Environ. Sci. Technol. 2008, 42, 8541–8546
A Systematic Investigation into the Extraction of Aluminum from Coal Spoil through Kaolinite X . C . Q I A O , * ,† P . S I , † A N D J . G . Y U ‡ School of Resource and Environmental Engineering, and State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Meilong Road 130, Shanghai, China, 200237
Received July 1, 2008. Revised manuscript received September 4, 2008. Accepted September 17, 2008.
This research has applied kaolin and active carbon (AC) to the investigation of the recovery of aluminum from coal spol (CS). The kaolin, AC-containing kaolin mixture, and CS have been calcined at 500, 600, 700, 800, and 900 °C for 15, 30, 60, and 120 min. The transformation of kaolinite and aluminum extraction that occurred in each calcined sample have been characterized using XRD, TG, IR, and hydrochloric acid leaching methods. The dehydroxylation of kaolinite and the decomposition of metakaolin were influenced by thermal treatment temperature and time. The metakaolin had kept a portion of OH- in its structure until it was calcined at a temperature of 800 °C. Under 60 min treatment, new SiO2 phase was able to be formed at 500 °C, kaolinite was totally converted to metakaolin at 600 °C, and the SiO2 rejoined the reaction at 800 °C. The decompositions of CS were similar to those of kaolin mixture containing 20 wt % AC (MKC). The combustion of combustible matter accelerated the decomposition of kaolinite in the CS and MKC. Higher AC content led to lower aluminum extraction. The treatment at 600 °C was optimal for both CS and MKC.
Introduction The aluminum industry is one of the fastest growing global sectors in the world, especially among developing countries. Increased worldwide demand for aluminum oxide, currently at more than 160 million tons per year, is expected to lead to a gradual global decline of high-grade bauxite, the ore from which alumina is mainly produced using the Bayer process (1). Many aluminum-producing countries have thus become increasingly interested in the investigation of producing alumina from nonbauxitic sources. Coal spoil (CS) is a byproduct generated during coal mining and beneficiation. It consists of various types of clay minerals and rocks, associated with variable amounts of coal contents (2). The unfavorable environmental problems caused by the CS are given more and more attention in China and many other coal-based energy-consuming countries (3, 4). Although the CS have been reported to be suitable for use as construction and filling materials (5-7), the low valueadded character of these utilizations limited their universal application. The amount of CS is about 15-20 wt % of raw coal, and more than 7000 million tons of CS remained unused * Corresponding author phone: 86-21-64252171; fax: 86-2164252826; e-mail:
[email protected]. † School of Resource and Environmental Engineering. ‡ State Key Laboratory of Chemical Engineering. 10.1021/es801798u CCC: $40.75
Published on Web 10/22/2008
2008 American Chemical Society
in China (7). It is hence necessary to extend research to develop novel applications so that the CS can be beneficially used as a resource. Clay minerals such as kaolinite are able to be calcined to form metakaolin, from which the aluminum can be dissolved. Therefore, clay has been mostly thought to be an alternative to bauxite (8). The feasibility of aluminum recovery from kaolinite contained in CS had been reported by some investigations. Livingston et al. reported that high kaolinite content and calcination in a fluidized bed combustor with controlled temperature range of 700-900 °C were necessary for the recovery of aluminum when using coal spoil as feed materials (9, 10). Bailey found that the addition of fluoride in the HCl solution led to a dramatic increase in the rate of aluminum extraction from CS (11). It is also reported that the aluminum chloride in the leachate was be able to be selectively precipitated by a HCl gas induced crystallizationtechnique (12). The majority of proposed routes for aluminum recovery from kaolin-based CS involved the calcination of feed materials under controlled conditions. However, the influence of calcination conditions on the reactivity of selected CS has not been clearly understood. This research systematically investigated the extraction of Al from coal spoil under different thermal treatment temperatures and times. Kaolin and mixtures containing kaolin and active carbon were used to simulate the decomposition of coal spoil.
Experimental Section Materials. The CS collected from Wuda Pinggou coal mine in Inner Mongolia (China) was crushed and wet milled to less than 10 µm in an agate mill. The milled CS was then oven-dried at 105 °C for 24 h. A commercially available kaolin (
