Eco-Technological Process of Glass-Ceramic Production From Galvanic Sludge and Aluminium Slag

Methods of purification of waste water which are most commonly used in the Republic of Serbia belong to the type of conventional systems for purification such as chemical oxidation and reduction, neutralization, sedimentation, coagulation, and flocculation. Consequently, these methods generate waste sludge which, unless adequately stabilized, represents hazardous matter. The aluminium slag generated by melting or diecasting aluminium and its alloys is also hazardous matter. In this sense, this paper establishes ecological risk of galvanic waste sludge and aluminium slag and then describes the process of stabilization of these waste materials by means of transformation into a glass-ceramic structure through sintering. The obtained product was analyzed with Fourier Transform Infrared Spectroscopy (FT-IR) and X-ray diffraction (XRD). The object of the paper is the eco-technological process of producing glass-ceramics from galvanic sludge and aluminium slag. The aim of the paper is to incorporate toxic metals from galvanic sludge and aluminium slag into the glass-ceramic product, in the form of solid solutions.


Introduction
Galvanic processing of metal and secondary processing of aluminium generate waste with high concentration of toxic metals.
Heavy metals are non-biodegradable and accumulate permanently in the environment, which is how they enter the food chain and indirectly affect human life and health.
Aluminium slag is very heterogeneous and thermodynamically and chemically unsTab.. Inside molten slag there is a possibility of numerous double exchange reactions, reactions typical for glasses, interaction between metal oxides and chlorides of the following type: 2MeF + MgCl 2 MgF 2 + 2MeCl These processes usually yield evaporable and hygroscopic products.After the removal and disposal of aluminium slag, mostly in open space, many processes occur, connected with crystallization of particular slag components, with solidification of amorphous slag components, and with the process of hydrolysis of numerous chlorides and fluorides.
Integral environmental protection requires that such waste should be stabilized prior to final disposal.
Stabilization of toxic metals from such waste is performed with various techniques, such as sintering [1,2], transforming sewage sludge and galvanic sludge into glass-ceramics [3], obtainment and properties of glasses and glass-ceramics from coal fly ash [4], vitrification [5], solidification of galvanic sludge by asphalt emulsions [6], and others.
Thus, chemically active substances (Cu 2+ , Cr 3+ , Cd 2+ , Ni 2+ , Pb 2+ , Zn 2+ ) are transformed, by phase and chemical transformations, into a sTab.structure, where pollutants cannot be set off even in critical conditions such as high temperature, acidic or basic influence, etc.

Ecological risk
Ecological risk of waste sludge and aluminium slag is determined on the basis of defining high-flow and low-flow fractions of a series of metals by a standard procedure U.S.EPA 1997/222.The primary ecological risk of waste sludge was determined on the basis of rinsing representative samples in distilled water and analyzing metal with atomic absorption spectrophotometry (Perkin Elmer AAM1100 and Varian AA-20).In the results the term water eluate has been adopted for this movable fraction.
For the sake of a more detailed insight into the pollution potential, and at the same time bearing in mind the chemical origin of waste sludge and aluminium slag, a low-flow fraction was also determined, which is more strongly bonded and thus represents a pollutant reservoir.A low-flow metal fraction was also isolated by a standardized procedure of rinsing with diluted nitric acid.This fraction is marked as HNO 3 -eluate.The criteria based on which the comparison of concentration of metals in water after rinsing the representative samples of waste sludge was carried out are taken from the Holland list 8935/1998.
The results are shown in Tab.sI and II where A is the referential value for standard groundwater and B is the concentration at which detailed testing and safety measures are recommended (EEC regulation) [7].The presented data indicate that the concentration of toxic metal after rinsing of composite samples of galvanic sludge and aluminium slag is higher than that in standard groundwater (criterion A).The Holland list 8935/1998 also recommends safety measurespurification of seepage water in case waste is disposed of unprocessed.
Apart from the basic waste materials, boron trioxide (B 2 O 3 ) was also used as a liquefier for glass-ceramics synthesis.
Since aluminium slag contains high-flow fluorides which could destroy glass-ceramics phase, it is rinsed with water in order to remove the fluorides.The slag is sunk in the water and left like that for 24 hours, during which the fluorides dissolve.The insoluble part of the slag is separated by filtration and fluorides are regenerated by evaporation and crystallization.The residue thus obtained is then processed further.
To obtain dehydration, it is necessary to dry the galvanic sludge.A low-temperature dry kiln can be used for this purpose.
After these operations all the elements (sludge, slag, glass, and borax) are ground in a coloid mill to facilitate melting.When the ingredients are ground, they are mixed until the mixture is homogenized and then the same mixture is poured into the melting mould.
The mould with the mixture is put into a blast furnace where it is heated at 1200-1400 o C. The melting temperature of the mixture depends on the quantity of several components included in the mixture.An increased quantity of glass will lower the melting temperature and reduce the solidity of the obtained ceramics.With the increase in the quantity of aluminium slag the solidity of the ceramic structure will also increase, but this requires a higher melting temperature, which sometimes presents a difficulty.
In order to get to phase transformations, the mixture has to be absolutely liquid, which is why the mould is kept in the blast furnace for about 40 minutes.After melting, the melted content is poured into a preheated graphite mould.The preheated graphite mould is preheated at 250-300 o C so that glass-ceramics would not crack due to sudden cooling in a cold mould.

Fig. 1. Glass-ceramic sample
The analyses of determining the concentration of metals in the obtained glassceramics show absence of metals in the water in which the sample is rinsed, which proves the efficiency of the method of stabilization of this waste.

Results and Discussion
The efficiency of the eco-technological procedure of transforming galvanic sludge and aluminium slag into a sTab.glass-ceramic structure was confirmed with Fourier Transform Infrared Spectroscopy (FT-IR) and X-ray diffraction (XRD).Infrared spectra of glass-ceramics were obtained with a BOMEM (Hartman & Braun) FT-IR spectrophotometer, model MB-Series, in a spectral range 4000 to 400cm -1 , with a resolution of 2cm -1 , by method of pressed pellets.The Potassium Bromide (KBr) Technique for sample preparation was used to record spectra.A quantity of 1-2 mg of tested sample is mixed with 150 mg of spectroscopically pure KBr.The mixture is then vacuumed and pressed at 200 Mpa, which forms thin permeable pellets.A reference pellet for BACKGROUND recording was prepared from pure KBr (fig.2).

Fig. 2. IR spectrum of glass ceramics
On the basis of the IR spectrum we observe lines at 560 and at 660 cm -1 as well as at 1350 cm -1 , which indicate the presence of Cr 2 O 3 .It is these lines that reveal that highly toxic chromium is bonded as an oxide with a silicate phase and that it is chemically completely inert.Apart from detoxication of chromium, glass-ceramics gets an aesthetic value because Cr 2 O 3 is a very well-known pigment and it gives a very beautiful green colour.Stronger in intensity and its form, the line in the range from 600 to 800 cm -1 confirms the presence of silicate phase with the dominant content of aluminosilicates in the form of Al 2 O 3 ×SiO 2 ×2H 2 O.Likewise, during melting at high temperature the elimination of crystal water in the structure due to no lines at 1630 cm -1 and the dominant presence of copper (II)titanate (IV) (CuTiO 3 ) are obvious, based on the form and intensity of the lines in the range from 400 to 800cm -1 .These lines (from 400-800 cm -1 ) reveal the transformation of highly toxic copper and titanium, which are bonded in the form of oxides as solid solutions and are completely chemically inert, which is very important from the environmental perspective [8].
The presence of aluminosilicates in the form of Al 2 O 3 ×2SiO 2 ×2H 2 O confirms the obtained glass-ceramic structure with the bonding of hazardous and harmful substances (CuTiO 3 , Cr 2 O 3 ), which qualifies them as chemically inert.
The XRD analysis of samples prepared from a compact piece or powder was conducted with a SIEMENS D500 powder diffractometer.Recording was done with CuKα radiation (λ =1,54184 Å) in angle ranges 5-90 º2θ at the speed of 0,02 º2θ/s.Identification of crystal phases in samples was conducted by means of Diffrac plus software and PDF (Powder Diffraction File, PDF 2) database [9].
Sample diffractogram (fig.3) indicates a presence of a significant quantity of amorphic phase, which causes a wide diffraction profile at about 28º 2θ.The ZnO⋅Al 2 O 3 crystal phase and substantial quantities of elemental lead were identified in the samples.

Conclusion
Through application of standard procedures U.S. EPA 1997/222, designation of lowand high-flow metal fractions, we identified the ecological risk of the disposal of galvanic sludge and aluminium slag, which are classified as hazardous waste.If they are not processed or properly disposed of, high-flow metal fraction is easily eluated with atmospheric precipitation, thus infiltrating and polluting the environment.The low-flow fraction is conditionally mobile and represents the reservoir of the high-flow fraction.Accordingly, waste galvanic sludge and aluminium slag were inactivated by sintering into a useful product -glass-ceramics.The FT-IR and XRD spectra confirmed chemical-phase transformations with the bonding of toxic metals (Zn, Pb, Cr, Cu, Cd, Ti, etc.) to the aluminosilicate phase in the form of solid solutions.