AU626888B2 - Process for improving the quality of upgraded brown coal - Google Patents

Description

626888 Form

AUSTRALIA

PATENTS ACT 1952 COMPLETE SPECIFICATION Short Title: PROCESS FOR IMPROVING THE QUALITY OF UPGRADED BROWN COAL Int. Cl: Application Number: Lodged: Complete Specification Lodged: Accepted: Lapsed: Published: 0 S Priority: Related Art: DIVIDED FROM AUSTRALIAN APPLICATION NO. 52422/86 0 9 TO BE COMPLETED BY APPLICANT Name of Applicant: THE UNIVERSITY OF MELBOURNE and CRA SERVICES LIMITED Address of Applicant: Parkville, Victoria, Australia o *and 9 55 Collins Street, Melbourne, 3000 Actual Inventors: Mrs Kaye Frances HARVEY Dr Theodore Vincent VERHEYEN 0 Dr Reginald Basil JOHNS Dr Alan Lloyd CHAFFEE Prof Alan Stuart BUCHANAN Mr David Anthony CAIN Address for Service: GRIFFITH HACK CO., 601 St. Kilda Road Melbourne, Victoria 3004, Australia Complete Patent Specification for the invention entitled: "PROCESS FOR IMPROVING THE QUALITY OF UPGRADED BROWN COAL" The following statement is a full description of this invention including the best method of performing it known to me: -1- *I -2- 0 0 0e 0099 o oo S 0 o o 0 p PROCESS FOR IMPROVING THE QUALITY OF additn UPGRADED BROWN COAL This invention relates to the production ofharder relatively densified coal pellets of improved strength and attrition 0resistance.

In our co-pending patent application 10 AU-24294/84 and in our application for a patent of addition thereto No. PG 9283 filed February 14, 1985, a process is disclosed for upgrading brown coal to harder relatively denser forms of solid fuel of smaller residual water contents and substantially enhanced calorific value per unit weight. The process disclosed has important

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i i r i _II~LY._1I~^- 0 0 0 0 0 0 o 0 00 4 4 0 o 0o o00 o a o o o o 0* 0 0 9 o o o oot -3 technical and economic advantages over the existing briquetting and solar drying processes for upgrading brown coal.

In essence the process of our co-pending application involves three steps:- 1. Firstly, there is an attritioning step in which the coal is subjected principally to shearing, as distinct from grinding forces.

This is achieved in a blending or kneading apparatus or another machine able to comminute soft materials by shearing rather than by crushing or abrading.

2. The second step involves compaction of the wet plastic mass produced in the first stage by 15 extrusion into pellets in an extruding or similar compacting device.

3. In the third stage of the aforementioned process the extruded pellets are dried at or near ambient temperature, preferably but not 20 necessarily with the assistance of some air flow to remove the water vapour evolved.

It is believed that a mechanism of chemical bridge bond formation is involved in the densification process. This centres on the irreversibility towards 25 dispersion in water, changed behaviour towards solvent extraction, changes in pyrolysis behaviour, sensitivity of the process towards pH, and solute species in the aqueous medium, as well as marked changes in the spectral characteristics of the densified solids. All of these 30 factors point towards the development of strong covalent and electrostatic bonds between the coal particles and, in particular, on these bonds involving small molecules and polyvalent ions forming bridges between reactive sites on adjacent faces of coal particles. An analogous, but not identical, situation for one of the types of 0 0 e a Lu i- r 4bonding is that in which formaldehyde forms bridges between phenols in generating thermosetting phenol-formaldehyde resins.

Brief mention has been made in our co-pending application to the participation in the bonding processes of molecular species in the coal structure with a natural acid ionization typically phenols and/or carboxylic acids. The reactivity of covalent bonds in such species is dependent on the state of ionization of the acidic groups, which is diminished by increasing the acidity of the medium and vice versa.

We have found that alkali and alkaline earth carbonates (in spite of the comparative insolubility of the latter) have proved to be comparatively efficient additives to most coals for increasing the bonding strength. This suggests that, in addition to the alkalinity of these additives, the carbonate ion or an -Oo aqueous species derived from it plays a significant role S, in the bonding process. Possible species are those .o 20 included in the following equilibria: o 0

CO

3 HOH HCO 3 OH

HCO

3 HOH H 2

CO

3

OH

H

2

CO

3 CO HOH i: S0° Carbon dioxide is of course an invariable constituent of all coals, so that any effect due to added carbonate will be supplementary to that naturally present.

Of the known major chemical species in brown 0 coals, as shown by pyrolysis-gas chromatography and other procedures, the phenols, and especially the polyhydroxy j phenols, are likely to be the most reactive towards carbon dioxide or related molecular species at ordinary temperatures and pressures in an aqueous medium.

i-i 5 The reaction most likely to be significant is that in which carbon dioxide participates in electrophilic substitution at an activated position on the benzene ring of the phenol. The type of reaction is as below:

OH

O6rH OH YH o ==O Pressure is required to achieve reaction with monohydroxy phenol, but when activation of ring positions is reinforced with two or three hydroxyls, as in the polyhydroxy phenols, then reaction will occur under ordinary conditions. The electron densities at various ring positions in hydroquinone, resorcinol and catechol illustrate this point.

OH OH OH I~1. 032 1.05~~ 069 .1.0 3

O

3

H-

*0 0 o 00 00 *0*0 o 0 0 00 0 00 0 0 0 0 O 04 00 0 004 0* @0 0 0 0 0 0 00*0 0 00 00 0 0 0 0 pa.

0* *0 0 0 0 0 0 OH 1.022 Of the three dihydroxy phenols, resorcinol is evidently the most reactive toward electrophilic substitution, with three positions having very high electron densities. Carboxylation with carbon dioxide yields the following two resorcylic acids:

COOH

O0H OH OH In general the electron densities at ring positions of the phenols are markedly influenced by the state of ionization of the hydroxyl groups. As the pH of the medium increases, the hydroxyls approach nearer to ionisation and the flow of electronic charge to the -6benzene ring will increase accordingly. Increase of pH will thus favour electrophilic substitution in suitable ring positions.

When the brown coal treated by the method of our co-pending application is in its wet plastic state it consists of many small fragments dispersed in an aqueous phase, each fragment having freshly cleaved faces exposing phenols which will be attached to the main polymer framework of the coal. When coal faces are in close proximity, electrophilic substitution by carbon dioxide could well involve a pair of phenols, one in each face, giving a bridging structure rather than a carboxylic group attached to one ring only. The reaction may now be represented as follows: OH OH oH 01 coa34 coal coaV C-O coal H 0E H0 O When coal faces are not close enough, reaction may still occur, but would be expected to result in 940 a 9 inactivation of phenolic sites without achieving densification bonding.

There will always be sufficient carbon dioxide present in any coal to permit substantial reaction to proceed. The aqueous medium is important In the plastic 9 0 state of the coal to facilitate the bridging bonding, In the manner of solvents In other chemical reactions.

It is an object of the present invention to ,30 enhance both the rate and extent of densification of brown coal. In one aspect the Invention attains such enhancement by the careful selection and controlled use S of additives.

Accordingly, the present invention provides a process for production of densified coal pellets of improved physical properties which includes subjecting coal to a shearing-attritioning step followed by extrusion and drying steps, characterised by 6a incorporating an additive into the coal that is subject to the said shearing-attritioning step, said additive being chosen from one or more of the group consisting of: alkali metal hydroxides, alkaline earth metal hydroxides, ammonium hydroxide, alkali metal carbonates, alkaline earth carbonates, oxides of base metals, oxides of transition metals, and carbonyl components having a molecular weight not substantially greater than that of urea.

The brown coal may also be e 1 0 0 6a a P oo O t 0 o b- 7 upgraded by the process employing shorter treatment times as disclosed in our copending Australian provisional specification PG 9283, including a process in which the coal is subjected to shearing and extruding in a continuous manner, for example in a Sigma Knetmaschine HKS 50 manufactured by Janke and Kunkel GmbH and Co., KG IKA-Werk Biengen.

Our investigation has firmly established the usefulness of a wide range of additives, and their effectiveness in improving the rate and extent of densification as measured in terms of the crushing strength and attrition resistance of the densified material produced.

It must be noted that various brown coals show 15 different responses to additives. These responses are 94 4 related to the particular properties of the coals, the most important of which is probably the natural pH value of a given coal.

*4 While the benefits obtainable from various o 09 0. 20 additives about to be described are especially relevant to the brown coal upgrading process disclosed in our co-pending application, benefits may also be expected from such additives with other brown coal upgrading processes, e.g. a process such as that being investigated 25 by the State Electricity Commission of Victoria, which o involves the production of a pumpable coal slurry which is solar dried over several months.

Additives useful according to the present Sinvention may-generally be of the following types: 1. Additives for raising pH.

S° 2. Additives for improving bridge bonding.

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8 3. Additives which provide electrostatic bridge bonding capability in the form of divalent cations such as are present in magnesium and calcium compounds, for example according to the formula coal o coal

M

g 0 Preferred additives are alkali metal hydroxides, ammonium hydroxide, alkali metal carbonates, the oxides of base metals and transition metals, aldehydes and certain carbonyl compounds.

We have found that alkali metal hydroxides, preferably in the range of 0.05 to 5% by weight, more preferably in the region of will significantly o improve the strength and attrition resistance of 2 densified brown coal pellets.

-1 20 Ammonium hydroxide a weak base preferably in °o o the range of 0.05 to 5.0% by weight, more preferably in the region of 0.5 to will have similar beneficial Eo effects to alkali metal hydroxides. In this case not only is the acidity of the coal neutralized with the previously described advantages, but is it also possible that the aqueous ammonia phase provides an enhanced 0o0o activity of those solute species, based on carbon dioxide, which function as bridging molecules between coal particles. This additive is especially advantageous 0 o o 30 in circumstances where the end use of the densified coal 0 6 1 I 1 8 9 precludes the addition of metal ions. The use of an ammonia gas atmosphere during kneading of the coal is also very beneficial.

Alkali metal carbonates e.g. sodium carbonate have also been found to improve the rate of hardening, the crushing strength, and the attrition resistance of brown coal pellets, the percentages added being preferably 0.05 to 5.0% by weight, more preferably 1 to This additive has a twofold action in that it reduces the acidity of acid coals and also provides a considerably enhanced activity of aqueous species based on the carbonate ion, i.e.

CO

3 HOH HCO 3

OH

HCO

3 HOH H 2

CO

3

OH

H

2

CO

3

CO

2

HOH

o OO O 0* o o 0a o oo0 o a 0 0 a a 0 oo 00 0 a O 0 0 Alkaline earth hydroxides have a similar effect to the additives mentioned so far when added preferably at concentrations of 1 to 5% by weight. This also applies to alkaline earth carbonates (either precipitated or natural), in this case the concentration range being preferably 1 to 20% by weight.

Finely divided magnesium and calcium hydroxides are very effective additives with a twofold action. The pH of acid coals will be advantageously increased, while 25 the divalent cations will form electrostatic bridge bonds, utilising acidic (including phenolic) groups on adjacent coal particles.

Finely ground natural magnesite and calcite, even though they are relatively insoluble, are still effective additives. Their functions include neutralisation of acids present in some coals, the

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i, i1* provision of magnesium and calcium ions for electrostatic bridge bonding, and the provision of an additional supply of carbon dioxide species in solution.

Simple aldehydes and other small molecule carbonyl compounds in association with various bases have also been found to improve the rate of hardening, the strength and the attrition resistance of densified brown coal pellets, the concentration of additive preferably being in the range 1 to 5% by weight. The preferred aldehydes are formaldehyde and acetaldehyde, especially the former.

In this case, small molecule carbonyl compounds should be able to supplement carbon dioxide in providing bridging bonding between coal particles. Efficiency will be a function of pH, which determines the activity of 0, polyhydroxy phenols towards electrophilic substitution.

GO" Other useful carbonyl compounds are urea and 9 formamide. Bases for use with these additives include .o the alkali hydroxides and ammonium hydroxide.

S 20 Hexamine (derived from formaldehyde and ammonium hydroxide) improves the rate of hardening, the strength, and attrition resistance of densified brown coal pellets, with addition rates preferably of 1 to i 0.:I Hexamine, which yields ammonium hydroxide and i formaldehyde slowly on hydrolysis, should provide both a "of. aneutralising action and a supply of bridging molecules: 6HCH.+ 4 NH 3

(CH

2 6

N

4 6H 2 0 ,u Base metals and transition metal oxides may also be used as additives in preferred concentration ranges of 5 to 70% by weight, yielding dried products i which are often hard, dark, vitreous solids, and in some instances showing considerable increases in strength t compared with the original coal.

Preferred embodiments of the invention will be illustrated by the following non-limiting examples, in which all percentages relate to dry weight:- Example 1 (Alkali metal hydroxides) Loy Yang medium-dark lithotype coal has a pH of 3.2 in the wet plastic state and might be expected to benefit particularly from base addition. Accordingly 200 g of this coal was subjected to attritioning for five hours in a sigma kneader with 2% (wt) of added sodium hydroxide. The resultant plastic mass (pH 5.7) was extruded in a hand operated screw extruder to produce 3 mm diameter rods which were cut in 5 mm lengths and permitted to dry for one week in still air at 20 C.

Compressive strength measurements on the dried aop 15 cylindrical pellets gave values averaging 48 MPa. By *contrast, pellets made under identical conditions but without added sodium hydroxide yielded an average strength of 11 MPa.

a B Morwell coal has a natural pH of about 5.4 a o 4 20 When treated experimentally as described above for Loy Yang coal with 2% sodium hydroxide addition, the resultant material hardened rapidly and became difficult to extrude. Repetition of the experiment with addition "oB" o of 2% extra water and reduction of kneading time from 5 25 to 1.5 hours enabled pellets to be extruded which, when dried, gave an average compressive strength of 41 MPa.

a.s Pellets with no additive gave an average compressive strength of 29 MPa.

Maddingley coal has a natural pH of 7.1 and, c 30 when subjected to the experimental procedure described above for Loy Yang coal, produced pellets showing little improvement in compressive strength when compared with pellets containing no added sodium hydroxide.

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12 Example 2 (Ammonium hydroxide) 200 g of Loy Yang medium-dark lithotype coal (natural pH 3.2 in the wet plastic state) was subjected to attritioning for 5 hours in a sigma kneader, with 1% of aqueous ammonia .880) as additive. The pH was increased by this treatment to 6.2. Pellets (10 mm diameter, 10 mm long) were then produced by extrusion with a hand screw extruder and dried in still air at 20 0

°C

for one week. These pellets gave an average compressive strength of 35 MPa, in contrast to similar pellets made with no additive, for which the average was 5 MPa.

Smaller pellets (3 mm, 5 mm long at extrusion) provided even greater compressive strengths, with an average of 63 S. MPa for material with 1% ammonium hydroxide additive.

Similar pellets with no additive gave an average compressive strength of 11 MPa.

The enhanced compressive strength of smaller o'o (diameter pellets compared with that of larger diameter o 0 in the extrusion device when the large diameter nozzle is replaced with that of a smaller diameter.

200 g of Morwell (N 3372, dark lithotype) coal 0°"0 of pH 4.0 had its pH increased to 7.6 on kneading in an o°0. atmosphere of ammonia gas. However hardening was very 0 64 rapid, and the plastic mass became too stiff for 0o successful extrusion. Considerable heat was developed in i: the kneading machine as work was performed, not only in breaking up the original coal structure but in destroying 0o °0 new structures formed as the densification processes proceeded rapidly.

Example 3 (Alkali metal carbonates) j 200 g of Loy Yang light lithotype coal (pH 3.4) was kneaded for 5 hours in a sigma kneader with an addition of 0.2% (wt) of sodium carbonate. The product

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13 was extruded in the hand operated screw extruder to provide 3 mm diameter pellets (5 mm long) which were air dried at 20 C for one week. The average compressive strength of the pellets was 19 MPa, compared with an average of 11 MPa for pellets with no additive. For pellets produced under similar conditions with 0.4% Na 2

CO

3 additive the average was 33 MPa.

Maddingley coal has a natural pH of about 7.1.

When 3mm diameter pellets were produced using the above experimental conditions but with 2% of added sodium carbonate, the average compressive strength was 38 MPa, compared with 29 MPa for pellets with no additive.

Example 4 (Alkaline earth hydroxides) 200 g of Loy Yang medium-dark lithotype coal o. o 15 was kneaded in a sigma kneader for 5 hours with 5% by o .weight of fine precipitated magnesium hydroxide. The product was extruded in the hand operated screw extruder to yield 3 mm diameter, 5 mm long pellets which were dried in still air at 20 C for one week. The average compressive strength was 61 MPa, compared with 11 MPa for similar pellets containing no additive. When calcium

"O

1 hydroxide was used as additive, average compressive So strengths 39 and 65 MPa were found for pellets containing 2% and 5% calcium hydroxide.

Example 5 (Alkaline earth carbonates) 200 g of Loy Yang medium-dark lithotype coal was kneaded in a sigma kneader for 5 hours with 5% by weight of fine magnesite. Extruded pellets (3 x 5 mm) when dried in still air at 20 0 C for one week gave an j average compressive strength of 20 MPa. By contrast, similar pellets with no magnesite additive gave an average compressive strength of 11 MPa.

NDi i 1~ i ICI*IIIIIIICCL-- 14 o o o 00 i a o o 0 C 0. p o o p o op

GA

0 0* 0ao° 0 0 A GA A o A A a <a a aa 200 g of Morwell coal (H 1317 borehole, pH 4.6) was converted into 10 mm diameter (on extrusion) densified pellets by the above procedure. These pellets gave an average compressive strength of 23 MPa. When similar pellets were prepared by kneading the coal with of fine precipitated magnesium carbonate, the average compressive strength proved to be 36 MPa.

Similar experiments were performed with Morwell coal from the N 3372 borehole. In two separate experiments 5% each of fine magnesite and fine calcite were used as additives, and pellets of 10 mm diameter were extruded. The dried pellets in each case gave an average compressive strength of 39 MPa. By contrast similar pellets with no additive gave an average of 22 MPa.

Example 6 (Formaldehyde and other simple aldehydes) In the following experiments the preparative procedures described in the examples 1-5 above were used (that is, kneading, extrusion to produce pellets of 20 either 3 or 10 mm diameter, followed by air drying).

Loy Yang medium-dark lithotype coal (pH 3.2) gave pellets of average compressive strength 5 and 11 MPa respectively for 10 mm and 3 mm pellets respectively.

Addition of 2% sodium hydroxide during mixing increased these values to 18 and 48 MPa respectively. Further preparations in which 5% formaldehyde was added as well as 2% sodium hydroxide increased these values to 32 and MPa respectively. The latter mixtures hardened very rapidly, and in some cases became impossible to extrude.

These products assumed the form of very tough resinous solids.

When acetaldehyde was substituted for formaldehyde, the average compressive strengths were 22 and 51 MPa respectively for 10 mm and 3 mm pellets.

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-a i i i 15 The neutral Maddingley coal (pH 7.1) did not require pH adjustment when aldehydes were used as additives. For example, with 1% of formaldehyde alone the average crush strength of 3 mm pellets was found to be 46 MPa, by contrast with 29 MPa for pellets with no additive.

An alternative measure of the strength of densified coals is provided by an attrition test in which a 5 g sample of pellets in 30 ml of water was tumbled 16,000 times in a 250 ml closed cylinder. Loy Yang medium-dark lithotype 3 mm pellets provided a recovery of only 9% of fragments exceeding 1 mm. With 2% sodium hydroxide additive, the recovery of fragments exceeding 1 0 4 mm increased to 81%, and with further addition of ,o 15 formaldehyde the recovery increased to 96%. The o ocorresponding values for Morwell coal were 79, 86 and o 91%.

o o a 00e Example 7 (Hexamine) o The reagent hexamine yields ammonium hydroxide and formaldehyde slowly on hydrolysis, and should therefore provide both a neutralising action and a supply o*"oa of bridging molecules. Experiments were performed with Loy Yang medium-dark coal using the procedures detailed in Examples 1 to 6 above, with 5% hexamine as additive.

25 The additive caused an increase in pH of the mixtures from 3.2 to 5.1, and in the average compressive strength of the 3 mm pellets from 11 to 29 MPa. The attrition resistance of the densified pellets was also mucL improved, from a 9% recovery of fragments exceeding 1 mm to a 68% recovery.

I i 16 Example 8 (Base metal and transition metal oxides) It has proved possible to incorporate quite high proportions of certain finely divided metal oxides in brown coals during the densification procedures as detailed in Examples 1 to 7 above. The dried products are often hard, dark, vitreous solids, and in some instances show considerable increases in strength compared with the original coal.

Morwell (N 3372) coal (pH 3.8) with 20% of additional water provided 10 mm diameter pellets of compressive strength about 21 MPa. With 10% cobalt oxide the strength increased to about 25 MPa, which increased further to about 31 MPa on pyrolysis to 500 C in an inert Go atmosphere. The coal alone does not show an increase in 15 strength on pyrolysis.

Loy Yang (dark) coal (pH 3.2) provided 3 mm diameter pellets of compressive strength about 11 MPa.

0o t With 10% CuO the dry pellets gave a compressive strength 00 o6 of about 47 MPa.

Loy Yang (dark) coal with 10% fine Fe 2 0 3 provided compressive strengths of about 22 MPa (3 mm diameter pellets). Further preparations containing S 50 and 75% Fe 2 0 3 all provided hard coherent pellets, °o 0 indicating that bonding was still effective in spite of the large proportions of inorganic material.

o o0: It will be clearly understood that the invention in its general aspects is not limited to the specific details referred to hereinabove.

This application is divided from our copending application No. 52422/86 and the entire disclosure in the complete specification and claims of that application is by this cross-reference incorporated into the present I specification.

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Claims (12)

1. Process for production of densified coal pellets of improved physical properties which includes subjecting coal to a shearing-attritioning step followed by extrusion and drying steps, characterised by incorporating an additive into the coal that is subject to the said shearing-attritioning step, said additive being chosen from one or more of the groups consisting of: alkali metal hydroxides, alkaline earth metal hydroxides, ammonium hydroxide, alkali metal carbonates other than sodium carbonate, alkaline earth carbonates other than magnesium and calcium carbonates, oxides of base metals, oxides of transition metals, and carbonyl compounds chosen from formaldehyde, acetaldehyde and formamide.

2. Process according to Claim 1 in which the additive is chosen from and/or used in an amount of 0.05 to 5% based on dry weight of the coal.

3. Process according to Claim 2 in which the additive comprises sodium hydroxide, added to the coal in 25 an amount of about 2% on a dry weight basis.

4. Process according to Claim 2 in which the additive comprises an alkali metal carbonate other than sodium carbonate, used in an amount of 1 to 2%. Process according to Claim 1 in which the additive comprises aqueous ammonia, added to the coal in an amount of 0.05 to 5% of 0.880 ammonia per dry weight of coal.

6. Process according to Claim 5 in which the amount of 0.880 ammonia used is from 0.05 to 2%.

7. Process according to Claim 1 in which the additive comprises an alkaline earth hydroxide used in an amount of 1 to 5% based on the dry weight of the coal. RA42, Lti- C) Nx Tv _i i 18

8. Process according to Claim 7 in which the additive comprises finely divided magnesium hydroxide and/or calcium hydroxide.

9. Process according to Claim 1 in which the additive comprises an alkaline earth carbonate used in an amount of 1 to 20% based on the dry weight of the coal. Process according to Claim 9 in which the additive comprises finely divided magnesite or calcite.

11. Process according to Claim 1 in which the additive comprises formaldehyde and/or acetaldehyde used in an amount of 1 to 5% based on the dry weight of the coal.

12. Process according to Claim 1 in which the additive comprises formamide used together with one or more hydroxides chosen from alkali metal hydroxides and ammonia.

13. Process according to Claim 1 in which the additive comprises hexamine.

14. Process according to Claim 1 in which the 'e "additive is chosen from base metal oxides and/or transitioh 0 metal oxides used in an amount of 5 to 70% based on the dry weight of the coal. DATED THIS 19TH DAY OF MAY 1992 THE UNIVERSITY OF MELBOURNE and CRA SERVICES LIMITED 25 oBy their Patent Attorneys: GRIFFITH HACK CO Fellows Institute of Patent Attorneys of Australia 'I 1^