|FR | ENGL|
|Recycling of granite sludges in brick-type and floor tile-type ceramic formulations|
|J.M.F. Ferreira, P.M.C. Torres, M.S. Silva, J.A. Labrincha
Ceramics and Glass Engineering Dept.
University of Aveiro
3810-193 Aveiro – Portugal
This work reports the use of sludge generated from nautral granite cutting processes as raw material in brick-type and floor tile-type ceramic formulations. The physico-chemical and mineralogical characterization of the sludge was carried out in order to identify the major technological constraints and to define the sludge pre-treatment requirements. First, a careful separation of different materials should be implemented, e.g. for marble or granite stones. Then, disposal and mixing steps of sets collected in different dates are needed, like normally practised for natural raw materials. High moisture content (above 50 wt%) of the as-processed sludge and the induced high shrinkage values observed on drying constitute the most important drawbacks for their direct use. A previous drying step, preferably conducted by the producer, will minimise those problems. Once following those requirements, the incorporation of high level (up to 50 wt%) of residue in brick-type (BT) or floor tile-type (FTT) formulations is possible, as shown from the comparative evaluation of the relevant functional properties of dried and fired bodies (e.g., mechanical resistance, water absorption, shrinkage).
In recent decades, the growing consumption and the consequent increasing of industrial production has led to a fast decrease of the available natural resources (raw materials or energy sources). On the other hand, a high volume of production rejects or sub-products is generated, most of them not directly recyclable. Traditionally, disposal as soil conditioner or land filling are the commonly used processes for their consumption. However, these ways are sometimes problematic, not only because land is limited, but also because there are strict regulations in terms of location, categorised site, gas emission, leaching characteristics, etc. Therefore, alternative ways to reuse several types of waste materials have been attempted in recent years, including the incorporation in clay-based ceramic products [1-7]. Heavy clay ceramic materials (namely, bricks and roof tiles or floor tiles) are generally very heterogeneous, since they consist of natural clays with a very wide-ranging overall composition . For this reason, such materials can tolerate the presence of different types of wastes, even in considerable percentages .
The present work aims at studying the recycling ability of a non-toxic sludge generated from natural granite cutting processes. The high daily-producing amounts and the difficulties in reducing their volume, by suitable filter-pressing methods, require high transportation costs for disposal. On the other hand, no interesting applications were implemented up to now. Its use as raw material in brick-type and floor tile-type ceramic formulations is reported. Such practical approach is encouraged by the relatively fine particle size distribution of this type of sludge, which will not require any further grinding step, and can potentially replace the low grade clay components used in the fabrication of BT products or the feldspar in FTT compositions.
2. Experimental Procedure
2.1. Raw materials
The raw materials used in the present work were a plastic red clay (PRC), and a low-grade clay (LGC) normally used in brick-type compositions, supplied by Fábrica Campos, Lda, Portugal; a shale clay (SC) from Bacia do Paraná, Brasil, that exhibits excellent fired properties when sintered at about 1100ºC ; and granite sludge supplied by Eurogranitos, Águeda, Portugal. The PRC is based on illite, kaolinite, montmorilonite and some quartz and feldspar, while the LPG clay is mainly based on quartz and illite, as detected by DRX (Rigaku Denk Co. USA). Their full characterisation involving thermal and XRD analysis, and evaluation of physical properties after firing at a maximum temperature of 1000°C in a complete cycle of 22 hours, was already reported . The Brazilian clay consists essentially of a mixture of quartz, mica (muscovite), kaolinite, illite and montmorilonite.
2.2. Characterisation of the sludge
The complete characterisation of the as-received sludge involved determinations of its thermal behaviour, average composition, average particle size and particle size distribution, and other relevant parameters in order to evaluate the needs of any previous pre-treatment steps and its suitability to be directly incorporated into brick-type or floor tile-type compositions.
Table 1 gives the average chemical composition of the dried sludge, confirming its silicious character. Aluminium, calcium, sodium, potassium and iron are the secondary oxides, being the last one the main responsible for strong red couloring of the fired products.
Table 1. Chemical composition (XRF) of the dried sludge, in terms of oxide components.
As expected, XRD revealed that quartz and feldpar (albite and microcline) are the main crystalline phases, with some ninor amounts of caulinite and illite. Its thermal inertness is easily proved by the low value of ignition loss up to 1000°C. Particle size distribution of the as-received sludge was measured using a Coulter LS230 particle size analyser (Coulter, UK).
Figure 1 shows the differential particle size distribution of the dried sludge. The average particle size, D50, is about 8 µm, while D10 and D90 are about 1µm and 49 µm, respectively. There are two small coarser populations centred at about 63 µm and 134 µm. The largest measured particle size is approximately 213 µm. This particle size distribution data shows that the fineness of the sludge is suitable for being directly incorporated into brick-type formulations.
2.3. Preparation and characterisation of samples
The clay raw materials were mixed with the granite sludge in different proportions, as reported in Table 2. The batch formulations containing a total amount of 2 Kg solids were prepared by a traditional method involving the mixture of the dry components followed by addition of controlled amounts of water, ≈ 20% (BT) and ≈ 18% (FTT) (on a wet basis) and extrusion under vacuum. Several cylindrical rod shapes with 10 mm diameter and about 120 mm long were extruded. The testing samples were carefully dried at room temperature for 24 h and then at 40ºC for another 24 h period and then completely dried at 120ºC for further 12 h. Linear drying shrinkage was determined from the dimensions of the samples before and after drying.
Table 2. Tested compositions
For BT compositions, about 20 testing pieces were fired at 950ºC in an industrial furnace, following a typical long cycle of about 9 h. The same amount of testing samples of the FTT compositions were fired at 1100ºC in an electrical laboratory furnace, using a heating rate of 5ºC/min, with a holding time of 1 h at the maximum temperature.
3. Results and discussion
The characteristics of fired materials like linear shrinkage, water absorption, and flexural strength were measured in a normalised way. The average results are summarised in Table 3.
Table 3. The relevant fired properties of the fired products
Comparing the BT formulations, it can be observed that increasing amounts of PRC tend to enhance the total shrinkage, as expected, except for the compositions 2 and 3 in which 35% or 50% of LGC were replaced by granite sludges, while water absorption is almost unaffected by the compositional changes. Such evolution of shrinkage values can be understood, since the PRC is mostly constituted by very fine clay particles of illite, kaolinite and montmorilonite. These fine clay minerals are very sensitive to drying and, after completely dried, they readily absorb water from the atmosphere. These characteristics made impossible to obtain undamaged testing samples from this raw material alone due to the stresses induced during the drying period. This high sensitivity behaviour to drying or water uptake might be responsible for the decreasing trend of the flexural strength with increasing amounts of PRC. These results suggest that the proportions of PRC should be kept at levels in the range of 30-40 wt.% or otherwise, the amount of water should be decreased, while the LGC could be partially or totally replaced by granite sludges. This was the reason why a lower amount of water was used in the FTT compositions. Table 3 shows that under these conditions the flexural strength increases with increasing amounts of either PRC of SC, while water absorption decreases. Water absorption values lower that 0.5% could be obtained for PRC/granite sludge = 50/50, while impervious materials can be produced using the same proportion of granite sludge but replacing the PRC by the SC. The better results obtained with the SC are according to the excellent densification properties of this raw material, as reported before .
4. ConclusionsThe results presented and discussed along the present article show that granite sludges derived from the cutting processes of this natural stone can be regarded as an interesting raw material for the fabrication of heavy clay ceramic products, such as bricks and roof tiles when fired at the normal temperatures for this type of products. The proportion of the granite sludges in BT compositions can be as high as 60%. For FTT products fired at 1100ºC, a level of sludge incorporation in the 50-60% range can be used when combined with the SC, or be limited to a maximum of 50% in the case of PRC. Since the heavy clay ceramic industries daily process several hundred tons of raw materials, these results show that this will be a good way to consume the whole amounts of granite sludges produced, avoiding expensive management of the residues with landfill, and preserving an equivalent amount of natural mineral resources.
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