RAPID MEASUREMENT OF COAL OXIDATION

Abstract

The present invention relates to a method for determining oxidation of coal, the method comprising: a) mixing a coal sample with an organic solvent and an inorganic solvent, to extract oxidised coal species from the coal sample into a liquid phase, and b) analysing the liquid phase from step (a) to determine a degree of coal oxidation. The inorganic solvent may comprise an inorganic compound such as a pyrophosphate. The present invention also relates to a method for controlling collector used in a coal flotation process, the method comprising determining a degree of oxidation of coal by the preceding method and controlling a ratio of non-polar collector to polar collector in the coal flotation process in response to the determined degree of oxidation of coal.

Claims

1. A method for determining oxidation of coal, the method comprising: a) mixing a coal sample with an organic solvent and an inorganic solvent, to extract oxidised coal species from the coal sample into a liquid phase, and b) analysing the liquid phase from step (a) to determine a degree of coal oxidation.

2. The method of claim 1, wherein the extracted oxidised coal species in step (a) dissolve in the organic solvent.

3. The method of claim 1, wherein the inorganic solvent comprises an aqueous solution of an inorganic compound.

4. The method of claim 3, wherein the inorganic solvent and the organic solvent are miscible.

5. The method of claim 1, wherein step (a) is conducted at ambient temperature.

6. The method of claim 5, wherein in step (a) there is a contact time of less than 5 minutes between the coal sample and the liquid phase.

7. The method of claim 5, wherein step (a) is performed with shaking or agitation.

8. The method of claim 1, wherein the coal sample comprises a coal slurry.

9. The method of claim 3, wherein the inorganic solvent comprises an inorganic compound comprising a carbonate, a pyrophosphate, or a tetraborate; or mixtures thereof.

10. The method of claim 3, wherein the inorganic solvent comprises an inorganic compound selected from K.sub.4P.sub.2O.sub.7 or Na.sub.4P.sub.2O.sub.7, or mixtures thereof.

11. The method of claim 3, wherein the organic solvent is selected from one or more of ethanol, methanol, 1-propanol, 2-propanol, dioxane, dimethyl sulfoxide, tetrahydrofuran, and dimethylformamide.

12. The method of claim 11, wherein the organic solvent is ethanol.

13. The method of claim 1, wherein step (a) produces a mixture in which the coal to liquid ratio is from 1 wt % to 5 wt %.

14. The method of claim 13, wherein step (a) produces a mixture in which the concentration of inorganic compound is 0.005M to 0.5M.

15. The method of claim 13, wherein step (a) produces a mixture in which the volume of the organic solvent is from about 3 vol % to about 40 vol %.

16. The method of claim 1, wherein the liquid phase from step (a) is separated from the coal sample prior to step (b).

17. The method of claim 1, wherein step (b) uses UV/VIS spectrophotometry to determine the concentration of extracted species in the liquid phase.

18. The method of claim 1, wherein the analysis of step (b) provides a value of the degree of surface oxidation of the coal.

19. A method for controlling collector used in a coal flotation process, the method comprising determining a degree of oxidation of coal by the method of claim 1 and controlling a ratio of non-polar collector to polar collector in the coal flotation process in response to the determined degree of oxidation of coal.

20. A method for controlling collector used in a coal flotation process, the method comprising determining a degree of surface oxidation of coal by the method of claim 18 and controlling a ratio of non-polar collector to polar collector in the coal flotation process in response to the determined degree of surface oxidation of the coal.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0053] Preferred features, embodiments and variations of the invention may be discerned from the following Detailed Description which provides sufficient information for those skilled in the art to perform the invention. The Detailed Description is not to be regarded as limiting the scope of the preceding Summary of the Invention in any way. The Detailed Description will make reference to a number of drawings as follows:

[0054] FIG. 1 shows XPS spectra of heavily oxidised coal;

[0055] FIG. 2 shows a schematic diagram illustrating diesel or other non-polar collector attaching to unoxidised coal surface and a polar collector attaching to oxidised coal surface;

[0056] FIG. 3 shows a graph of degree of oxidised carbon determined by XPS spectroscopy vs spectroscopy oxidation index obtained by analysing coal using the NaOH-based technique described in the background art section of this specification;

[0057] FIG. 4 shows a graph of UV absorbance determined from experimental work in accordance with an embodiment of the present invention plotted against UV absorbance determined using the NaOH-based technique described in the background art section of this specification;

[0058] FIG. 5 shows a graph of UV Spectroscopy readings using a portable UV spectrophotometer vs UV spectroscopy readings using a laboratory UV spectrophotometer;

[0059] FIG. 6 shows a graph of combustible recovery vs flotation time using a heavily oxidised coal feed and varying ratios of diesel collector and polar collector;

[0060] FIG. 7 shows a graph of combustible recovery vs flotation time using a medium oxidised coal feed and varying ratios of diesel collector and polar collector;

[0061] FIG. 8 shows a graph of combustible recovery vs flotation time using a lightly oxidised coal feed and varying ratios of diesel collector and polar collector; and

[0062] FIG. 9 shows a graph of UV absorbance of flotation feed in a plant processing old tailings at different times and dates over a one month period.

DESCRIPTION OF EMBODIMENTS

[0063] Preferred embodiments of the present invention were developed with a view to providing a more user-friendly and safe coal oxidation measurement technique that can be used on-site in a plant, such as a coal flotation plant, without requiring laboratory facilities. Preferred embodiments of the present invention require no heating, require only a short reaction time and can use portable apparatus, whilst also requiring minimal operator training.

[0064] FIG. 1 shows the XPS spectra of heavily oxidised coal. Oxidised coal is widely present in coal mines. Oxidation occurs once coal is in contact with oxygen. It occurs during mining, in stockpiling, during transport and in tailings dams. As can be seen from FIG. 1, a number of oxidised species are present on the surface of oxidised coal.

[0065] Oxidised surfaces of coal are generally hydrophilic, whereas non-oxidised surfaces of coal are generally hydrophobic. As a result, different types of flotation collectors are attracted to the surfaces. FIG. 2 shows that a diesel collector (or indeed, other oily or non-polar collectors) is attracted to the non-oxidised surface of coal. In contrast, non-polar collectors are attracted to the oxidised surfaces of the coal. Therefore, strategies to increase the flotation performance of oxidised coals should ideally include use of a mixture of non-polar collectors and polar collectors, with the dosage of non-polar collectors being determined by the degree of surface oxidation. This flotation process would desirably include a rapid method for measuring coal oxidation in order to be able to rapidly respond to changes in the degree of oxidation of the coal feed to the flotation plant.

[0066] Preferred embodiments of the present invention were developed with the aim of being able to measure oxidation of coal in time periods of 10 minutes or less.

[0067] The first step of the method of preferred embodiments of the present invention involves mixing a coal sample, which will typically be a coal slurry that is being fed to a coal flotation plant, with an inorganic solvent and an organic solvent. The inorganic solvent of the preferred method of the present invention comprises potassium pyrophosphate, K.sub.4P.sub.2O.sub.7. Sodium pyrophosphate may also be used but potassium pyrophosphate is preferred as it has a higher water solubility.

[0068] The organic solvent may comprise ethanol. In preferred embodiments of the present invention, the organic solvent is miscible with water and does not interfere with the UV spectroscopy readings that are subsequently taken. DMSO (dimethyl sulfoxide) and several other organic solvents previously named in this specification may also be used in preferred embodiments of the present invention.

[0069] In order to measure the UV absorbance of the liquid phase following extraction of coal, a portable UV254 instrument was purchased from Photonic Measurements Ltd in the United Kingdom. This instrument is designed to measure organic carbon in water treatment plants by measuring UV absorbance at 254 nm. Coal oxidation species are mainly aromatic organic carbons which also have strong UV absorbance at 250 nm through to 270 nm, making an instrument that operates at a wavelength of 254 nm suitable for purpose. This instrument is portable, can be directly used in the plant, relatively inexpensive, resistant to dust and water and powered by a battery.

[0070] The following procedure was used to measure the degree of coal oxidation: [0071] 1) bring the portable UV spectrophotometer and sampling bottles to the coal flotation plant. [0072] 2) collect coal slurry sample in the plant and add 25 ml of coal slurry into a 50 ml plastic tube or bottle (other size bottles may also be used). [0073] 3) add 2 ml ethanol and 0.8 ml 0.5M K.sub.4P.sub.2O.sub.7 solution into the bottle containing the slurry sample and shake for 1 minute. [0074] 4) turn on the UV254 portable spectrophotometer. [0075] 5) add the extraction solution having the same ratio of ethanol and K.sub.4P.sub.2O.sub.7 as used in step (3) into the cuvette to use the UV254 portable spectrophotometer to measure the UV absorbance of the blank solution. [0076] 6) use a 10 ml syringe to take around 5 ml slurry from the plastic tube or bottle and use a 0.45 ?m filter head to filter it, then add the filtrate to the cuvette for UV measurement. [0077] 7) measure the UV absorbance of the filtrate.

[0078] The above test can be done in the plant and takes around 5 minutes in total. This can be very beneficial for the plant when processing oxidised coals. Plant operators can measure the oxidation degree more frequently and optimise operating conditions based on the degree of coal oxidation. In addition, the extracting agents (ethanol and K.sub.4P.sub.2O.sub.7) are much safer to use compared to 1M NaOH solution. Further, the above method does not require any heating.

[0079] FIG. 3 shows a graph of degree of oxidised carbon determined by XPS spectroscopy vs spectroscopy oxidation index obtained by analysing coal using the NaOH-based technique described in the background art section of this specification. The results shown in FIG. 3 demonstrate that there is a good correlation between the spectroscopic oxidation index obtained using a laboratory UV spectrophotometer following extraction with hot NaOH for 45 minutes and the degree of oxidised carbon determined by XPS. FIG. 4 shows a graph of UV absorbance determined from experimental work in accordance with an embodiment of the present invention plotted against UV absorbance determined using the NaOH-based technique described in the background art section of this specification. As can be seen from FIG. 4, the UV absorbance using the method of preferred embodiments of the present invention provides a good correlation with the UV absorbance measured using the NaOH solvent technique. Therefore, the results of FIGS. 3 and 4 demonstrate that there will be a good correlation between the UV absorbance determined in accordance with preferred embodiments of the present invention and the degree of oxidised carbon in the coal determined by XPS. Further, the correlation between the spectroscopic oxidation index obtained using the NaOH-based method and the degree of oxidised carbon determined by XPS can be used to provide a correlation between the UV absorbance determined in accordance with embodiments of the method of the present invention and the degree of oxidised carbon.

[0080] FIG. 5 shows that the portable, inexpensive UV spectrophotometer used in this experimental work gives results that are closely correlated with the laboratory UV spectrophotometer used in previous work. Therefore, the portable, inexpensive UV spectrophotometer used in this experimental work is capable of providing good quality results.

[0081] FIG. 6 shows combustible recovery against flotation time for a highly oxidised coal obtained from old tailings reject. The coal sample contains 63% ash in the flotation feed and has a spectroscopy oxidation index of 0.236. The flotation results shown in FIG. 6 use a collector that comprises diesel alone as the collector (the lowest line in the graph) or a mixture of diesel and polar collector, with the amount of polar collector being indicated on the graph. The total collector dosage was fixed at 300 g/t. As can be seen from FIG. 6, adding a polar collector to the diesel increases combustible recovery. The optimum polar collector ratio for the highly oxidised coal was around 15%.

[0082] FIG. 7 shows combustible recovery against flotation time for a coal having medium oxidation. The coal sample used in the experimental work shown in FIG. 6 contains 36% ash in the flotation feed and had a spectroscopy oxidation index of 0.190. The flotation results shown in FIG. 7 use a collector that comprises diesel alone as the collector (the lowest line in the graph) or a mixture of diesel and polar collector, with the amount of polar collector being indicated on the graph. The total collector dosage was fixed at 300 g/t. As can be seen from FIG. 7, adding a polar collector to the diesel increases combustible recovery. The optimum polar collector ratio for the medium oxidised coal was 5% to 10%.

[0083] FIG. 8 shows combustible recovery against flotation time for coal having low oxidation. The coal sample used in the experimental work shown in FIG. 8 contains 33% ash in the flotation feed and has a spectroscopy index of 0.122. The flotation results shown in FIG. 8 use a collector that comprises diesel alone as the collector (the lowest line in the graph) or a mixture of diesel and polar collector, with the amount of polar collector being indicated on the graph. The total collector dosage was fixed at 300 g/t. As can be seen from FIG. 8, adding a polar collector to the diesel increases combustible recovery. The optimum polar collector ratio for the highly oxidised coal was around 5%.

[0084] The above results demonstrate that the preferred method of the present invention can provide a rapid and easy to use method for determining the degree of coal oxidation. The method can be used at a plant site by a plant operator. The method can provide an accurate determination of the degree of coal oxidation. As the method is rapid, it provides the option of adjusting the ratio of non-polar collector to polar collector used in a flotation plant to optimise flotation recovery.

[0085] In another example, the extraction procedure that was tested is as follows: [0086] (1) add 10 ml dimethyl sulfoxide to a tube containing 1 g dry coal, then shake the tube for 30 seconds. [0087] (2) add 25 ml 0.1M K.sub.4P.sub.2O.sub.7 water solution to the tube, shake for another 30 seconds.

[0088] In step 1, all organic species on the coal surface are dissolved in dimethyl sulfoxide.

[0089] In step 2, because a larger volume of water was added, the un-oxidized organic species became insoluble in the solution, while the oxidized organic species can react with K.sub.4P.sub.2O.sub.7 and therefore remain soluble and the liquid component can be tested for the content/concentration of oxidised species.

[0090] In further examples, experiments were conducted with various inorganic chemicals to test their extraction efficiency. This is illustrated in the table below, which used 25 ml aqueous coal slurry (5% solid). The mixture of 4.8 ml 0.5M K.sub.4P.sub.2O.sub.7 (shaking 1 min) provided superior extraction over 100 ml 1M NaOH (heating at 90? C. for 15 min). Even though the mixtures with Na.sub.2CO.sub.3 and Na.sub.2P.sub.2O.sub.7 were not as effective as the NaOH mixture, both of the Na.sub.2CO.sub.3 and Na.sub.2P.sub.2O.sub.7 mixtures were still able to extract the oxidised coal sample, and under conditions that were significantly faster than the NaOH sample (1 min versus 15 min) and at a lower temperature (ambient temperature versus 90? C.). The Extraction Ranking in the tables below was determined from a measurement of the absorbance of the solution at 270 nm (after separating solid matter), as the higher the absorbance the more material was extracted into the liquid phase.

TABLE-US-00001 Extraction Experiment conditions (with Highly Oxidised Coal sample) Ranking 100 ml 1M NaOH (Heating at 90? C. for 15 min) 2 12 ml Ethanol + 4.8 ml 0.5M K.sub.4P.sub.2O.sub.7 (Shaking 1 min) 1 12 ml Ethanol + 4.8 ml 0.5M Na.sub.2CO.sub.3 (Shaking 1 min) 4 12 ml Ethanol + 4.8 ml 0.5M Na.sub.2P.sub.2O.sub.7 (Shaking 1 min) 3

[0091] In other examples, experiments were conducted with different organic chemicals to test their efficiency on extraction. This is illustrated in the table below, which used 25 ml aqueous coal slurry (5% solid). The extractions using methanol, dimethylsulfoxide (DMSO) and tetrahydrofuran (THF) all worked better than the extraction with ethanol. However, all of the organic solvents tested were superior to the NaOH conditions. Ethanol has advantages over THF, DMSO and methanol, as ethanol is less toxic and/or less flammable than these solvents.

TABLE-US-00002 Extraction Experiment conditions (with Highly Oxidised Coal sample) Ranking 100 ml 1M NaOH (Heating at 90? C. for 15 min) 5 12 ml Ethanol + 4.8 ml 0.5M K.sub.4P.sub.2O.sub.7 (Shaking with 1 min) 4 12 ml THF + 4.8 ml 0.5M K.sub.4P.sub.2O.sub.7 (Shaking with 1 min) 1 12 ml DMSO + 4.8 ml 0.5M K.sub.4P.sub.2O.sub.7 (Shaking with 1 min) 2 12 ml Methanol + 4.8 ml 0.5M K.sub.4P.sub.2O.sub.7 (Shaking with 1 min) 3

[0092] In yet more examples, experiments were conducted with different inorganic/organic ratios to test their efficiency on extraction. These results are illustrated in the below tables, which used 25 ml aqueous coal slurry (5% solid). Again, even though the mixtures with 12 ml Ethanol+2.4 ml 0.5M K.sub.4P.sub.2O.sub.7 on an oxidised coal sample and 6 ml Ethanol+4.8 ml 0.5M K.sub.4P.sub.2O.sub.7 on oxidised coal from a tailing dam from a different geographical location were not as effective as the NaOH mixture, both of these solvent mixtures were still able to extract the oxidised coal sample, and under conditions that were significantly faster than the NaOH sample (1 min versus 15 min) and at a lower temperature (ambient temperature versus 90? C.).

TABLE-US-00003 Extraction Ranking Experiment conditions (with Highly Oxidised Coal sample) 100 ml 1M NaOH (Heating at 90? C. for 15 min) 2 12 ml Ethanol + 4.8 ml 0.5M K.sub.4P.sub.2O.sub.7 (Shaking with 1 min) 1 12 ml Ethanol + 2.4 ml 0.5M K.sub.4P.sub.2O.sub.7 (Shaking with 1 min) 3 6 ml Ethanol + 4.8 ml 0.5M K.sub.4P.sub.2O.sub.7 (Shaking with 1 min) 1 3 ml Ethanol + 4.8 ml 0.5M K.sub.4P.sub.2O.sub.7 (Shaking with 1 min) 1 Experiment conditions (with oxidised coal from a tailing dam from a different geographical location) 100 ml 1M NaOH (Heating at 90? C. for 15 min) 2 12 ml Ethanol + 4.8 ml 0.5M K.sub.4P.sub.2O.sub.7 (Shaking with 1 min) 1 12 ml Ethanol + 2.4 ml 0.5M K.sub.4P.sub.2O.sub.7 (Shaking with 1 min) 1 6 ml Ethanol + 4.8 ml 0.5M K.sub.4P.sub.2O.sub.7 (Shaking with 1 min) 3 12 ml Ethanol + 1.2 ml 0.5M K.sub.4P.sub.2O.sub.7 (Shaking with 1 min) 1

[0093] In a further example, experiments were conducted with different concentration of inorganic compounds. The results are illustrated in the below table, which used 25 ml aqueous coal slurry (5% solid). Again, even though the mixture with 12 ml ethanol+4.8 ml 0.1M K.sub.4P.sub.2O.sub.7 was not as effective as the NaOH mixture, this solvent mixture were still able to extract the oxidised coal sample, and under conditions that were significantly faster than the NaOH sample (1 min versus 15 min) and at a lower temperature (ambient temperature versus 90? C.).

TABLE-US-00004 Extraction Experiment conditions (with Highly Oxidised Coal sample) Ranking 100 ml 1M NaOH (Heating at 90? C. for 15 min) 3 12 ml Ethanol + 4.8 ml 0.5M K.sub.4P.sub.2O.sub.7 (Shaking with 1 min) 1 12 ml Ethanol + 4.8 ml 0.25M K.sub.4P.sub.2O.sub.7 (Shaking with 1 min) 2 12 ml Ethanol + 4.8 ml 0.1M K.sub.4P.sub.2O.sub.7 (Shaking with 1 min) 4

[0094] As discussed above, for best flotation performance, a polar collector should be used together with diesel (or other non-polar collector) and their dosages should be determined by the degree of surface oxidation of the coal. A rapid technique that may allow for on-site determination of coal oxidation therefore may be extremely advantageous. To illustrate this point, FIG. 9 shows the UV absorbance (an indicator of the degree of coal oxidation) of flotation feeds in a coal preparation plant reprocessing old tailing dams. A large number of samples were collected at different times and dates over a one month period. As illustrated in the figure, the degree of oxidation of the flotation feeds varied significantly even in the same day.

[0095] In the present specification and claims (if any), the word comprising and its derivatives including comprises and comprise include each of the stated integers but does not exclude the inclusion of one or more further integers.

[0096] Reference throughout this specification to one embodiment or an embodiment means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases in one embodiment or in an embodiment in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more combinations.

[0097] In compliance with the statute, the invention has been described in language more or less specific to structural or methodical features. It is to be understood that the invention is not limited to specific features shown or described since the means herein described comprises preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted by those skilled in the art.