Research On Preparation Of Ceramsite Using Tannery Sludge

In the tannery wastewater treatment process, a great deal of tannery sludge will be produced. Tannery sludge contains a large number of pathogens, such as sulfur and heavy metal Cr, if not be effectively deal with, it will be harmful to environment. At present,there are many ways to dispose tannery sludge, for instance, land treatment, composting and the production of construction materials. And it is a novel and effective approach for disposal of sludge that using tannery sludge as one of raw metals to make ceramsite by high temperature sintering and puffing. Ceramsite is a lightweight aggregate used in concrete architecture which has a number of special features, such as light weight, high strength, low thermal conductivity and high water absorption for its honeycombed internal structure. Traditional ceramsite is sintered by clay and shale, whereas mining clay will destroy the land, and the exploitation of shale mines will destroy the natural environment, thus it is contrary to the principles of sustainable development. In this paper, tannery sludge as the principal material was blended with a certain amount of clay and fly ash to product ceramsite with considerable strength by high temperature sintering. This method can not only stabilize Cr in sludge by high temperature, but also consume large amounts of tannery sludge. It can also alleviate the increasingly serious problem of sludge treatment and broaden the types of raw materials for making ceramsite. Furthermore, Because of tannery sludge is rich in organic matter andu has some caloric value, so decarburization and the heat released during sintering will be effectively reused, which also reduce fuel consumption.It is essential that the preparation of ceramsite made by tannery sludge is a behavior of solidifying waste by high temperature, so the basic thermal behavior of three different sources of tannery sludge were studied in this paper. Futhermore, sludge A derived from a tannery using the wet blue leather as raw materials; the sludge B came from a tannery which produces cattle hide leather and the Cr of wastewater was recovered by the alkali, however, the Cr of sluge C from a tannery which produce cattle hide leather and pig-skin has not been recovered. And thermogravimetric experiments of tannery sludge from three different sources were studied by a thermal-gravity analyzer(TGA). The pyrolysis characteristics of different stages under the temperature rise rates of 10℃/min and 20℃/min were observed by analyzing the TG and DTG curves. The results showed that the pyrolysis process of tannery sludge can be broadly separated into three stages, namely evaporation of water, release of volatile, decomposition of carbonate and other inorganic mineral phase. The second stage is the main phase of the weight loss of sludge and volatile is released in a wide temperature region. Most of pyrolysis process of tannery sludge was second-order reaction by using of Coats-Rdefern integral method. It was found that the pyrolysis process of sludge B and C were similar to each other and the heating rate has little effect on curves of sludge B and C, however the more effect on curves of sludge A. When the heating rate was changed from 10℃/ min to 20℃/min, the finally weight loss of three sludge had different level of decline, moreover the weight loss of sludge A decreased obviously. It also can be seen that the activation energy of sludge A increased slightly with the increasing of heating rate but activation energy of sludge B and C declined.In this paper, novel ceramisite have been produced by sintering mixes of dried tannery sludge A and ordinary clay between 1120℃and 1190℃. Six ceramic samples with different ratios of clay/sludge were produced in our study, which of loose bulk density, 1h water absorption, particle density, compressive strength and leaching of chromium were determined and compared. According to the test results, after ceramsite made by high levels of tannery sludge was sintered by more than a thousand centigrade, its compressive strength is lower than before. To attain certain strength, temperature and time must be strictly controlled during decarburization and sintering. Larger proportion of sludge or lower sintering temperature would cause a dramatic decline in strength of ceramsite sample. On the contrary, much lower proportion of sludge or too longer sintering time would lead ceramsite samples to be molten and hardening. Appropriate conditions for decarbonization is that temperature is controlled at 350℃, and heat preservation time is 20 min, sintering temperature ranges are between 1130℃and 1180℃, and according to the ratio of clay and sludge, sintering temperature should be controlled strictly.The study also found that the heavy metal Cr can be well solidified in novel ceramsite produced by appropriate sintering process and appropriate ratio of materials, and Cr concentrations of the leachate of sludge ceramsite were in line with the national standards. This paper also examines the characteristics of the ceramsite produced by sintering mixes of tannery sludge B and ordinary clay between 1080℃and 1140℃. The research shows that compared with ceramsite made by sludge A, the compressive strength of ceramsite made by sludge B is smaller, and water absorption of which is higher, hence the performance of ceramsite made from different tannery sludge is quite different. Considering that the relatively high loose bulk density and small compressive strength of ceramiste only made by sludge and clay, during the following study, tannery sludge, clay and fly ash were chosen as the initial components for preparing raw balls of 10~15 mm in diameter, which were used to make ceramsite as a lightweight aggregate by drying, preheating and firing processes. The results shows that the loose bulk density of ceramsite was decreased and the compressive of ceramsite increased significantly, but water absorption of ceramsite is still relatively high after the addition of fly ash. And Cr concentrations of the leachate of ceramsite were in line with requirements of the national standards.