Undersökning av deponerat ferrokromstoft
Utvärdering av ny behandlingsmetod
The production of ferrochromium alloys at Vargön Alloys AB contributes to formation of unwanted ferrochromium dust, which contains toxic Cr(VI). The dust is since summer 2005 treated with an iron-sulphate solution to reduce Cr(VI) to the non-toxic Cr(III). A year later the treatment technique was improved and disposal occurs in cells on the landfill. The cells can be up to 300 m3 and the edges can be made out of fro example concrete. Tests are conducted on the treated dust when it is disposed. The analyses show low contents of Cr(VI), which indicates that the reduction is successful. Analyses of leakage water from a small scale cell however showed an increasing content of Cr(VI) and pH under the first two months.This thesis has evaluated the new treatment method. An investigation was conducted to determine if the same changes have occurred in the full scale cells. Half of the 18 collected samples from the disposed dust showed high or very high contents of Cr(VI). The other half had a almost no detectable content. pH was over 12. No correlation between Cr(VI)-content and pH, conductivity, age or water content in the disposed dust could be shown.An experiment was also performed in small scale to reduce the number of influencing parameters. The results showed that if the dust has access to oxygen the Cr(VI) content increases during the first days from a few mg/kg dry substance to over 20 mg/kg dry substance, a level which was maintained during the rest of the experiment period. The content of Cr(VI) was almost constant at 4 mg/kg dry substance when oxygen was eliminated during the whole experiment period. When the water content was below 5 %, the content of chromium did not increase more.Since almost all newly created Cr(VI) is reduced to Cr(III) by the addition of ferrous sulphate, recreation of Cr(VI) occurs. Today there is now possibility to follow the changes in the dust from untreated to after disposed. The addition of iron-sulphate is based upon an average Cr(VI) content in the dust and the variation of disposal occasions can be significant. To determine if the increase in Cr(VI) is caused of an incorrect addition of solution a smaller disposal cell can be constructed. The initial content of Cr(VI) can be controlled and the dust can be examined during the time it ages. If the dosage could be improved the addition can be optimized. It is also important to minimize the infiltration and accelerate the drying of the slurry since the Cr(VI) content can increase the as long as the water content is high the Cr(VI) content can increase.