The formation of barium sulfate scale is one of the major challenges of the drainage of coal mines, as barium containing pit water is mixed with sulfate containing water before being pumped aboveground. Traditionally this has been overcome by the addition of scale inhibitors based on phosphoric acids. However, due to the inforcing legislation there is a need for a scale inhibitor with improved ecological behaviour. A new scale inhibitor based on polyaspartic acid is as safe and effective as conventional phosphorous containing inhibitors, but has a much higher environmental compatibility. Results of a three year field trial are presented.
Mining operations extract more than just mineral resources such as coal and ores. Also there is a huge amount of pit water, which has to be pumped above ground. This water comes from different levels and varies significantly in composition, as it passes different geological formations. Table 1 shows the water composition of two veins of the same pit. This drainage water is collected and mixed underground before being pumped aboveground. Thereby, the volubility of poorly soluble salts can be exceeded, thus causing precipitation. Due to the very low volubility of barium sulfate, that has a volubility product of 1.05 l 10-10mo12/12,a precipitate is formed in almost any case, where barium containing water is mixed with sulfate containing water. Besides this, also precipitation of strontium sulfate and calcium carbonate can occur in the drainage water. Since their volubility products are considerably higher than barium sulfate (strontium sulfate 3.42 l 10-7 molY12; calcium carbonate: 4.95 l 10-9mo12/12),these solids only form in pit water to a minor extent or under special conditions.
These precipitates cause serious problems, as they all tend to form very hard scale. Especially barium sulfate scale is very difficult to remove. In practice this has to be done by mechanical means. Indeed, the formation of barium sulfate scale in pipes and water raising pumps is one major problem of the drainage of coal mines in Germany. Table 2 shows the analysis of scales from two different mines. Thus, if no measures are taken against the formation of barium sulfate, frequent maintenance of the water raising pumps and even an exchange of the rising mains can be the consequences. Disassembling of pipes and pumps and installing new ones underground is a very time-consuming process. At the same time high expenditure is connected therewith. By the addition of scale inhibitors to the pit water the formation of barium sulfate precipitates and scale formation can be avoided. For more than 15 years this has been general practice in those mines that had to raise barium containing pit water. These inhibitors are, however, pumped aboveground together with the pit water and are discharged without further treatment into surface waters. Consequently, only inhibitors with a low aquatic toxicity could be used to prevent any detrimental effect on the aquatic community of life. Therefore, approvals for chemicals added to the pit water by both the mining authority and the local water authority are necessary, to ensure that only chemicals are used, that are in accordance with this requirement.
For the purpose of barium sulfate inhibition products based on phosphoric acids are widely used. They are distinguished by a good efficacy and a considerably low aquatic and human toxicity. Furthermore, their ecological behaviour is well-known, as it has been subject to intensive research for many years 1-7.Recently, general risk assessments of some phosphoric acids proved, that they represent no risk for the environment 8?9. However, their poor biodegradability and the p