Liu Shuqin, Zhao Limei, Yu Xuedong, Yu Li
(UCG Research Centre of Chemistry & Environmental Engineering College,Beijing Campus of China University of Mining & Technology, Beijing 100083)

1 INTRODUCTION
UCG (underground coal gasification) is a process in which underground coal, without mining and transportation, is directly converted into combustible gas. It offers a potentially means of clean coal conversion into high-energy fuels^[1]. However, the process generates massive amounts of solid waste in the form of gasification residue, which is left underground different from surface gasification. The composition of the gasification residue has been a subject of environmental interest because it may contain toxic components such as trace hazardous elements which, if leached, may contaminate underground water since UCG is usually conducted in the environment of fresh water^[2].
    An extensive research has been carried aimed at various types of combustion ash and combustion-ash leachate originating with coal-fired power plants. However, only limited information has been obtained about UCG residue. With the future trend toward greater utilization of UCG as an energy source, there will be a need for such information.
   The objective of this investigation was to study the leachable levels and leaching behavior of 8 trace elements from UCG residue. The elements studied were Cr, Co, Ni, Cu, Mo, Cd, Sr, As. It is expected to provide basic data for pollution protection of underground water around the underground gasification zone.

2 EXPERIMENTAL
2.1 Sampling
Simulation test of UCG of lignite was conducted in the UCG central laboratory of China University of Mining & Technology. The test lignite was provided by Dayan Coal Mining Group. Block coal, rock, sand, and clay were used to simulate in-site underground coal deposit condition. The test lasted for 150h through blowing oxygen and steam simultaneously with production of middle caloric gas. After the UCG simulation test, samples were taken along the gasification tunnel through opening and investigation of the gasifier. UCG residue of 5kg was collected and reduced to 1kg by quartering. The obtained sample was crushed less than 100 order and prepared for use.
2.2 Leaching procedure               
HNO₃ was used to adjust the pH value of deionized water into 2.0, 4.0, and 6.5 respectively and served as leaching solution. Glass column with length of 70mm and diameter of 25mm was processed for leaching experiments and dipped in HNO₃ before use.
    Representative 15g portions of residue sample were filled into the leaching column and leached with deionized water maintained at different pH 2.0, 4.0, 6.5 for 80h. Flow rate controlled was 4.0ml/h to keep the liquid surface 10cm higher than solid surface and liquid/solid ratio 3:1. Leachates were collected with 50ml bluck with interval of 5h or 10h, diluted to 50ml, filtered, and analyzed by Platform ICP-MS (inductively coupled plasma mass spectrometer). As was determined by atomic absorption spectrometry. The true value of trace element concentration was obtained by subtract the blank value of the leaching solution.

3 RESULTS AND DISCUSSION
3.1 Leaching rate and leaching intensity                   
The concentration levels of the 8 trace elements in the leachates at different pH value were obtained. In order to compare the leaching trend and leaching levels of the trace elements from UCG residue, leaching intensity was introduced and calculated through the following model^[3]:

    In which, Lx, the leaching intensity of certain trace element; a_x, concentration of trace element in the leachate, mg/L; V, total volume of the leachate, ml；A_x, content of trace element in the sample of UCG residue, mg/g；M, weight of the total sample, g；t, leaching time. The calculated leaching intensity and leaching rate are shown in table 3.1.

Table 3.1 Leaching index of trace elements in UCG residue of lignite at different pH

3.2 Influence of trace element existence form on leaching behavior
Leaching behavior of the trace elements depends heavily on their existence form^[5]. It is usually considered that trace elements leaching becomes difficult following the order: exchangeable, bound to carbonate and Fe & Mn oxides, bound to organic matter, and residual, since in this order trace elements are more tightly combined^[6]. Trace elements in exchangeable form are easily to be leached because they are combined to the particulate surface only through adsorption-desorption process. Leaching of trace elements bounding to carbonate is remarkably influenced by the pH value of leaching solution, which affects the solubility of carbonates. In the case of trace elements bounding to organic matter, only under oxidizing conditions in natural waters, organic matter can be degraded, leading to a release of soluble trace metals. The residual should contain mainly primary and secondary minerals, which may hold trace metals within their crystal structure. These metals are not expected to be released in solution over a reasonable time span under the conditions normally encountered in nature.
    The partitioning of the element existence form in UCG residue of lignite through sequential chemical extraction is shown in table 3.3. It is observed, the exchangeable form of Mo takes up more than 20%, thus Mo shows strong leaching. Sr represents higher leaching rate and leaching intensity since 45.62% of Sr exists in carbonates and Fe-Mn oxides. The fraction of Cd bounding to carbonates and Fe-Mn oxides accounts for more than 30%, which leads to higher leaching rate. But its leachable amount is lower. Proportion of Ni found in residue is higher than 70%, thus it is weak leached. Mo and Sr take on higher leachable amount and represent a large fraction in carbonates, thus their leachable amount are remarkably affected by pH of the leaching solution.

Table 3.3 Partitioning of the trace element existence form

4 CONCLUSION
Sr and Mo give priority to strong leaching, Cr, Co, Cu, Cd. As mainly behave as middle leaching, and Ni shows hardy leaching. Leaching intensities of the trace elements are remarkably influenced by the pH value of leaching solution. Leaching behavior of trace elements depends primarily on their existence form. Presence of certain amount of carbonates and exchangeable form in UCG residue increased the leaching tendency of trace elements. Comparison of trace element levels in leachates and concentration standard of underground water shows that long-term leaching of UCG residue has the possibility of contaminating surrounding underground water, which should be attached more importance. The basic leaching characteristics of the above trace elements would provide basic dates for pollution prevention of underground water environment near UCG site.