solvent has been heated to the temperature required.

The paint is completely removed after 10 min. In case the paint is not removed after expiration of the time, the test is repeated either with a higher temperature or a larger concentration of the cleaning solvent.

Alkaline bath: COMBIREN is a leaching-based combined cleaning solvent, which is widely used within the food processing industry. COMBIREN is mainly used in cases where most other alkaline CIP-cleaning materials cannot remove the paint.

COMBIREN contains: Sodium hydroxide, potassium hydroxide and complex agents.

Acid bath: NOVASYRE is based on sulphamic acid and a combination of organic acid. The product is used within the food processing industry where a strong phosphate free cleaning is required. The composition of the different organic acids provides a high acid strength without utilizing traditional acid such as phosphoric acid and nitric acid.

NOVASYRE contains: Sulphamic acid, organic acids and tensides.

Neutral bath: NOVALAK is a N-methyl-pyrrolidone based paint remover which is specially developed to comply with possible problems with leaching solution and acid. The product does not give rise to anxiety as regards corrosion and can be used on all metals.

NOVALAK contains: N-methyl-pyrrolidone and tensides.

The results carried out on steel sheets appear from enclosure 1.

Beyond the tests on steel, a number of tests have been made on teflon-coated aluminium. These sheets have been delivered from ACCOAT A/S.

However, results from these tests are not included. During the tests the aluminium corroded and after 1-2 preliminary tests it was impossible to make further tests.

IAT Kolding

The preliminary tests are carried out with a view to investigate if it is possible to remove the paint with water, pressure and heat.

The CIP-plant performs a constant pressure of 110 bar and the temperature can be adjusted from 8-100 degr. C.

The tests are carried out on teflon-coated vessels and steel vessels. Two types of teflon-coatings are used ACCOFAL 2G and ACCOPON 2G.

ACCOFAL 2G is softer and seems smoother (more dirt-repellant) than ACCOPON 2G. Alkyd simply crawls off when the coating is applied. It is possible to clean the vessel for paint having dried up for approx. 30 min. at ambient temperature. During the tests it appeared that it is very important with a cold rinse before the actual cleaning is carried out. In case the pre-rinse is left undone the paint will polymerize and become rubbery and it is then impossible to remove the paint (the results appear from enclosure 2).

ACCOPON 2G seems very hard and is rather rough to look at. After the cleaning a small edge was left, however it was easy to remove with a nail. During the test of removing the paint edge it turned out that the coating of the vessel left green pigments in the white lacquer. Later, ACCOAT A/S investigated the vessel and had to admit that the coating was not carried out properly - the teflon was undercured.

Generally speaking, the teflon-coating sticks very poorly and is easily removed. The coatings are "very" soft and that is why the production management in the reference company has expressed their doubts that teflon-coatings can be used in mobile mixing vessels. Today, the company utilizes teflon-coatings on stationary mixing vessels.

A test has been made on a steel vessel according to above conditions. Altogether the water had no effect on the paint (the results appear from enclosure 2).

O & J Højtryk, Esbjerg

Tests have been made with pressure from 400-1150 bar. It clearly appeared from the tests that it is impossible to remove a paint film from a steel surface with water and pressure being undercured (the paint was moved around in the vessel).

Tests on undercured epoxy were also carried out. At a pressure of 1150 bar it was very easy to remove the lacquer.

IAT Kolding

IAT Kolding undertakes cleaning of tankers and pallet tanks. We are in a position to rent the plant for cleaning of pallet tanks. The plant is a modified Kärcher high-pressure cleaner for industrial cleaning with a rotating spud.

First a pre-rinse for 5 min. The plant performs 18-20 litres of water/min. and a pressure of 90-100 bar. It is not possible either to adjust the pressure or the concentration of cleaning solvent on the plant.

Alkyd: After a total cleaning time of 30 min. a ALKAREN 45 concentration of 1.2% and a bath temperature of 70 degr. C the paint was completely removed from the steel surface.

Epoxy: After a cleaning time of 60 min. 75% of the paint was removed. The remaining 25% was easy to remove with a slight mechanical effect. The concentration of the cleaning solvent and the bath temperature was 1.2% ALKAREN 45 and 70 degr. C respectively.

Similar tests were carried out on the teflon-coatings. It was not possible to remove any of the paint from the teflon-coatings - even after 60 min.

Brüel International A/S

Brüel International manufactures tunnel washers and cabins for the chemical, pharmaceutical and food processing industries. The company has experiences with cleaning of mixing vessels from a Norwegian producer of paint and lacquer.

The Norwegian paint and lacquer manufacturer uses an alkaline combined cleaning solvent corresponding to ALKAREN 45 from NOVADAN A/S. The concentration of the cleaning solvent is approx. 2.5%, the bath temperature is 75 degr. C, and the pressure is approx. 7 bar.

Brüel International’s own experiences draw attention to the fact that a too high pressure (above 10 bar) makes the cleaning difficult. This fact is identical with our results with high pressure.

The tests are carried out on a Wemag washer. The machine is a cabinet washer, manufactured for washing butcher’s vans within the slaughterhouse industry, the so-called Wemag van (machine type CB1-42).

The machine performs 100 litres per min., the pressure is 7 bar and the temperature can be adjusted from 8-95 degr. C.

After some unsuccessful attempts a short pre-rinse for 300 sec. is chosen, the cleaning time is set to 900 sec. and the bath temperature is set to 80 degr. C.

During the unsuccessful attempts with temperatures below 80 degr. C, the ALKAREN 45 concentration was gradually increased from 3% to 5.7%. It was not possible to thin the leaching solution in the plant and due to lack of time it was not possible to start up a new attempt (the plant had to be sent to Sweden).

Tests with above cleaning procedure were carried out on alkyd, epoxy, acrylic, and polyester. All types of lacquer were completely removed after the cleaning (the results appear from enclosure 5).

8 Discussion

The investigations show that the temperature plays an essential part as to the cleaning results. Thus, it was not possible to achieve a reasonable cleaning result without a cold pre-rinse. At a bath temperature below 80 degr. C it is impossible to remove the paint completely.

The total cleaning time of 20-25 min. must be considered as too long. However, it should be noted that neither time nor leaching solution is optimised. Therefore, future work should be concentrated about optimising these parameters.

The pre-rinse was set to 300 sec. in order to ensure a sufficient cooling of the paint film. During normal operation this time is probably too long. The reason for the longer rinse time is that the tests are carried out on the same vessel. Thus the vessel was not cold at the beginning of the test and due to this the drying time of the paint was also reduced. The vessel was approx. 80 degr. C by the time it was taken out of the cabin.

The cleaning cycle of 15 min. is also too long. A satisfactory cleaning result within a shorter period with the concentration of ALKAREN 45 should be obtainable.

It should be investigated how fast it is possible to clean a vessel with the chosen parameters. In case the time of the cleaning cycle is reduced to less than 300 sec. the following tests should concentrate about reducing the concentration of ALKAREN 45.

After optimizing the cleaning at 80 degr. C an investigation should take place aiming at the influence of a possible change of temperature on the cleaning result.

After optimizing the total cleaning procedure a test on all colour and lacquer systems in the production at the reference company should be carried out (see enclosure 1).

During tests on the cabinet washer it should be investigated further how large a dirt load the cleaning solvent manages before it has to be replaced. This condition will not only affect the wastewater amount and the water amount on the plant, but also the usability of the procedure as such. If the cleaning solvent needs to be replaced more than two times a day this may affect the capacity of the plant considerably. It may be very time-consuming to heat up the 800 liters of cleaning solution. Pumping of liquid from and to the machine also takes a long time.

The optimal solution would be to replace the cleaning solution one time every 24 hours. The regeneration of the liquid may take place by means of filters.

9 Conclusion

It is conceivable that a considerable low leaching solution may clean the mixing vessels from the production of paint and lacquer.

However, the cleaning time of 20-25 min. per vessel is too long. The future work must therefore be concentrated about optimizing the cleaning procedure. The total cleaning time per vessel must be between 5 and 7 min. corresponding to a capacity of 8-12 vessels per hour.

The concentration of the cleaning solvent is reduced so that the cleaning solvent is non-corroding. Thus, the ALKAREN 45 concentration must be below 3% corresponding to a NaOH concentration less than 1% (local irritating).

The used cabinet washer must also be considered applicable for the purpose.

Reference list

1. NOVADAN A/S, results from laboratory tests
2. IAT Kolding, results from cleaning tests with water
3. Data sheet on ALKAREN 45
4. IAT Kolding, results from cleaning tests with leaching solution
5. Brüel, results from cleaning tests with leaching solution in cabinet washer

Enclosure 1

Solubility tests carried out by NOVADAN A/S

The test results carried out on steel plates on 19 February 1997 by Mr. Ole Christensen.

Enclosure 2

Cleaning tests with water at IAT on 10 May 1997

The plant performs 18-20 litres of water/min. and performs a pressure of approx. 110 bar.

The tests have been carried out on a vessel coated with ACCOFAL 2G from ACCOAT. The cleaning time shall be considered with reservation as all the tests have been carried out on the same layer of paint (accumulated).

The tests have been carried out on a vessel coated with ACCOPON 2G from ACCOAT.

The test has been carried out on steel.

The tests have been carried out on a vessel coated with ACCOFAL 2G from ACCOAT. The cleaning times shall be considered with reservation as all the tests have been carried out on the same layer of paint (accumulated).

The tests have been carried out on a vessel coated with ACCOPON 2G from ACCOAT. The cleaning times shall be considered with reservation as all the tests have been carried out on the same layer of paint (accumulated).

Enclosure 4

Cleaning tests with leaching solution at IAT on 7 June 1997

The plant performs 18-20 liters of water/min. and performs a pressure of 90-100 bar.

The test has been carried out on steel. The cleaning times shall be considered with reservation as all the tests have been carried out on the same layer of paint (accumulated).

The test has been carried out on steel. The remaining 25% of paint shall easily be removed by means of mechanical effect (a nail). The cleaning times shall be considered with reservation as it has not been registered when the paint has become soft.

Similar tests have been carried out on teflon-coatings, but the leaching solution had no effect.

Enclosure 5

Cleaning tests in cabinet washer at Brüel on 18 June 1997

The cabinet washer performs a constant pressure of 7 bar.

The test has been carried out on steel. The cleaning times shall be considered with reservation as all the tests have been carried out on the same layer of paint (accumulated).

The test has been carried out on steel.

The test has been carried out on steel.

The test has been carried out on steel.

The test has been carried out on steel.

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