• Vol 9, No 2 (2018)
  • Electrical, Electronics, and Computer Engineering

Preparation and Characterization of Phosphate-sludge Kaolin Mixture for Ceramics Bricks

Jackson Muliawan, Sotya Astutiningsih

Cite this article as:

Muliawan, J., Astutiningsih, S., 2018. Preparation and Characterization of Phosphate-sludge Kaolin Mixture for Ceramics Bricks. International Journal of Technology. Volume 9(2), pp. 317-324

Jackson Muliawan Department of Metallurgy and Materials Engineering, Engineering Faculty, Universitas Indonesia
Sotya Astutiningsih Department of Metallurgy and Materials Engineering, Engineering Faculty, Universitas Indonesia, Jakarta , Indonesia
Email to Corresponding Author


Phosphate Sludge (PS) waste has been a problem in metal surface finishing industry. The waste cannot be dumped in landfill due to the metal content. Valorisation of the waste will be beneficial in a way that it conserves natural reserves and reduces energy consumption. This paper describes the attempt of utilization of PS by mixing it in kaolin in preparation of ceramic bricks. A series of experiments showed that mixtures containing between 25–50 mass % PS sintered at 1200oC attained the highest compressive strength of >25 MPa. X-ray diffractions (XRD) showed that the presence of PS hindered the formation of mullite, the phase that contribute to strength in Al2O3–SiO2 kaolin system. In the mixture of 1:1 kaolin: PS fired at 1200oC, cristobalite was formed, instead of mullite, as observed in the XRD patterns.

Ceramics; Firing; Kaolin; Phosphate-sludge


Zinc phosphate is widely used in the finishing process of metal industries such as automotive, steel furniture, houseware, electrical equipment, construction, etc., due to its capability to protect the base metal from corrosion and to enhance paint adhesion.  However, the use of zinc phosphate produces waste in the form of liquid, which can be treated and disposed to the environment, and solid sediment known as phosphate sludge (PS) which is harmful if not properly treated. PS is a mixture of iron and zinc salt of phosphoric acid with the general formula ZnFe7/3(PO4)3, also called as phosphophylite. PS mass range consists mainly of, in weight percentage, ~20 iron, 50-55 phosphorus, and ~10 zinc (Narayanan, 2005). According to the US Environmental Agency (USEPA) PS is considered hazardous and therefore must be encapsulated before disposing in landfills. The high cost of PS handling that cannot be disposed into drains and the environment directly, has been a problem that demands an urgent solution.

Effort to recover PS involved leaching and extraction processes in physics and chemistry or solidification (Kuo, 2012; Navarro-Blasco et al., 2015). Reuse and recycling of the materials was focused on the recovery of zinc, iron and phosphate. Several methods for reclaiming phosphate has been patented (Narayanan, 2005), however none can be claimed to be the most effective method in terms of benefits versus energy consumption or cost. Utilization of this solid waste into building materials becomes an alternative to recovery. Unlike the ordinary building materials, the capability of, especially hazardous, metals containment within the   method used is of prime important. Some of use of PS reported were as an aggregate for concrete (Do?an & Karpuzcu, 2010; Loutou et al., 2013); as an ingredient in the manufacture of clinker in cement industry (Caponero &Tenório, 2000; Lin et al., 2009); and in clay bricks manufacture (Bersch et al., 2012).

The presence of phosphate and iron oxides affects sintering temperature hence phase(s) formation, mechanical and physical properties of the products. A study found that the addition of phosphate compounds into kaolin decreased  sintering temperature of up to 200oC without decreasing rupture strength (Little et al., 2008).  Little and colleagues reported an experiment by adding 10 weight % of dried PS of tri-cationic phosphate solution into fly ash ceramic, resulted in significantly lower sintering temperature of up to 75oC (Little et al., 2008).  Besides lowering the sintering temperature, the addition of PS also reduced the level of arsenic leaching (Little et al., 2008).  In addition, the presence of a metal with a low melting point such as zinc, contributed to the increase of liquid phase at lower temperatures (Little et al., 2008). PS could also be added by 10?20 weight % in concrete for applications that do not requires high strength (Ucaroglu & Talinli, 2012). The presence of iron oxides in kaolin lowers the sintering temperature of up to 50°C (Nana et al., 2013) and resulted in solidified phase of mullite and cristobalite (Little et al., 2008).  It was also found that mullite crystal in lath-like and needle-like shape occurred in samples containing iron oxide (Guo, 2010). Flexural strength of samples with a high content of iron oxide found to be higher than the samples with a low content of iron oxide (Michot et al., 2008).

In this work, kaolin- based ceramic brick with the addition of PS has been prepared and characterized. The mass optimization of PS utilization is the main focus in this study. Sintering temperature related to optimum mixture composition of PS-kaolin was varied to obtain the best compressive strength at the lowest sintering temperatures, hence the minimum energy consumption.

Experimental Methods

2.1       Raw Materials

PS used in these experiments were supplied from a metal surface treatment company in Banten Province, Indonesia, while kaolin was purchased from PT RAK Minerals Indonesia under the trademark of Cerafill. The kaolin, with a size of 325 mesh, was originated from Bangka island, Indonesia. Chemical compositions, analysed using X ray fluorescence spectroscopy, of both raw materials are presented in Table 1 and 2.

2.2.     Specimens Preparation

PS was cloth-filtered to reduce excessive water, washed and then dried at 110oC until sufficiently dried to be milled.  The milled PS was mashed by hand mortar and sieved to 100 mesh. PS and kaolin at specified mixing ratios were mixed using hand rotating drum and casted in molds with dimensions according to the tests. After casting, the specimens were dried in an oven at 110oC for at least 24 hours. Cubes of 50×50×50 mm3 were prepared for compression test. Compressive strength values presented were from three best specimens. For each composition and sintering condition, materials from broken compression test samples were taken for x-ray diffraction and scanning electron microscopy (SEM). Variations of composition, sintering temperature and sintering time were performed to obtain optimum compressive strength with minimum of energy consumption, i.e. the lowest sintering temperature and time. Sintering was conducted in an electric furnace with a heating rate of 5oC/min to 500oC, at 10oC/min from 500 to 925oC and at 15oC/min from 925oC to the specified sintering temperature.

2.3.     Characterization Techniques

Phase analysis using XRD and SEM observation were carried out to understand the phase(s) formation during sintering. Density, water absorption and initial rate of suction (IRS) of the products with the optimum strength were then measured.

XRD was performed using XRD-7000 X Shimadzu Maxima, equipped with a graphite monochromator and using Cu radiation Ka, wavelength (?) of 1.54 Å. JEOL JSM-6390A SEM equipped with Energy-Dispersive X-ray Spectroscopy (EDS) was utilized for microstructure observation. Density of the specimen was calculated by dividing the mass measured with volume calculated from the specimen dimension. Dimensional measurements were carried out using a calliper while mass of the specimens were weighed using an analytical balance with an accuracy of ± 0.01 gram.   Water absorption was calculated using Equation 1 as follows,


% absorption = {100(ws – wd)}/w                                                (1)


where ws counts for saturated mass after soaked in cold water for 24 hours, and wd is dry mass of specimen before soaking.

Initial Rate of Suction (IRS) was calculated using Equation 2 as follows,


                                                  IRS = (m1-m2)K                                                                   (2)


where IRS is for initial rate of suction in gr/minute/30in2 or in gr/minute/193.55 cm2, in which K=30/Area or K=193.55/Area, respectively.


The fact that many of the specimens cracked and the results of compressive strength suggested that PS – kaolin mixture specimens were sensitive to heating/firing parameters. Before firing at high temperatures, the specimens should be dried, and the firing cycle should be carefully controlled to avoid shrinkage. Based on the experiment results, highest compressive strength of >25 MPa was achieved by specimens prepared with the addition of 25 to 50% of PS fired at 1200oC. The melting of silica was believed to occurred in kaolin – PS mixture and that the melt binding the particles together was the mechanism of strength development. The addition of PS in kaolin induced porosity. SEM observation showed that pore size within the specimens increased with PS. The porosity was believed to be the cause of high values of water absorption and IRS which was below the standard.


This research has been funded by The Ministry of Research and Higher Education of the Republic of Indonesia through The University of Indonesia under the grant Excellent Research of Higher Education (Penelitian Unggulan Perguruan Tinggi) 2017 No. 2760/ADD/UN2.R3.1/HKP05.00/2017. The authors wish to thank the Laboratory of Metallurgical Process of the Department of Metallurgy and Materials for facilitating the experiments.


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