|Publication number||USH478 H|
|Application number||US 06/779,716|
|Publication date||Jun 7, 1988|
|Filing date||Sep 24, 1985|
|Priority date||Sep 24, 1985|
|Publication number||06779716, 779716, US H478 H, US H478H, US-H-H478, USH478 H, USH478H|
|Inventors||George C. Blytas|
|Original Assignee||Shell Oil Company|
|Export Citation||BiBTeX, EndNote, RefMan|
|Non-Patent Citations (6), Referenced by (3), Classifications (9)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The present application is related to my commonly assigned and concurrently filed applications Ser. Nos. 779,718 (now abandoned) and 779,717 on "Reduction of Sodium in Coal by Water Wash and Ion Exchange with Weak Electrolyte" and "Reduction of Sodium in Coal by Water Wash Followed by Ion Exchange Within A Pipelne." The disclosures of the related applications are incorporated herein by reference.
The present invention relates to upgrading coal by removing sodium ions. More particularly, the invention relates to removing sodium ions from coal by means of water-washing and ion-exchanging.
The presence of large amounts of sodium in coal is undesirable as it contributes to fouling of combustion facilities. The fouling problems can arise if sodium exceeds about 3% w (as Na2 O), yet several important deposits of coal contain considerably more than that much sodium. Thus, a process which can economically remove, for example, 30 to 70 percent of the sodium, can be very desirable and can be a prerequisite condition for exploitation of sizeable deposits.
The levels of sodium oxide in the ash at which coal combustion can lead to fouling problems are not yet clearly defined, and can be different for different coals. Nevertheless, levels in excess of about 3% w are not desirable and coals with more than 4% w in the ash, are generally difficult to market. Some Powder River Basin coal samples have been found to yield over 6% w of sodium oxide. And, coals with sodium oxide contents in excess of 8 or 9% w have also been found and documented.
U.S. Department of Energy publication DOE/GFETY/RI-82/3 "Conceptual Design of a 1.6 MM Tons/Year Lignite Preparation Facility for Sodim Reduction" describes ion exchanges of coal with 0.11 normal sulfuric acid for various times and solid to liquid ratios and indicates that a sodium oxide content decreased from 8.5 to 0.99% and the selectivity of the ion removal varied with variations in residence time.
A publication from Pennsylvania State University Fuel Science Program "Ion Exchange in Selected Low Rank Coals, Part I Equilibrium, Part II Kinetics," Solvent Extraction, Ion Exchange, 1:4, 813-825 (1983), describes measurements of equilibrium ion exchange behavior for metal cations with hydrogen ions and indicates the exchange to be a linear function of pH regardless of the cation concentration of the solution; with the extent of exchange being a function of available hydrogen ions.
The present invention relates to an improved ion exchange process for reducing the sodium content of coal. The coal is first washed with water that is substantially free of sodium ions, to remove water-soluble materials containing those ions. The washed coal is then ion exchanged with an aqueous solution of strong electrolyte solution in which both the kinds and concentrations of ions are correlated with the kind and concentration of ion exchangeable cations on the coal. The electrolyte solution used in the ion exchange (a) is relatively free of sodium ions, (b) is sufficiently free of substances capable of forming atmospheric pollutants during the oxidation of coal so that the resultant total amount of those substances in the ion exchanged coal is insufficient to cause significant pollution, and (c) contains cations capable of exchanging with ion exchangeable cations on the coal in an amount which is less that the stoichiometric amount of the cations on the coal but is an amount which causes sodium ions to be removed selectively from the coal. The amount of the cations capable of exchanging with cations on the coal is preferably about 20 to 80% less than the stoichiometric amount of the cations on the coal.
The advantages of this invention are many fold. Some of these advantages are: (a) the consumption of exchanging reagent can be kept at an economically acceptable low level; (b) the process is selective in removing sodium rather than calcium and magnesium, which are often useful as sulfur oxide capturing species; (c) the spent solutions do not leave a very low pH, and thus are relatively non-corrosive and easy to dispose of; and (d) the process can be adapted or tailored to suit a wide variety of coals, by determining their cationic concentration and adjusting the concentration of the ion exchanging solution as described herein.
We have found that sodium may occur in coals in three forms; (a) water-soluble sodium, (b) ion-exchangeable sodium, and (c) fixed sodium. The ion-exchangeable sodium can be about 30 to 80 percent of the total, with the balance split between water-soluble and fixed sodium. Many low-rank coals can be rendered marketable and usable by the present process, which is often capable of removing 30 to 70 percent of the sodium ions (measured as Na2 O) economically. This invention is illustrated by using a coal sample from the Powder River Basin. However, the invention is not limited to coals of this origin, and the method disclosed herein is readily adaptable to essentially any coal or lignite characterized by a high sodium level.
A sample of coal from the Powder River Basin was ground into three fractions as follows:
(A) 3/4" by 28 mesh
(B) Less than 14 mesh
(C) Less than 28 mesh
Sample fractions (B) and (C) were dried to 8-9% w water (down from about 25%) by heating in a vacuum at 100° C. Analysis of sample fractions (B) and (C) has yielded Na+ Mg++ and Ca++ contents and ash levels which were less than 3% apart. Accordingly, for the metal removal calculations, we have used the average of analyses obtained on sample fractions (B) and (C).
We can consider the coal sample as an ion-exchanger in which negative fixed sites (believed to be R--COO- or similar moieties) are counterbalanced by ion-exchangeable cations, primarily by Na+, Ca++ and Mg++ ions. Relatively minor amounts of K+ ion and other metal ions are also present, but will be neglected in the present context. The form in which iron occurs has not been determined, although it is expected to be primarily as pyrite. In Table 1, along with the analysis of Na+, Ca++ and Mg++, we show the percentage of base capacity contributed by each of these cations.
TABLE 1______________________________________Metal and Ash Contents of the Coal Tested Equivalent Fraction Aver- Base 14-28 Mesh <28 Mesh age Capacity %______________________________________Sodium (ppm) 2370 2430 2400 17Calcium (ppm) 7570 7800 7685 64Magnesium (ppm) 1330 13700 1350 19Ash % w 4.79 4.77 4.78 --______________________________________
Contact times from a few hours to several days were studied, the former corresponding to mine-mouth conditions, the latter corresponding to pipeline transport conditions. Coal particle sizes and solid/liquid ratios were selected accordingly.
The degree of sodium, calcium and magnesium reduction in coal, resulting from various ion-exchange treatments, was estimated by monitoring the increase in the level of these ions in the treating solution. In general, Na+ analysis was done by ion-specific electrode methods, whereas Ca++ and Mg++ analysis was done by titration methods.
Sodium reduction by exchange with strong electrolytes was studied using sulfuric acid, H2 SO4, and calcium chloride, CaCl2. Three levels of stoichiometry were investigated, in three sets of experiments, as follows:
Set 1: Using 200% of the stoichiometric amount of ion-exchangeable cations on the coal, i.e., using twice as many equivalents of H+, or Ca++ as the total Na+ +Ca++ +Mg++ equivalents in the coal.
Set 2: Using 68% of the stoichiometric amount, which is less than the total equivalent present in the coal but more than the most abundant cation, calcium, which accounts for 64% of the coal capacity, and
Set 3: Using, 23% of the stoichiometric amount, which is still 35% more than the equivalent of sodium present in the coal.
The removal of Na+ or other ions achieved in these three Sets was compared with the extent of removal achieved by water washing. This latter figure yields the amount of sodium associated with water-soluble salts adhering to the coal as crystalline salts or dissolved salts.
Tables 2 and 3 summarize the results obtained in experimental Set-1, at 20% of stoichiometric addition of the exchange agent. The Tables also show the effect of washing with water only. It should be noted that the equilibrium pH with the H2 SO4 experiments (i.e., ion-exchanges with an aqueous solution containing a 100% excess of cations capable of exchanging with ion-exchangeable cations on the coal) was less than 1, whereas the water and CaCl2 treatments yielded solutions of nearly neutral pH. All of these tests were carried out using 3/1 liquid/solid ratios. The large excess of reagents in these tests make them useful as "reference tests, " but too high for consideration in practical sodium reduction process.
Table 2 shows that 78 to 81% of the Na+ is removed with excess H2 SO4 reagent in two days. Since water wash alone brings about 20±1% reduction, we can conclude that the maximum exhangeable Na+ ion is Ca. 60% of the total. From the data in Table 2, we also conclude that about 19% of the sodium is associated with species which are insoluble even in excess 0.2 N H2 SO4 solution.
In experimental Set-1 (200% Stoichiometric) CaCl2 was marginally less efficient than H2 SO4 in sodium reduction. It is not clear whether this is a real difference in reagent effectiveness, or a reflection of the difficulty associated with Na+ determinations in the presence of a large excess of Ca++ ions.
In set-1 experiments, water wash yielded essentially no Ca++ or Mg++ ion leaching. Thus, those species either are not present as water soluble salts in the aqueous phase adhering on the ground-up, partially dried coal samples or, if they are present as soluble salts, there is a high affinity for absorption of such salts on the coal.
Table 3 shows the extraction of Ca++ ions by H2 SO4. An interesting observation is noted for samples B and C in this Table, i.e., with the samples which had been ground-up. In these samples, the calcium ion concentration goes through a maximum in 1 to 2 hours. This trend must be due to changes in solubility and adsorptivity of CaSO4 species. Apparently, the solubility of CaSO4 decreases with decreasing acidity of the solution, i.e., decreases as the exchange of base metal by H+ proceeds to completion after the first 2 or so hours. The coarser sample did not show this effect.
TABLE 2______________________________________Sodium Reduction (%) by Treatment with 100% ExcessH2 SO4 and CaCl2 and With Water Sample: (A) (B) (C) 3/4" by 28 Mesh -14 Mesh -28 MeshContact Time Solution(hours) I II III I II III I II III______________________________________0.8 48 25 16 73 60 16 78 59 182.4 53 33 18 75 60 17 80 60 1824 61 65 19 -- -- 19 81 -- 1948 77 66 21 78 66 21 81 66 22______________________________________ Conditions Liquid to Solid Ratio: 3 to 1 Solutions: I = 0.4 N H2 SO4 II = 0.4 N CaCl2 III = Deionized water Na+ analysis in ppmw Ambient temperature with shaking
TABLE 3______________________________________Calcium Reduction (%) by Treatment With100% Excess of H2 SO4 Sample: (A) (B) (C) 3/4" by 28 Mesh -14 Mesh -28 MeshContact time (hrs) % Reduction______________________________________0.2 3 38 370.8 17 50 502.4 18 46 4024 22 24 2348 23 24 23______________________________________ Conditions: Liquid to Solid Ratio: 3 to 1 Solution: 0.4 N H2 SO4 Shaking at 23° C.
In experimental Set-2 (with 68% of the stoichiometric amount of reagent), the amount of cations is still 4-times the amount needed to exchange all of the Na+ ion present at high selectivity. In this set, only -14 mesh coal was used, and long contact times were studied to simulate pipeline transport conditions more closely.
The results from experimental Set-2 are shown in Tables 4 and 5. The Na+ reductions achieved with this dosage compares favorably with those achieved with the four times higher reagent dosages in Set-1 (200%). From a process standpoint, an additional advantage over Set-1 is that the pH levels at equilibration are higher in Set-2 than in Set-1. Thus, the final pH region with H2 SO4 exchanges shown in Table 2 is ca. 0.8 after 48 hours, whereas the pH obtained with H2 SO4 exchange in Table 4, is 2.9 after 10 minutes, 4.1 after 45 minutes, and 5.3 after 2.4 hours. This last high pH value also suggests that the normality of the treating solution was reduced from 0.1N initially to ca. 0.00001N, thus indicating that essentially all of the H+ ion available was consumed to exchange cations from coal.
With regard to sodium reduction, Set-2 results agree closely with those of Set-1 (compare to Tables 2 and 4). A notable difference is that 0.1N CaCl2 appears to be a more effective exchanger than 0.4N CaCl2. It is, however, also possible that the Na+ analysis in Set-2 is more accurate, because the Ca++ level at the end of exchange was very low (unlike in Set-1).
With regard to the exchange of Ca++ and Mg++, Table 5 shows again a maximum of exchange in the first 2 to 3 hours, with a subsequent gradual reduction over the next 2 to 3 days. This trend has been observed with other samples as well and appears to be a characteristic of Ca++ and Mg++ exchange by H2 SO4. This type of behavior should be considered when selecting conditions for an optimal ion exchange of coal.
Table 6 shows further how the level of Na+ ion in solution increases with time, whereas the levels of Ca++ and Mg++ in solution decrease with time. The percentage of Na+ +Mg++ +Ca++ which migrate from the coal into the solution decrease from 50% of the total coal base in 0.8 hours to 40% in 92 hours. In Table 6, the sodium present in solution is 82% of the total, about the same as reflected in Table 4.
TABLE 4______________________________________Na+ Exchange with 68% of StoichiometricH2 SO4 and CaCl2 Solutions and Water Solution: 0.1 N H2 SO4 0.1 N CaCl2 H2 OContact Time (Hrs) Percent Na+ Exchanged and/or Washed______________________________________0.2 63 51 180.8 76 76 192.4 76 77 1824 80 80 2048 80 80 2292 81 81 23______________________________________ Liquid to Solid Ratio: 3 to 1 Particle Size less than 14 mesh
TABLE 5______________________________________Ca++ and Mg++ Exchange Using 68% ofStoichiometric H2 SO4 SolutionContact Time (Hours) Ca++ Exchange % Mg++ Exchange %______________________________________0.2 36 940.8 46 962.4 47 ˜10024 26 9148 25 8392 24 57______________________________________ Liquid to Solid Ratio: 3 to 1 0.1 N H2 SO4 Solution Particle Size <14 Mesh.
TABLE 6______________________________________Percentage of Coal Base Found inSolution after 0.8 and 92 hoursTime (Hours) Sodium Calcium Magnesium Total______________________________________0.8 13 29 17 5992 14 15 11 40Maximum 17 64 19 100______________________________________
The results obtained in Experimental Set-3 (with 23% of the stoichiometric amounts) are summarized in Tables 7 and 8. These experiments were performed at a 3 to 1 liquid to solid ration with 0.03N H2 SO4 or CaCl2. These dosages correspond to a 35% excess over the sodium level present.
TABLE 7______________________________________Na+ and Ca+ Exchange with H2 SO4at 23% of Stoichiometric RatioContact Time (hours) Na+ Exchange % Ca++ Exchange %______________________________________0.2 56 120.8 60 122.4 62 1124 65 1148 68 11______________________________________ Liquid to Solid Ratio: 3 to 1 0.03 N H2 SO4 Solution
TABLE 8______________________________________Displacement of Na+ by 23% of Stoichiometric CaCl2 SolutionContact Time (hours) Percent Exchanged______________________________________0.2 540.8 592.4 6048 65______________________________________ Liquid to Solid Ratio: 3 to 1
Tables 7 and 8 show that Na+ is displaced more completely than Ca++ by H2 SO4. However, both, H2 SO4 and CaCl2 can displace over 60% of the Na+ in 2.4 hours (Table 8).
In Table 7, the displacement of Na+ corresponds to 12% of the total available base, whereas that of Ca++ corresponds to 8%. We can define a selectivity for Na+ versus Ca++ +Mg++ removal by comparing the relative amounts of Na+ removed to those of Ca++ +Mg++. Such a definition would suggest that the lower concentrations of H2 SO4 are more effectively utilized in Na-reduction than the higher concentrations of H2 SO4.
Quantitatively, we can define the selectivity (β) for Na+ over Ca++ +Mg++ as follows: ##EQU1##
In Table 9, this ratio, β, is given for the three sets of experiments, using the longest contact time results in each one. The selectivity goes from 1.94 at 200% stoichiometric, through 2.54 at 68% stoichiometric, to 8 at 23% stoichiometric. The corresponding reductions of sodium achieved are 80%, 80% and 68%. These results show clearly that we can achieve a significant degree of sodim reduction, using only moderate amounts of ion exchange reagent. The concept of selective use of a reagent for sodium reduction can be used to arrive at near optimal process conditions, either for mine-mouth or for coal slurry pipeline application.
TABLE 9______________________________________Selectivity of H2 SO4 for Na+ versusCa++ + Mg+ RemovalH2 SO4 Excess over Stoichiometric.sup.(a) β % Na+ Reduction______________________________________200% 1.94 8068% 2.54 8023% 8.0% 68______________________________________ .sup.(a) Estimated on the basis of Total Base Capacity (TBC) of Na+ + Ca++ + Mg+
In order to assess the effect of background Na+ ions on the washing and ion exchange capacity of water and H2 SO4 solutions we have added background concentrations of 0.04% w Na2 SO4 (130 ppm Na+) and 0.1% w Na2 SO4 (320 ppm Na+) in deionized water or in 0.01N H2 SO4 reagents and studied Na+ reduction in 1 and 2 hours. The results shown in Table 10, show that at low reagent concentrations (justified by the high liquid/solid ratios), the Na+ would have an adverse effect on Na+ reduction in a mine-mouth operation (short contact times, high liquid/solid ratios).
The results shown in Table 10 were obtained at a relatively high Na+ level (130 and 320 ppm) and with low H2 SO4 normality. A second experiment was carried out in which both Na+ and Ca++ were added as background cations at 100 ppm each and the concentration of H2 SO4 was raised to 0.025N and to 0.05N. A liquid to solid ration of 6 was used. The results, summarized in Table 11, show that sodium reduction in this case is in the range of 60 to 70% in three hours, i.e., acceptable for most process applications.
TABLE 10______________________________________Effect of Na+ Level in Solution on Na+ Reduction % Na+ RemovalAqueous Phase 1 HR 2 HRS______________________________________Deionized Water 23 25Deionized Water + 0.04% w Na2 SO4 19 23Deionized Water + 0.1% w Na2 SO4 <10 120.01 N H2 SO4 50 590.01 N H2 SO4 + 0.04% w Na2 SO4 39 56(0.0056 N Na2 SO4)0.01 N H2 SO4 + 0.1% w Na2 SO4 20 32(0.014 N Na2 SO4)______________________________________ Liquid to solid ratio: 6 to 1
All values are ±14% of the reported removal, due to analytical difficulties resulting from the use of high levels of Na+ background in the sample.
TABLE 11______________________________________Sodium Reduction Using H2 SO4 in the Presence ofSodium and Calcium Ions Background cations inreagent solutions: 100 ppm Na+, 100 ppm Ca++ Solution: 0.025 N 0.05 N % Na+ % Ca++ % Na+ % Ca++Contact Time (Hrs) Removal Removal Removal Removal______________________________________1 49 16 60 312 59 14 66 353 61 14 71 37______________________________________ Liquid to solid ratio: 6 to 1 Reduction results are ± 8% of the reported value.
Comparison of the results in Tables 10 and 11 suggests that the ionic composition of the water used to make the exchange solution should be considered in some process applications. However, at high ratios (H+ +Ca++)/(Na+) the effect of background sodium on sodium displacement becomes less critical.
In general the present invention can advantageously be applied to substantially any type of coal and/or lignite, or other solid carbonaceous fuel having an undesirably high sodium content such as 4% or more sodium oxide in the ash. Such materials are referred to herein by the term "coal".
The aqueous solution of strong electrolytes with which the coal is ion exchanged can be compounded in substantially any manner as long as the resulting solution contains a kind and amount of ions which are correlated as specified with the composition of coal being treated. As known to those skilled in the art, particularly suitable cations for use in such exchanges comprise hydrogen, calcium, and magnesium ions. Particularly suitable anions for such use comprise sulfate and, whenever the chlorine concentration in non-water soluble components of the coal is suitably low, chloride ions. Nitorgen-containing substances such as ammonia ions or nitrate or other nitrogen-containing anions are typical of substances capable of forming atmospheric pollutants during oxidation of the coal and should be avoided.
An ion exchanging of coal in an aqueous electrolyte solution generally leaves the coal wetted with an aqueous solution of salts which tend to become deposited on or otherwise attached to the coal as the water evaporates, unless the ion-exchanged coal receives an additional washing with substantially deionized water. Where the coal contains a relatively low concentration such as less than about 0.3% w of chlorine in non-water-soluble components, an aqueous electrolyte comprising calcium chloride or mixtures of it with acids which form water-soluble calcium salts are particularly suitable electrolyte solutions for uses in situations where the pH of the coal ion exchanging solution must be kept relatively non-acidic. Since most coals contain less than 0.3% w chlorine it follows that CaCl2 and its mixtures can be useful exchanging agents in numerous applications.
In a preferred procedure coal ground to particles of less than about 3/4" diameter is water-washed by a process involving immersing the coal in an aqueous liquid which is substantially free of sodium ions. The washed coal is mechanically separated from the liquid in at least two fractions. Fraction (A) comprises relatively large coal particles capable of being settled by gravity from the liquid in a feasibly short time. Fraction (B) comprises the particles which are too fine for such a desirably rapid mechanical separation. The particles of fraction (A) are separately ion-exchanged with an aqueous electrolyte, then mixed with an aqueous slurry of the particles of fraction (B). The resulting mixture is preferably subjected to an additional ion-exchange.
|1||Beneficiation of Lignite by Sodium Removal Using Ion Exchange, Baria et al, Chemical Abstracts, vol. 98:3740h, 1983.|
|2||Ion Exchange in Selected Low Rank Coals. Part II: Kinetics, Bobman et al., Chemical Abstracts, vol. 100:54204w, 1984.|
|3||Reduction in Ash Fouling Potential of Lignite by Ion Exchange, Baria et al, Chemical Abstracts, vol. 98:128905q, 1983.|
|4||Reduction of Sodium in Lignite by Ion Exchange: A Pilot Plant Study, Paulson et al, Chemical Abstracts, vol. 93:134825b, 1980.|
|5||Removal of Sodium from Illinois Coal by Water Extraction, Neavel et al, Chemical Abstracts, vol. 88:25374e, 1978.|
|6||Removal of Sodium from Lignite by Ion Exchange with Calcium Chloride Solutions, Paulson et al, Chemical Abstracts, vol. 95:189825y, 1981.|
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|U.S. Classification||44/621, 209/4, 208/403, 423/461, 210/681, 208/428|