THE ELUTION OF HEAVY METALS (Cd, Cu, Pb AND Zn) FROM USED MOULDING SANDS VS CONDITIONS OF THE ELUTION PROCESS

摘要

The paper presents the results of studies on the elution process of some selected heavy metals (Cd, Cu, Pb and Zn) from the used moulding sands in respect of the sand type (its composition and chemical reaction) as well as the chemical reaction and duration of the effect of an eluant. The studies were carried out on two different types of the sand, i.e. urea-furfuryl resin sand used for casting of CuZn36AlPb brass and sodium silicate sand used for casting of B555 bronze. Water eluats of these sands were prepared following the specifications valid in this respect in Poland. Additionally, eluats obtained with various times of contact between the sand and eluant (2, 6, 24, 72 and 168 hours) and using eluants characterised by various pH values (distilled water, solutions of pH = 4 and pH = 3) were analysed. The concentrations of the examined metals were determined by the method of potentiometric stripping analysis (PSA). The results of the studies have proved that elution of metals from the used sands depends to a great extent on the acidity of the process environment, i.e. on the chemical reaction proceeding between the sand and eluant. In systems characterised by low pH values (the sand of low basicity and an eluant of acid reaction), the rate of metal elution was much higher and followed by increasing concentration of the eluted metals with prolonged time of contact between the sand and eluant. In the case of sands characterised by a strongly basic reaction, under the examined conditions the effect of time on the volume of the eluted metals was rather insignificant. Irrespective of the conditions under which the process of elution was run, in the group of the metals examined the least resistant to elution has proved to be Zn, the most resistant Cu. In all the produced eluats the concentration of at least one of the examined metals exceeded the admissible value determined by the Polish Standard for water of purity class I. This means that the used moulding sands when disposed to dumping grounds can be a threat to the environment. One fact should be particularly emphasized here, namely that even when the disposed waste materials give water eluats with the content of metals permitted by standards, later this content may increase due to the power of these metals to cumulate in water and soil and contaminate them both.