Characterization of Sludge from the Process of Steel Tubes Chemical Treatment for Hot Galvanizing

Inadequate industrial waste management in Croatia is reflected in the non-sanitary waste disposal, low recycling levels, negligible share of waste processing technologies, insufficient control of its flows, etc.Generated industrial wastes are most frequently disposed of at producers’ own, mostly illegal landfills. There are many such landfills on the Croatian territory, and the disposed types of waste often include those that can be hazardous and represent a considerable source of environmental pollution.Past waste management in all industrial branches can be characterized in this way, which at the same time may result in the harmful impact on human health and the environment. It also represents economic loss due to low utilisation of material and energy potential of some industrial wastes. The metallurgical industry collects its production waste separately. Only a part of the generated waste is returned to the production process and some waste is occasionally used by other industries as secondary raw materials, but the largest part of it ends at producers' own landfills on site. Hazardous wastes (dust containing heavy metals, waste oils etc.) are mostly disposed of in a controlled and lawful manner. Past handling of metallurgical waste was unacceptable both from the environmental and economic point of view. Therefore a systematic resolving of this important issue was initiated at the beginning of this decade. Sisak Steelworks galvanized steel pipes in the hot-dip galvanizing procedure by immersing in molten zinc. Between 1970 and 2000 Sisak Steelworks produced approximately 900 000 tonnes of galvanized pipes this way and generated around 70 000 m3 of neutralisation sludge, which was subsequently disposed of in the landfill on site. The paper presents the results of examination of physical-chemical properties of neutralisation sludge generated as waste material in the process of neutralisation of waste sulphate acid bath used in Sisak Steelworks for pickling of steel pipes before hot galvanizing. The results obtained by search electronic microanalysis of sludge samples led to identification of particles of average diameter ranging from d ~1 to 200 µm of predominantly angular in shape. Qualitative search electron microanalysis of sludge samples identified the presence of Ca, Fe, S, O and C. Furthermore, the analysis of the distribution of these elements in sludge samples and the analysis of cluster shapes of individual elements have shown that they correspond completely, which indicates that they occur together in the observed sample particle. This indicates that the sludge sample contained compounds of the Ca–S–O and Fe–O type and their exact bonding form is defined by the x-ray diffraction technique. The identified phases appearing in all samples and their fractions are: CaSO4 · H2O – gypsum;CaSO4 · 0,5 H2O – basanite and Fe3O4 – magnetite. Results of quantitative chemical analysis of sludge samples and their fractions obtained by colour separation indicate that the samples mostly contain calcium (w = 15.55–17.33 %), sulfur (w = 13.38–15.63 %), iron (w = 9.48–11.04 %), followed by Al (w = 5.00–5.26 %), K, Ti, Mn, Zn, Cr, Sr, Ni, Co, Pb and Zn – all in concentrations below 0.1 %. Examination of physical-chemical properties of eluates thereof has proven that there is no risk of admixtures present in this waste and the sludge can be disposed of on the ground in compliance with legal regulations.The obtained results provide grounds for the conclusion that the basic component of the analysed neutralisation sludge is gypsum (w ~ 85%). Referring to the good agro-technical practice of agriculturally developed countries, it is justified to consider its possible application for calcification ofagricultural soil. The purpose of this paper was to identify the physical-chemical properties of this type of waste and use its valuable components to find new options for its use in other branches of economy and to stop its uncontrolled accumulation in the environment, improving thus the management of this and similar types of non-hazardous industrial waste