The wastes from industry and households contain high concentrations of toxic substances, heavy metals, organic substances and soluble salts. Their processing by reduction of the noxious and toxic substances is the basic method for environment preservation.
Rrecycling and encapsulation of the chemical wastes are carried out for environment protection against pollution by toxic elements. Nature detrimental components are transformed into inert ones by the technological processes of melting atomization, electrochemical extraction. The glass phase which is the component of many wastes materials plays the role of an encapsulating coating. By mixing the glass with the nature detrimental substances and subsequent heat treatment, products of structures similar to those of the nature minerals or glass are produced.
The glass is characterized by isotropy and high viscosity in all the phases of its production and the processes of its melting and hardening are reversible. The glass is produced by rapid cooling of melts of certain oxides or mixtures thereof. All glasses are amorphous non organic materials produced by melting and cooling without crystallization. They are corrosion resistant.
The silicate glass by its nature is multi-component system of silicate modulus Si/O=0.39. If Al-ions are part of the structure then the glasses are resistant in acidic media if the molar ratio Na2O/Al2O3 is >1.
The waste glasses from industry and households after recycling is used for manufacture of glass ceramic materials. For this purpose, glasses of windows, containers end TV monitors are used. In many European countries approximately 26% of the waste window lass recycled and returned for remelting. The glass of containers is recycled as raw material and the percent of the recycling varies depending on the color: 82.0% for green, 44.5% for brown and 43.8% for the white glass. From an ecological point of view the glasses of windows and containers are harmless and can be used as initial material for blocking noxious and dangerous elements in the environment. For the glass used in the glass ceramic industry the most important features is to possess constant chemical composition particularly in full-scale production. The glass of TV monitors contains Pb, Sb, Sr, etc. and could be harmful for the environment.
The glass of computer monitors and TV sets may also contain heavy metals in its composition in the form of oxides of Fe, Ni, Co, Cu, Mn, V, Ce, etc. The glass is an excellent material for bonding the oxides of the heavy metals. They bond with SiO2 of the glass matrix which retains them in itself and decreases their solubility in acidic media.
The quantity of the dangerous oxides in the composition of the glass waste is controlled prior to its use.
The waste glass can be also used as an additive for clays; it decreases the synthesis temperature which results in decrease of the rejects in the production of tiles. Mixed glass scrap can be also used in the production of glass asphalt (90%asphalt+10% waste glass).
The glass ceramic composite materials produced from waste glass possess many times better mechanical characteristics than the glass itself. The glass ceramic is a polycrystalline ceramic material produced by controlled crystallization of glass. It resist chemical and atmospheric affects, especially in the case of glass ceramics on a silicate base. The glass ceramics is also called vitro-ceramics.
The inherent porosity of the solids considerable affects both the mechanical properties of the materials and the final product structure. The porosity of the latter is controlled by the granulometric and chemical composition, the pressing force, the synthesis temperature, etc. A general dependence that connects the glass ceramic microstructure with its mechanical properties is not known. According to Ryskewitch, the bending strength (σ) decreases exponentially when the porosity of the material grows:
σ=σ0·exp(-n·V) ,where: V – volume of the pores; n – a constant which characterizes the material (n=4-7).
The bending strength depends also on : grain size (σ depends in inverse proportion on √dgrain, d – grain diameter), the degree of synthesis between the grains, the sample surface, etc. According to Kingery, the pores have zero Young’s modulus, and the variation of the elastic properties of a closed porous system in an infinite matrix is described by the equation: E=E0·(1-1.9·θ+0.9·θ2 ), where: E - Young’s modulus; E0 – modulus of a non porous body; θ – coefficient of porosity.
The size of the pores and their distribution directly affect the elasticity of the glass ceramics: the pores of elliptic shape are the cause for high material elasticity. If the size and shape of the pores do not affect directly Young’s modulus they are the cause for secondary effects, the most important of which is the occurrence of micro cracks. The presence of micro cracks after the synthesis depends on the grain size and leads to a several times decrease of Young’s modulus.
The grain sizes as well as other microstructure characteristics of the glass ceramics directly affect the composite thermal expansion. The thermal expansion on the micro cracks affects the expansion of the composite material.
The high porosity materials (θ>70%) are characterized by: high hardness and strength, resistance to thermal effects, chemical and thermal stability, large specific surface, low heat conductivity, low dielectric constant, etc.
EXPERIMENTAL
The chemical composition of the raw materials is determined by the atomic absorption spectrophotometer Rank Hilger, Atom-Spek H-1580.
The powder specific surface was determined by the BET method by the apparatus BET Micromerits Gemini. Nitrogen was used as gas carrier. The powder morphology was studied with an electron microscope SEM Leica S 440i at magnification of 50-2000 times.
The differential thermal and thermogravimetric analyses were performed in air atmosphere by derivatograph Netzsch STA-409 in the range from 20-1400°C at a heating rate of 10°C min-1. The apparatus Philips PW 1820 with the use of CuK-radiation and a Ni-filter was applied for carrying out the X-ray structural analysis of the powders in the range 2·θ=6-50°.
The milting and mechanical activation of the raw materials was performed in a planetary-motion mill Fritsch pulverisette 5 (balls of Al2O3; milling time – 1-3 hours; medium – alcohol + water). Mash analysis by meshes Retsch STR 36 D-42781 Haan was carried out for determination of the powder granulometric composition. The uniaxial press Weber Pressen KIP 100 with a pressing force of 10-400 MPa was used for pressing the samples in the first sintering stage. Molds of the shapes of :parallelepiped (dimensions: a=60 mm, b=c=4-6 mm) and cylinder (Ø=35 mm) was used.
The thermo microscope Leitz 5 (heating rate - 10°C min-1, examination range – 20-1500°C) was used for determination of the sintering interval and the temperatures of softening and melting of the raw materials.
The mechanical tester Netzsch 401/3 was used for determination of the elasticity modulus and bending strength. The dilatometer examinations of the samples were performed in the range 20-500-20°C of the dilatometer Netzsch 402E at rate of heating/cooling 2°C min-1.
The sample density was determined by the hydrostatic method in water as the operation liquid. The sample porosity was determined by the formula: θ=1-ρ/ρt, were : ρ – actual sample density; ρt – theoretical density of the waste glass.
RESULTS END DISCUSSION
Three types of glass were used as a glass phase source: of TV monitors, windows and containers.
The window and container glasses were directly processed by crushing and milling. The monitor glass was pre-treated with a 12% HF solution in order to eliminate the chemical inclusions occurring in every TV screen.
The chemical analysis of the TV glass shows high content of the nature detrimental lead oxide (PbO=8.18%).
In the SEM and optical micrographs it is seen that the particles have irregular shape, sharp edges and various sizes in the range from 5 to 50 µm.
In all glasses over 50% of the particles have dimensions < 0.063 mm. The window glass has the highest degree of geometric activity because almost 80% of the particles have dimensions < 0.045 mm.
The thermal characteristics of the wastes glasses show that the TV glass has a much lower sintering range and the lowest melting temperature which is due to the presence of PbO and K2O in it. Because of that the TV glass in the most suitable for subsequent usage.
The powder fraction of particle size < 45 µm was used for strengthening of the waste glasses. Plasticize PVA was used for the preparation of the press powder and the pressing was performed under pressure of 30 MPa. The raw pre-forms produced in the shape of parallelepiped with dimensions 60×5×5 mm3 undergo sintering in a chamber furnace at temperature of 600-800°C (air atmosphere, heating rate - 10°C min-1 and hoding for 1 hour).
The elasticity modulus, bending strength (σ), density (ρ) and technical coefficient of linear expansion (α) are show in Table 1.
Table1. Density, mechanical properties and technical coefficient of thermal linear expansion of the sintered glasses.
| Glass type | ρ, g/cm-3 | E-modulus, GPa | σ, MPa | α, 10-6 °C-1 |
|---|---|---|---|---|
| TV glass | 2.61 | 72±8 | 136±10 | 10.6 |
| Window glass | 2.66 | 50±7 | 125±10 | 10.1 |
| Container glass | 2.54 | 52±7 | 136±10 | 10.8 |