Acid etching, economic and environmental consideration
Federico Vincenzi
Italtecno, Italy
ABSTRACT
Acid etching is a well-known technology that provides an impressive matt finishing in short time and with very low aluminum removal. The drastic reduction of aluminum removal offers great economic advantages but unfortunately the pollution, the water emission limits and the material incompatibility make the traditional process not universally applicable.
In this paper we illustrate all the different technological achievements that make the acid etching available even in new fields where it was not possible until now (for example where titanium racking is a must). The latest developments on emissions reduction in waste waters treatment are described in detail, in particular: ammonium content is very important in environmental standards of different countries and it is becoming more and more stringent.
1 – ACID ETCHING
Acid etching has been known and used since 1930; Arthur Brace (6) reports a smooth “white” etching obtained in a solution of 2.5% by wt of Ammonium Bifluoride and 1,5% by wt of ammonium sulphate.
The technology, abandoned for many decades for many different technical reasons, has been recently brought on the market again with revised formulas mainly due to the positive environmental impact compared to alkaline etching.
This research was carried out to develop a new system of acid etching, with new formulas and new technologies to replace, with great advantages, the alkaline etching.
These are the requirements expected for the development of the new acid etching process:
- Low aluminium dissolution.
- Significant reduction in the caustic soda consumption.
- Significant reduction in the volume of sludge for disposal.
- Reduction of the treatment time to increase productivity.
- Less viscous bath solution that makes the treatment appropriate for profiles with small cavities.
The developed process requires a first step in acid etch for 3 to 5 minutes at 45 °C followed by a brief immersion (about 1 minute) in a caustic soda bath at 55 °C with low concentration of dissolved aluminium and with the addition of a special additive called MG 39.
The process has been named Beauty Etch because of the beauty of the obtained finishing degree and the formulated product is called BE 10.
Graph 1: example of treatment cycle.
DESCRIPTION OF THE EXPERIMENTAL PART
In short the research was developed as follows:
- Formulation of preliminary product
- Preliminary tests.
- Refining and tuning of the product formula based on the results of the performed tests.
- Experimental tests for monitoring and evaluation of the new product.
- First industrial tests.
- Evaluation of the results.
- Mass marketing of the new product.
The first part of the tests concerned the formulation of the new product. The formula was gradually refined during the numerous tests that lasted several months.
Once the formula was developed we proceeded to carefully evaluate all the parameters and results.
Picture 1: Before and after treatment with BE 10
The study of the consumptions and the degree of aluminium removal were definitely the first parameters to be assessed because by means of this information it is possible to deduce a number of considerations, especially economic.
Graph 2: degree of aluminium removal from Beauty Etch treatment.
From Graph 2 we can deduce that the degree of aluminium removal is strongly influenced by temperature but much less by the time of treatment.
It is also possible to note that the degree of aluminium removal is about 10 times lower than that obtained by alkaline etching.
We then proceeded to calculate the consumptions of the process, in comparison with the alkaline etching, evaluating both the reaction consumption and the drag-out consumption.
Graph 3: Comparison of viscosity and solution drag-out between acid and alkaline etching.
Graph 3 shows that the average drag-out from the acid etching bath is about 5 times lower than that of alkaline etching. This excellent result was achieved thanks to the lower solution viscosity of acid etching. Please note that the drag-out of the solution of alkaline etching shown in Graph 3 must be referred to long-life baths with high concentration of dissolved aluminium but not to the alkaline etching bath used after acid etching (as this bath has a treatment time of 1 minute and then the concentration of dissolved aluminium will never be very high and therefore the viscosity and the drag-out will be on average of 0.2 l/m2).
Graph 4: Evaluation of the anodizing costs
From Graph 4 we can see that the total cost of the anodizing process depends by more than 35% on the alkaline etching treatment. The alkaline etching cost is mainly due to the following reasons:
- High quantity of “wet” sludge from water treatment (with consequent disposal costs).
- Chemical consumption of caustic soda and additive
- High percentage of dissolved aluminium during treatment
Graph 5: Comparison of sludge composition between Beauty Etch and alkaline etching.
Graph 5 shows that the sludge produced by the Beauty Etch process is very dry, with very little water content, in contrast with that obtained by alkaline etching that contains a lot of water. Due to the fact that the sludge is very dry and that the Beauty Etch process dissolves a small amount of aluminium during the treatment, consequently the volume of sludge to be disposed of is significantly lower than that produced by alkaline etching.
Graph 6: Comparison of the sludge formation to be disposed of between Beauty Etch and alkaline etching.
From Graph 6 it is possible to deduce that the formation of sludge to be disposed of by the Beauty Etch process is about 10 times less than traditional alkaline etching.
The next step was cost evaluation, concerning the cost of chemicals, the cost for the disposal of the by-products, and the cost due to the loss of aluminium removal.
Graph 7: Example of cost comparison between Beauty Etch and alkaline etching.
From Graph 7 it is possible to deduce that the savings using the Beauty Etch process may be more than 40% compared to traditional alkaline etching.
Combining the cost savings and the advantage of the better finishing that may be achieved with this treatment, with a significant reduction and masking of extrusion lines, almost similar to that achieved by mechanical etching, this means that the process can be considered a valid alternative to the existing alkaline etching process.
5 – FLUORIDE AND AMMONIUM ELIMINATION
The greatest concern in the development of this technology was the impact that the chemicals used in the treatment could have on the environment.
Most of the research was therefore focused on the separation of these chemicals from waste water and sludge.
The target of pollution elimination from water can be achieved using different treatments, with no expensive equipment. In this paper three different methods are described:
- Disposal of a static rinse after acid etching
- Chemical treatments of the first rinse after acid etching obtaining valuable fertilizer
- Special filtration through selective membranes of the first rinse after acid etching
All these systems can easily reduce the emissions under the required limits.
6 – DESCRIPTION OF THE TREATMENT METHODS
Disposing of the static rinse:
The first method depends on the amount of water available on the anodizing line (litres per square meter). According to the amount of water it is possible to estimate the maximum level of pollution (ammonium) that can be reached on the static rinse in order to respect the emission limits.
Chemical treatments of the first rinse after acid etching
The second method consists in the following steps:
- Batch treatment of a small amount of the static rinse (concentrated).
- Aluminium precipitation by chemical addition of dedicated formula
- Filtration of the aluminium salt
- Ammonium precipitation as a valuable fertilizer by chemical addition of a second dedicated formula
- Filtration of the ammonium salt
Chemical treatments of the first rinse after acid etching
The rinsing solution after acid etching treatment is treated as follows:
1. Chemical pre-treatment in order to obtain fluoride precipitation
2. First filtration.
3. pH- adjusting (using caustic soda)
4. Special treatment for ammonium separation trough special membranes system
5. Recovery of the concentrated ammonium solution (pure)
A very pure ammonium salt solution is obtained.
7 – CONCLUSIONS
Acid etching is a well-known technology that provides an impressive matt finishing in a short time and with very low aluminum removal. The drastic reduction of aluminum removal offers great economic advantages but unfortunately the pollution, the water emission limits and the material incompatibility make the traditional process not universally applicable.
In this paper we focalized the different technological achievements that make acid etching environmental friendly thanks to three different possibilities:
1. Disposal of a static rinse after acid etching
2. Chemical treatments of the first rinse after acid etching obtaining valuable fertilizer
3. Special filtration through selective membranes of the first rinse after acid etching
In comparison with acid etching the new technology allows an easy conversion from the conventional alkaline technologies without high investment costs for new equipment (tanks, filtration systems, equipment for fluoride treatment, etc...)
REFERENCES
(1) Ninth International Aluminium Extrusion Seminar & Exposition ET, USA, 2008
(2) A.W. Brace, “The technology of Anodizing Aluminium”, Chapter 5, www.interall.it
(3) (4) A.W. Brace, “The technology of Anodizing Aluminium”, 3rd edition, Chapters 7-8, www.interall.it
(5) A.W. Brace, “The technology of Anodizing Aluminium”, Chapter 5, page 72, www.interall.it
Italtecno Seminar, January 2010, Modena, Italy, This email address is being protected from spambots. You need JavaScript enabled to view it.
(6) A.W. Brace, “The technology of Anodizing Aluminium”, 3rd edition, page 32, www.interall.it
(7) Eighth World Congress Aluminium Two Thousand, Milan, ITALY, 2013 – This email address is being protected from spambots. You need JavaScript enabled to view it.
