The Phases Change in the Sludge from Aluminium Production in the Biochemical Leaching Process

Pednáška bola prednesená na 12.medzinárodnom Al Symposium v Bratislave, v dňoch 7-10 septembra 2003
Kušnierová Mária, Vašková Helena, Kafka Richard
Introduction
The microbiology has allowed and continues allowing us to discover a new face of nature with its
planetary symbioses which also includes human beings and their activities. From the historical point of view it is possible to state that the formation of life and two thirds of its development on our planet were ensured by microbes. They gradually developed one of the most important electron transport systems - photosynthesis. The photosynthesis had considerably changed conditions on the planet. The atmosphere started to gradually change from the reducing state into oxidising one, what resulted in the formation of ozone shield protecting the life against the deadly radiation. The life started spreading from the shallow waters to the mainland. The microbial communities were formed ensuring both reducing and oxidising biological-chemical processes in the element circulation. Many of them still act in nature in its original or developed form. Their existence and activity have both positive and negative effects. The positive one is the irreplaceable service of microbes in the decomposition of wastes produced by people and the return of the artificially put elements in the natural circulation. In many cases the pollution caused by industrial production is so critical that the traditional procedures are not sufficient to solve the problem and new principles have to be sought. At present there are a lot of biotechnological procedures known, which ensure the degradation of different types of wastes, their detoxication and in some cases also recycling. That is why our research was focused on the possibility of applying biotechnological procedures in the processing of waste sludge originated from the aluminium production.
Except for some single cases, waste sludge from aluminium production is not currently used and represent a serious environmental danger. Depending on the geographical location of individual production plants, waste sludge is either dumped directly in the sea or stored on the man-made waste disposal sites occupying vast soil areas. The individual sludge boxes occupy in average 900-1300 hectares. In Slovakia there are about 10 000 000 tonnes of waste dumped next to production plants. The first attempts at a complex use of sludge from aluminium production date back to the period before the World War II. Up till now there have been a number of research projects focused on the possibility of re-use of utility elements (Fe, Al, Ti, Ga) and on the massive direct use of this material in the production of various kinds of construction materials, ceramic materials, pigments and fillers. Several different technically interesting technologies have been developed, but in most cases, they were not viable due to economic reasons. A lot of procedures showed to be partially viable, e.g. in the production of cement, ceramic floor tile materials, bricks, fillers and iron production charge additives
providing the receiving plants are situated close to the source of produced waste. From the relative area of the use of bio-chemical processing of this type of sludge it is possible to use results available from the research of chemolithotrophic bacteria Thiobacillus ferrooxidans, carried out by Hungarian researchers in form of pilot operation experiments, which were aimed at obtaining Na2O and Al2O3 and achieving relatively good yield parameters, but this procedure has not dealt with the issue of detoxication, processing and use of large quantities of acid sludge with Fe, Ti and V concentrations.
Results
Our research was concentrated on the aluminium production sludge deposited in the premises of
ZSNP a. s. Žiar nad Hronom. To get more details about the material being researched, we took 6
samples with different period of deposition ranging from 3 to 37 years. From the basic analyses of samples (chemical and phase analyses, grain size composition, density, pH and specific surface) it followed that the deposited material probably changes with the period of deposition and under the effect of exogenous factors influencing the material under “in situ” conditions, although the possible changes in the technology and the quality of raw material being processed could have a certain effect on the inhomogeneity of deposited material. The change is demonstrated by the gradual recrystallisation of amorphous phase as documented by Fig.1 showing the X-ray records of sludge deposited for 3 and 37 years. Due to gradual re-crystallisation the content of amorphous phase in the long-term deposited material decreases and the structures of böhmite and goethite appear. The presence of cryolite in these samples is probably connected with the technology and raw material processed at that time. Another change observed was the content of Na, which decreases mainly in the first years of deposition from 2.8 % after 3 years of deposition to 0.5 - 0.8 % after 10 years of deposition. The period of deposition has also effect on change in the grain size composition of sludge and its density (Tab. 1).
Fig. 1 Comparison of X-ray analysis records of sludge deposited for: / A / - 3years, / B / -37 years

Table 1. Comparison of grain size and density of selected samples of deposited sludge.

On the basis of these data it is possible to assume that the deposited sludge is secondarily
compacted forming the relatively solid pellet, which often have the shape of independent grains.
However, their solidity is seeming as confirmed the results of granularity analyses carried out by dry and wet methods (Tab. 2).
Tab. 2 Comparison of results of dry and wet granularity analyses of deposited sludge

Comparing the results shown in Tab. 2 it is possible to deduce that the deposited material is quasicolloidal and its grains float in contact with water the similar way as clay materials.
The biological-chemical treatment experiments used average homogenised sample formed by
mixing samples with different period of deposition. Several microbial cultures were used in the
biological-chemical leaching of sludge. Then only one the most suitable species of microorganisms was selected for the research of the technology of complex waste sludge processing. The result of relatively extensive research including model experiments is the technology described in Fig. 2.
The set of technological knowledge on the possibility of a complex use of sludge from aluminium production has been extended with the results of study carried out in our institution focused on the usability of combined bio-chemical procedures based on the application of selected species of chemolithotrophic microorganisms [4], which resulted in the technological procedure design protectedby the patent of the Slovak Republic.
Fig. 2. Technology of complex treatment of waste sludge from the aluminium production
The set of technological knowledge on the possibility of a complex use of sludge from aluminium
production has been extended with the results of study carried out in our institution focused on the usability of combined bio-chemical procedures based on the application of selected species of
chemolithotrophic microorganisms [4], which resulted in the technological procedure design protected by the patent of the Slovak Republic.
The most significant changes have been observed during application of leaching agent based on
metabolic products of microorganisms, which are considered to be involved in the proposed
procedure. The full decomposition of original waste sludge components was achieved by sludge
processing applying an optimised procedure, while it consequently and partly parallely came to a
gradual precipitation of autigenous compounds. The designed and by the laboratory model testedtechnology of bio-chemical processing of sludge from aluminium production (Figure 2) enables totransform the phase composition of sludge and to incorporate majority elements into forms ofcompounds industrially usable in the building industry and production of plastic materials. The designed procedure represents a waste free technology with a high degree of recycling oftechnological extraction media. The presented technology is protected by the Bureau of Industrial Property of the Slovak Republic. It allows to process sludge in form of two to four final products. The most simple two-product variant allows to process sludge into hemihydrate and red pigment.
After corrective treatment of radioactive properties, hemihydrate can be used in the complex
gypsum plasterboard program. (In Slovakia this program is currently fully covered by import and its introduction in Slovakia in co-operation with the corrective anhydrite material producers could contribute to the creation of jobs). On the basis of preliminary tests, red pigments can be used in plastic material processing. The world demand for quality red pigments is not sufficiently satisfied by industrial production and it is expected that they will represent a profitable article on the commodity market.
Other final products can be:
- silica gel which can be obtained by the separation from the gypsum product,
- ordinary alum K Al/SO4/2.12H2O which can be obtained by the crystallisation of filtrate after the leaching of sludge,
Ti-product the separation of which has not been solved yet, but its recovery is theoretically
possible by processing the liquid phase after the crystallisation of alum.
Conclusion
The presented results of the research of biological-chemical processing of waste sludge from the
aluminium production confirm the possibility of using biotechnological procedures in the processing and complex use of this refractory material. The presented design of technology has certain reserves, that is why further basic, applied and operational research is required. At present there are certain possibilities arising for the implementation of the project of the transfer of the presented technology of complex processing and use of waste sludge from the aluminium production within the co-operation of researchers, producers (owners) of this type of waste with the use of pre-accession EU Funds