48    FACTORS AFFECTING THE PERFORMANCE OF
HYDROXIDE PRECIPITATION OF METALS
Olcay Tiinay, Professor
Ruya Tasli, Research Assistant
Derin Orhon, Professor
Istanbul Technical University
Maslak, Istanbul, Turkey
INTRODUCTION
Heavy metal ions have a serious impact on the environment such as toxicity and accumulation. The
principal source of metal ions is industrial discharges which are controlled by discharge standards.
Assessment of discharge standards involves many factors to be evaluated. One of the important
factors considered is the performance of treatment technology. The performance of the treatment
technology is evaluated considering the factors affecting the efficiency of the system. This evaluation
also makes use of statistics whose degree depends on the uncertainties and undefineable parameters
that take part in the treatment system of concern. However, the use of statistics and the expression of
statistical data, especially if the statistical evaluation does not account for the limitations defined by
the treatment system, may result in inadequacies. Recently the micropollutants have gained importance in the control of pollution. Heavy metal ions are included in the lists of priority pollutants. The
discharge standards for priority pollutants often require the application of reliable and highly efficient technologies. The reliability of a system can be deduced considering the predictability of the
performance under different circumstances. Hydroxide precipitation is a well defined and proven
technology which is the basis of many discharge standards issued for heavy metals.
However, the theoretical evaluation of the system poses problems to obtain conceptual guidelines
for the optimal operational conditions on which the standard can be based.
This paper attempts to evaluate the applicability of rational bases which are derived through the
theoretical considerations to predict the performance of hydroxide precipitation.
THEORETICAL APPROACH
The performance of the hydroxide precipitation depends on factors which can be divided into two
groups:
• solubility of the metal hydroxide
• separation of the solid phase from the solution
The latter can mainly be manipulated by physical factors and various means of separation have
been defined i.e., gravity settling, coagulation, flotation and filtration by which most of the separation problems faced in the metal precipitation process can be coped with. The former phenomenon is,
however, neither well-defined nor controllable by simply physical or chemical means.
Solubility, on the other hand, determines the real performance of the system since the basic
separation mechanism relies on solid formation while some physical processes such as adsorbtion on
solid surfaces would help the removal in a lesser extent. The main factors affecting the solubility can
be summarized as follows:
• solubility determining solid phase
• crystal structure of the solid phase
• solubility product of the solid phase
• complex formation and stability constants
• species of hydroxo complexes and their stability constants.
46th Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 48118.
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