Obtaining of Dense and Highly Porous Ceramic Materials from Metallurgical Slag

Glass–ceramics in a dense and highly porous form can be obtained from metallurgical slag and waste glass of TV monitors. Using polyurethane foam as pore creator, a highly porous system with porosity of 65 ± 5 %, E-modulus and flexural strength of 8 ± 3 GPa and 13 ± 3.5 MPa, respectively can be obtained. This porous material had durability (mass loss) of 0.03 % in 0.1 M HCl that is identical with the durability of a dense composite.


Introduction
Recycling of industrial waste is one of the tasks in the field of protection of the environment that should be solved in the near future.One kind of industrial waste such as metallurgical slag from the metallurgical factory in Republic of Macedonia contains as the main constituents Fe 2 O 3 , Al 2 O 3 , SiO 2 , MgO, Na 2 O and a small amount of ecologically risky oxides such as Cr 2 O 3 , NiO, MnO.
A combination of the metallurgical slag with waste glass of TV monitors under a controlled sintering procedure gives bulk or highly porous materials with surface or/and bulk crystallization.The structure of glass-ceramics built this way may prevent the migration of ecologically risky elements into the environment due to corrosion or friction.
The aim of this paper is to make a dense and porous composite between metallurgical waste and glass.In this way the following advantages are present: sintering leads to reduction of energy consumption and ecologically risky components from the waste are fixed molecularly in the silicate phase and inserted additionally in the matrix based on waste, which either have no toxic inserted components or have them in ecocompatible concentrations.The porous glass-ceramics obtained in this way which possess a foam like structure can be used as filters, thermal insulation, lightweight structural laminates, diffused aeration, dust collectors, acoustic absorbers etc.
The principle of this procedure was presented as a multibarrier-concept by Ondracek [1] and basically investigated for various waste combinations [2][3][4].This paper deals not only with the chemical inertisation process and combination of waste materials but also with a new application of these waste materials as highly porous glass ceramics.

Experimental Procedure
The metallurgical slag was supplied by the factory for production of ferro-nickel alloys and the waste glass was obtained from TV monitors.
Chemical analysis of the materials was carried out using an atomic absorption spectrophotometer and wet chemical methods.The crystalline phases were identified by powder X-ray diffraction (XRD) patterns using a Philips PW 1130.Identification of the crystalline phases was made using MPDS (mineral powder diffraction search).The thermal changes of waste compositions were determined by high temperature microscopy.
The material was ball milled (Fritsch pulverisette 5) for 180 min.The particle size distribution of the sample was determined by sieve analysis.
A Mixture of the ferronickel slag and waste glass in the ratio 4:1 and 9:1 was prepared by a powder technology, uniaxially pressed at 50 MPa using PVA as binder, and sintered at 1100 and 1150ºC/2 h using a heating rate of 10ºC/min.
Bulk densities of the sintered samples were determined by the water displacement method.Interactive image analysis was used for the determination of porosity.Thermal studies of polyurethane foam were performed by TG (Netzsch STA 409) in the temperature interval RT-500ºC.
Open-celled macrostructures were fabricated by coating the struts of polyurethane foam with ceramic slurry and then firing the resultant structure to pyrolyse the substrate and sinter the ceramic system [5][6].Commercial polyurethane foam with a density of 25 kg/m 3 was used as a substrate that was coated using slurry.The slurry contained 78 % solid, 19 % water glass and 13 % Dolapix (4 % water solution).It coherently coated the polyurethane substrates.The foam was squeezed and dipped into slurry, looking in that case like a sponge.During expansion to the original shape and size the mentioned slurry impregnated the foam.The load of the solid material was 1.2 g/cm 3 sponge.After drying at 105ºC/8 h, the coated substrates were heated up to 1100/1150ºC in a schedule that minimised disruption during pyrolysis and allowed the ceramic to achieve high density.This heating schedule consisted of a heating rate of 1ºC/min up to 500ºC and heating of 6ºC/min from 500 to 1150ºC, 2 h hold at 1100/1150ºC and then cooling in the furnace.
Creation of a porous structure using C-fibres was made by the following procedure: C-fibres (Toko-Rayon C. Ltd) in the form of bundles with the diameter of 400 ± 100 µm were embedded in a pulp consisting of 78 % solid phase and 22 % Dolapix CE 64 (4 % water solution).The number of fibres was ca 50 ± 10 per cm 2 .After drying at room temperature for 24 h and at 105ºC/6 h, the system was sintered under the following conditions: in the temperature region from 20 to 1150ºC, the heating rate was 1ºC/min, and holding time at 1150ºC was 2 h.
Linear thermal expansion of the dense material was measured with a dilatometer Netzsch 402E in air atmosphere and temperature interval RT-650-RT.The measurements were performed with a heating rate of 2ºC/min.
Investigations of mechanical properties (E-modulus and flexural strength) of the dense and porous samples were investigated at room temperature on specimens (10 pieces, 50x5x5 mm) polished with diamond paste of 9 µm and subjected to the 3-point bending tester Netzsch 401/3 with 30 mm span and 0.5 mm/min crosshead speed.
Durability of the glass ceramics was tested using standard methods both for glass and ceramics.The durability was determined as a mass loss in 0.1 M HCl and 0.1 M Na 2 CO 3 and in distilled water.After treatment for 24 h in this solution, Fe 3+ , Cr 3+ , Mn 3+ , Ni 2+ , Zn 2+ , ions were removed from the tested materials and analysed by atomic absorption spectroscopy.

Results and Discussion
The chemical composition of ferronickel slag refers to the multicomponent silicate system (Tab.I).
Particle size analysis of the milled ferronickel slag and glass showed that ca 65 % of the particles are < 63 µm.The particle size analysis of the powders is given in Tab.II.
Taking into account results of DTA and high temperature microscopy, sintering studies were carried out in the temperature range 1100-1150ºC.Metallurgical slag (S) reached the highest relative density after sintering at 1150ºC/2h of 83 %.Sintering at a higher temperature of 1150ºC exhibits side effects such as bending.
Technical coefficients of thermal expansion of ferronickel slag and TV glass are 11.57and 10.61⋅10 -6 /ºC, while E-modulus of ferronickel slag (17 % porosity) and TV glass are 29 and 73 GPa, respectively (Tabs.III-IV).These two conditions are needed for keeping the level of internal stresses in the composite low after fabrication at high temperatures [7][8] In order to encapsulate the solid phase into a matrix compatible with the environment, waste TV glass in the quantity of 10 (S10) and 20 wt.% (S20) was used.These composites were sintered at 1100 and 1150º/2h.The relative densities of the systems S10 and S20 were 85 and 87 %, respectively.
Flexural strength and E-modulus of the dense compositions are shown in Tab.III.The addition of 10 % glass (S10) at both sintering temperatures of 1100 and 1150ºC had no significant influence on the flexural strength but the value of the E-modulus was increased ca.50 % for both temperatures.Addition of 20 % glass (S20) evidently increased the flexural strength and E-modulus.___________________________________________________________________________ Thermal expansion characteristics of all investigated systems in the interval RT-650-RT showed absence of the hysteresis effect that proves that the systems are in thermal equilibrium.The temperature dependence of thermal expansion in the interval RT-650ºC can be represented by a III order polynomial form.By differentiation of this polynomial, the temperature dependence of the physical coefficient of thermal expansion α could be derived (Tab.IV).Durability of the investigated dense materials was determined for ceramics containing only waste (S) and waste with glass addition (S10, S20).The highest durability was shown by glass ceramics S20 (0.03 % in 0.1 M HCl), S10 (0.06 % in 0.1 M HCl), whereas in absence of an additive (S) the durability was 0.24 % in 0.1 M HCl.The changes of mass in 0.1M Na 2 CO 3 and distilled water were very low, 0.004-0.006% for all specimens.Atomic absorption spectroscopy didn't show the presence of harmful elements in the solution.

Tab. IV
The durabilities obtained allow categorization of these materials to definite classes according to DIN EN 122: all three materials (waste and with addition of TV glass) -to the class AA (no change in colour).The durabilities show that the ceramics developed meet the requirements of dense, unglazed ceramic tiles according to building ceramic norms DIN EN 106 [9].
By use of a polyurethane foam as the creator of a porous structure, samples with a porosity of 65 ± 5 % were fabricated (Fig. 1).The pores with diameters from 600 to 800 µm are interconnected.The E-modulus of this system was 8.1 ± 3 GPa, flexural strength was ___________________________________________________________________________ 13.2 ± 3.5 MPa.The durability of this porous system (mass loss) was 0.03 % in 0.1M HCl and 0.004 % in 0.1 M Na 2 CO 3 , which is identical with values obtained for the dense sample.
Using C-fibres as creators of a porous structure, according to the diameter and number of fibres in the bundle, a correspondent macro porous structure was created.In this case an integral porosity of 55 ± 5 % was achieved.The pores had a cylindrical form with the diameter of 400/700 µm (Fig. 2).Fractures among the pore walls were not evident.This system was characterized with the following mechanical properties: E-modulus of 14 ± 2 GPa and flexural strength of 17 ± 3 MPa.

Conclusions
Ecologically harmless materials (glass ceramics) can be created from metallurgical slag and waste of TV glass monitors.The addition of 20 % TV glass increases the flexural strength from (51 for the material (S) to 75 MPa for material S20) and E-modulus (29 for material (S) to 44 GPa for material S20).Chemical durability (mass loss) for the metallurgical slag (S) is 0.25 % in 0.1 M HCl while the composition containing 20 % waste TV glass addition (S20), had durability of 0.03 % in 0.1 M HCl.Using polyurethane foam and C fibres as pore creators, a highly porous system (porosity 65 ± 5 and 55 ± 5 %, respectively) can be obtained.E-modulus and flexural strength of the porous system obtained by polyurethane foam was 8.1 ± 3 GPa and 13.2 ± 3.5 MPa, respectively.The chemical and physical properties of the dense material, makes them suitable for a wide range of applications in the building industry.

Fig. 1
Fig. 1 Macrostructure of cell foam, using polyurethane foam as a pore creator.
. ___________________________________________________________________________ Tab.I Chemical composition of the ferronickel slag and glass of TV monitors