Date of Award

4-2002

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Environmental Science

First Advisor

Dr. M. S. Ibrahim

Second Advisor

Dr. M. S. EL-Maazawi

Third Advisor

Dr. Yehia S. Mecbref

Comments

Every year, aluminium producers have to dispose of thousands of tons of waste products, which is known as spent potlining (SPL) materials. The SPL material often contains excessively high concentration of contaminants as well as compounds that are industrially valuable and could be recycled. The components of SPL of greatest concern environmentally are cyanide and leachable fluoride. It is well known that CN- and F- are harmful to human beings. Hence SPL is listed as hazardous waste by more and more countries. The overall goal is to solve environmental and economic problems for aluminium producers.

In the present study, the chemical cleaning method used to remove the CN- concentration from SPL, which based on the use of peroxygen compounds, such as hydrogen peroxide and ammonium persulfate. Peroxygen compounds are known to oxidize CN- to cyanate (CNO-), followed by decomposition of the CNO- into ammonia and carbon dioxide. Unlike cyanide, fluoride present in the SPL material cannot be destroyed by oxidation; so, the F- removed by treatment the leachable F- with high calcium lime. This treatment converts most fluoride compounds to the insoluble CaF2, which suitable for disposal in a conventional industrial landfill.

In what follow, a brief description of the different chapters reported in this thesis is given:

In chapter I, of this thesis, a general literature survey of the previous research programmes have carried out to treat spent potlining (SPL) with the aim of producing an inert material or better to re-use some of the fractions of SPL is reviewed. Moreover, the aim of the present work is included.

In chapter II, the experimental part involves the preparation of the various solutions were described in detail. It also includes description of the instrumentations used in the present measurements. The general procedures for all the measurements were also explained in detail.

In chapter III, the destruction process of cyanide through treatment with peroxygen compounds (H2O2 or (NH4)2S2O8) in crushed and uncrushed SPL solution was studied. A systematic study of the various experimental parameters, that affect the destruction process of cyanide, was carried out e.g., pH of the solution and amount of oxidizing agent. In this context, we studied the effect of the presence of acetic acid and hydrochloric acid in the rate of destruction of cyanide, whether in crushed or uncrushed SPL samples. At pH 9.0 (in presence of HCl or acetic acid) the destruction process gave a maximum response compared to media with other pH' s. The results show also that (NH4)2S2O8 is much more effective (97.2%) than H2O2 (44.9%) in the destruction of cyanide from SPL, under the same conditions. Furthermore, the ammonium persulfate is more effective in the destruction of CN- in presence of HCl than acetic acid. For example, the addition of 33.36g (NH4)2S2O8 to the crushed bricks sample lead to the CN- concentration reduced to 82.1% and 94.7% in presence of acetic acid and HCl, respectively. In general, the rate of destruction of CN- in the uncrushed samples in highly hindered in compared to that for the crushed samples. In that the cyanide concentration was reduced to 92.5% and 37.1% for crushed and uncrushed SPL samples, respectively, in presence of (NH4)2S2O8 with acetic acid.

In chapter VI, the removal of fluoride as calcium fluoride (CaF2) by the treatment of leachate with high calcium lime was investigated whether, in crushed and uncrushed SPL solutions. In order to find out the optimum conditions for collection of fluoride as CaF2, several experimental parameters were studied, e.g., collection time and amount of calcium lime. The results lead to the conclusion that the bulk concentration of F- was removed within 2 or 3 days whether in crushed or uncrushed SPL samples, and no significant change was observed after 3 days. The results show also that the treatment of leachable fluoride with 21g calcium oxide for about 24 hours at room temperature can reduce the amount of F- by almost 98%.

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