Anyone who has experienced a kidney stone attack knows that it is a painful experience one wants to avoid in the future. A kidney stone normally consists of calcium oxalate and when leaving the body it can cause a lot of pain. This can, at least metaphorically, illustrate how parts of the pulping process may feel when it is hit by calcium oxalate precipitations.
Increasing environmental demands and needs to reduce water and energy consumption have lead to more closed process systems. Oxalic acid, as well as calcium, are present in the wood and is also formed during pulp bleaching. In certain process steps, where the chemical, temperature and pH conditions are favorable, the oxalic acid forms an only slightly soluble precipitation with calcium. This is why many pulp mills experience problems due to calcium oxalate precipitation in tubes and heat exchangers as well as on washing filters.
During end of 1980s and 1990s, MoDo, among others, worked hard to close the bleaching processes. The company succeeded in doing that in the Domsjö mill in Örnsköldsvik, which still has the only closed loop bleaching plant in the world, but could only partly close the bleach plant for birch pulp in the Husum mill, which could run in a closed-loop mode during shorter periods.
Karlstad University, Domsjö Fabriker, EKA Chemicals, Holmen Paper, M-real and Novozymes have participated in the project Control of oxalic acid in pulp manufacturing and in active packaging with Stora Enso, SCA and Södra supporting the research. This project was part of forest products industry related research at Karlstad University with financial support from the Swedish Knowledge Foundation and the participating companies.
The project's aim was to avoid calcium oxalate precipitation in the pulp and paper industry using a biotechnical method based on oxalate acid degrading enzymes. Other goals were to contribute to a better understanding of how and when oxalate acid is formed during the bleaching processes as well as to investigate the extent of the oxalic acid precipitation problem in pulp mills and biorefineries.
Nils-Olof Nilvebrant shows a pipe filled with calcium oxalate incrusts
To get a grip on the extent of the problem information was gathered in a couple of rounds from in total 78 pulp mills around the world. Mills in Europe, America, Asia and Africa participated in the study. The first set of answers, based on information from 11 European pulp mills, showed that:
- Many mills are planning to reduce water consumption in the next five years, which will increase the risk for calcium oxalate precipitations.
- Seventy percent of the mills had frequent problems with calcium oxalate precipitations.
- Advantages of reduced risk of these precipitations mentioned are for example: better possibilities to close process streams, increased production capacity, production of a more bleached pulp and better heat exchange efficiency.
- The costs for the problems caused in the different mills were difficult to estimate and figures between Euro 50,000 and 3 million per year were mentioned.
The biggest study covered 78 mills and showed that 29 had problems due to calcium oxalate precipitation whereas 11 had precipitation of other compositions. A general outcome was that mills in Asia, Brazil and Northern America appeared to have little or no problems, while mills in Europe in many cases stated that they had problems. One reason for this could be that the European mills in question had more closed systems. Another difference was that the proportion of mills producing peroxide bleached mechanical pulps was considerably higher in Europe.
The study showed that many mills lack knowledge about the precipitation problem and about the amounts of substances involved in the precipitations. Other mills, on the other hand, had done very careful mapping. Mapping of the presence of calcium and oxalate in process streams in different positions in these mills is done as part of the work to reduce problems caused by calcium oxalate precipitation.
The project has given both suppliers and mills an increased knowledge of how the oxalate problem can be reduced, says Eva Wackerberg
In one mill where careful mappings have been made, two important facts had been observed:
- Calcium coming to the process via wood chips gave a calcium amount of about 1,000 kg/day in the black liquor. This calcium was removed when filtering the green liquor, which hence acted as the process kidney. This experience shows that the green liquor filtering operation is very important in order to control the incrust situation.
- Oxalic acid enters the process via wood chips but is also formed in the bleaching process.
Increased knowledge about the calcium oxalate problem increases the possibilities to find efficient means and to identify critical process points where countermeasures can be used. Such measures can be conditions causing a reduced formation of oxalic acid or introduction of oxalic acid degrading enzymes.
One way to solve the problem is to degrade the oxalic acid by oxalic acid degrading enzymes like oxalate oxidas and oxalate decarboxylase. This requires that the enzymes function in the actual industrial process conditions and that it is possible to produce these enzymes on a larger scale.
The result showed that the oxalic acid degrading enzymes developed in the project were superior to enzymes available earlier. It was also shown that enzymatic degrading of oxalic acid in real process conditions is a technically realistic scenario.
To be continued ... Read Part IIhere.