REDUCTION OF HALOGENS IN CARBONACEOUS MATERIAL

Abstract: A process for the reduction of fluorides and chlorides from carbonaceous matter, such as coal, which has been leached by hydrochloric and hydrofluoric acid. The process comprises heating the leached solids (2), to initially dry the solids and then drive off the fluorides and chlorides (4) in a stream of purge gas (6). The purge gas (6) may be recycled (3) following separation from the entrained fluorides or chlorides (9). The treated carbonaceous matter now having a low halide concentration can be cooled (10) and used.

Description: "REDUCTION OF HALOGENS IN CARBONACEOUS MATERIAL" This invention relates to a process for the reduction of fluorides and chlorides from carbonaceous matter, particularly coal, which has been leached by hydrochloric or hydrofluoric acid. BACKGROUND ART The leaching of coal and other carbonaceous solids with aqueous hydrofluoric acid (HF) and/or hydrochloric acid (HCL) is known as a means of reducing the mineral content of the solids. Much of the mineral matter is soluble in the acids, whereas the carbonaceous base is insoluble. Thereby HF and/or HCL treatment is a potential means of de-ashing coal and other carbonaceous solids. However, this type of acid leaching results in levels of fluoride and chloride in the leached solids which are well in excess of the tolerable levels for most applications of the leached solids. For example, acid-leached coal can be applied as a fuel in a wide range of combustion equipment, this equipment is generally intolerant to high levels of fluoride and chloride in the fuel due to consequential corrosion problems, and environmental release of fluoride and chloride from combustion equipment is closely controlled by governments. The reduction to tolerable levels of fluoride and chloride content in acid-leached carbonaceous solids is the major technical problem preventing their wide-spread application. Washing the leached solids with water or other aqueous solutions is not successful when used over the practical range of temperature and time. Washing (or leaching) with oxidising aqueous solutions can result in low fluoride and chloride levels, but under these conditions the carbonaceous solids are strongly oxidised with consequential loss of heating value. However, in some cases it is not necessary to retain the carbonaceous volatiles but is more important to reduce the chloride or fluoride levels. One object of the present invention is to provide a practical technique for the reduction of fluoride and/or chloride, which does not significantly degrade the carbonaceous solids. A further object is to provide a method of substantially removing fluoride and/or chloride residues from carbonaceous matter, where it is not necessary to retain the volatile carbonaceous components. DISCLOSURE OF THE INVENTION In one broad form the invention provides a process to substantially reduce the fluoride and chloride content of carbonaceous solids which have previously been leached with hydrofluoric and/or hydrochloric acids; whereby the acid-leached solids are heated to a- temperature less than that at which the solids lose substantial amounts of their carbonaceous volatiles, with the heated solids being purged by gas in order to carry away the released fluorides and chlorides. In a further broad form the invention provides a process to substantially reduce the fluoride and chloride content of carbonaceous solids which have previously been leached with hydrofluoric and/or hydrochloric acids; whereby the acid-leached solids are heated to a temperature sufficient to reduce the chloride and/or fluoride levels to the desired value, with the heated solids being purged by gas in order to carry away the released fluorides and chlorides. BRIEF DESCRIPTION OF THE DRAWING The drawing is a schematic flowsheet of the process according to the invention. BEST MODE OF CARRYING OUT THE INVENTION Our technique is based on thermal treatment of the acid-leached solids. The solids are passed 1 to a vessel 2 where they are dried, and then heated to a temperature suitable for the particular carbonaceous base under treatment. The maximum temperature is normally set by the volatilisation characteristics of the solids. Often it is undesirable to devolatilise the solids as this degrades them, particularly in fuel applications. Most coals for example can be heated to 250-350°C without excessive loss of carbonaceous volatiles. This data can be easily obtained using conventional thermogravimetric analysis. The degree of fluoride/chloride removal increases with increasing temperature, so a compromise must be made between fluoride/chloride removal and the degree of devolatilisation. For a wide range of coals tested, we have found that a satisfactory temperature compromise is achievable in the range of 100-400 C. Whilst heating the solids it is necessary to purge 3 them with a suitable gas, thereby carrying away 4 the released fluorides and chlorides. In most cases it is preferred to use an inert purge gas, since at the temperatures used the carbonaceous solids can be oxidised by non-inert gases, thereby degrading them. Suitable inert gases include nitrogen, carbon dioxide and steam. The pressure within the thermal treatment apparatus is not significant within the range from atmospheric to vacuum. Thus pressure reduction means 5 are preferably used to lower the pressure to induce release of chlorides/fluorides at lower temperatures. The time required to effect adequate fluoride/chloride removal varies depending on the carbonaceous base, but typically is from 15 minutes to 2 hours. A variety of apparatus can be used, either batch or continuous over the range of commonly used solids - gas contacting equipment. This includes rotary kilns, fixed beds and fluidised beds. The source of heat 6 can be indirect, or direct from the purge gas, or radiant. The preferred internal materials of construction of vessel 2 are carbon-based, as these provide suitable corrosion resistance under the conditions. The purge gases containing the released fluorides/chlorides would normally be separately treated 7 to remove these components, thereby allowing recycle 8 of the purge gases or disposal to the atmosphere 9. Suitable treatment includes scrubbing with water or alkaline solutions, or condensation in the case of steam purge gas, or contacting with alkaline solids such as limestone. After thermal treatment, the carbonaceous solids are cooled 10 to ambient temperature, with the opportunity for heat recovery 11. Conventional solids cooling equipment is suitable. The following are examples of our technique:- Example 1 A high volatile bitumenous coal from N.S.W. was leached with HF, then thoroughly washed with water. The solids were dried in an oven at 105°C, then thermally treated in a batch fluid bed apparatus, at 300°C for 60 minutes, using carbon dioxide at 300°C as the fluidising purge gas. Results are shown in table 1: Table 1 Fluoride Content of Solid (P.P.M.) Raw coal 100 HF leached, water washed coal 1500 Coal after thermal treatment 200 Example 2 The same raw coal as in Example 1 was leached with HF, then leached with HCL, then thoroughly washed with water. The solids were dried in an oven at 105°C, then thermally treated in a batch fluid bed apparatus, at 300°C for 60 minutes, using carbon dioxide at 300°C for the fluidising purge gas. Results are shown in Table 2: Table 2 Fluoride Chloride Content of Content of Solids Solids (P.P.M.) (P.P.M.) Raw Coal 100 200 HF Leached, HCL Leached Water Washed Coal 700 5000 Coal after thermal treatmen Ltt 200 600 Example 3 A sub-bitumenous coal from New Zealand was leached with HF, then leached with HCL, then thoroughly washed with water. The solids were driLeedd iinn aann oovveenn aatt 110055°°CC,, then thermally treated on a pan in a purged oven, at 280 .oC for ttwwoo hhoouurrss uussiinngg nnitrogen purge gas at 280°C. Results are shown in Table 3: Table 3 Fluoride Chloride Content of Content of Solids Solids (P.P.M.) (P.P.M.) Raw coal 150 100 HF Leached, HCL Leached Water washed coal 400 1900 Coal after thermal treatmentt 150 350 _ O PI

Claims: Claims 1. A process to substantially reduce the fluoride and chloride content of carbonaceous solids which have previously been leached with hydrofluoric and/or hydrochloric acids; whereby the acid-leached solids are heated to a temperature less than that at which the solids lose substantial amounts of their carbonaceous volatiles, with the heated solids being purged by gas in order to carry away the released fluorides and chlorides. 2. A process to substantially reduce the fluoride and chloride content of carbonaceous solids which have previously been leached with hydrofluoric and/or hydrochloric acids; whereby the acid-leached solids are heated to a temperature sufficient to reduce the chloride and/or fluoride levels to the desired value, with the heated solids being purged by gas in order to carry away the released fluorides and chlorides. 3. The process of claim 1 or 2, wherein the carbonaceous solids are heated to a temperature of 100 to 400°C. 4. The process of any one of claims 1 to 3, wherein the purge gas is selected from nitrogen, carbon dioxide or steam. 5. The process of any one of claims 1 to 4, wherein the residence time of the carbonaceous solids in the heat treatment is carried out from 15 minutes to 2 hours. 6. The process of any one of claims 1 to 5 wherein the heat treatment is carried out at a pressure from vacuum to atmospheric pressure. OMPI

Inventors: LLOYD ROBERT, TURNER MAXWELL JAMES, LLOYD, ROBERT, TURNER, MAXWELL, JAMES
Applicants: KINNERET ETS LTD, KINNERET ENTERPRISES LIMITED
Application year: 1984
Main classification: C10L9
IPC codes: C10L9