Recycling of CRT panel glass as fluxing agent in the porcelain stoneware tile production

Abstract

In the present work, the feasibility to substitute feldspar raw material in a porcelain stoneware body with Panel Cathode Ray Tube (CRT) glass was investigated. A standard batch and a composition, where 35 wt.% Na-feldspar was substituted by CRT glass, were sintered at different temperatures in the range of 1000–1250 °C. The degree of the densification was studied by evaluation of the closed and total porosity, while the sintering rate was estimated by non-isothermal dilatometric measures. The variation of the crystalline phase composition was evaluated by XRD analysis. From the preliminary study other ceramic samples with different percentages of CRT glass (i.e. 2.5, 5 and 10 wt.%) were prepared and fired in industrial kiln. The sintering parameters, the microstructure and the mechanical properties were measured and compared with the standard composition.

Introduction

The porcelain stoneware tile is a product having high aesthetical qualities and very good technical characteristics are obtained by fast single firing (35–45 min) at about 1200–1250 °C. It is used in larger quantities and in increasing application fields. The Italian production of this typology was 360 million of square meters in 2004 corresponding to 61% of the total tile production [1]. The porcelain stoneware tile shows specific characteristic: extremely low open porosity (WA% < 0.05%), a high abrasion resistance and improved mechanical properties [2], [3], [4].

Porcelain stoneware bodies are primarily composed of kaolin and kaolinitic clays, sodium and potassium feldspars and quartz, heat treated to form a mixture of glassy and crystalline phases. The clay-based minerals are the main components (40–55 wt.%). They confer plasticity and workability in green state and furnish the main oxides involved, with fluxes and sintering aids, in the consolidation mechanism during firing. Feldspars and talc are to be considered as fluxing agents which provide, during firing, liquid phase with an adequate viscosity that will constitute the glassy structure of the final product. Quartz helps to balance the viscosity and has a structural function in the finished products. Chemical composition for the typical bodies can be presented graphically as a portion of the (Na₂O, K₂O)–Al₂O₃–SiO₂ plot [5]. Among the above-mentioned raw materials, the feldspar is the most expensive and therefore its replacement would represent a significant reduction in final costs.

During firing the main part of crystalline phases corresponding to the minerals by the green body disappear and are replaced by new amorphous and crystalline phases. Most of these reactions are kinetically governed and since the industrial firing cycle is very short they do not reach thermodynamic equilibrium. Hence, it is very common for the finished porcelain stoneware to contain residual crystalline phases of quartz and feldspars that have not been entirely transformed. The plastic clay components disappear completely during firing to form mullite crystalline phase (3Al₂O₃·2SiO₂).

The cathode ray tube of TV or PC monitor is composed of 85% CRT glass, which is possible to be separate and cleaned by using different suitable techniques. Recycling techniques for metals, plastics, and the other electronic components already exist, while the utilization of end of life (EOL) CRT glass is quite problematic. This is due to the fact that CRTs are normally made of several glass components divided into four typologies (screen or panel, cone or funnel, neck and frit junction), each of them with a different chemical composition and properties [6]. Glasses as cone and neck contain principally lead an other dangerous elements instead panel glass has other heavy metals (Ba, Sr, etc.) that forbid their recycling in the glass industry for the production of containers, domestic glassware and glass fiber. For these reasons, there is an increasing urgency to develop new applications for CRT glass in agreement with Directives 2002/95/EC on the restriction of the use of certain hazardous substances in electrical and electronic equipment (RoHS), and 2002/96/EC and 2003/108/EC on waste electrical and electronic equipment (WEEE). There is also growing concern about the environmental impact of disposing CRTs in landfill sites due to the presence of hazardous elements in the glass.

In this work, it has been tested the feasibility of panel glass to act as alternative material to replace partially the feldspar in the porcelain stoneware body, as demonstrated by other authors but with different kinds of minerals (nepheline, syenite, feldspatic rocks, zeolites, …) or cullet glass (soda-lime float and container glass) [7], [8], [9], [10]. This opportunity derives from both the CRT glass chemical composition rich in alkali and alkali earth oxides (about 14 and 20 wt.%, respectively) and the feature of porcelain material to develop a high glassy phase amount in the end ceramic product (60–70%). In particular, in the paper, the results on the sintering process and on the technological and mechanical properties (as water absorption, apparent density, linear shrinkage and Young’ modulus) are discussed.