CA1256848A - Process for the production of activated carbon - Google Patents

Abstract

A B S T R A C T
A process for the production of activated carbon from upgraded brown coal which has been produced by subjecting brown coal to shearing forces, compacting the mass so produced, and drying the mass to form a hard, relatively dense product, comprises the steps:
(a) pyrolysing the said upgraded brown coal, preferably at a temperature of 350 to 500°C;
(b) activating the pyrolysed product of step (a) by treatment with steam at elevated temperature, preferably 700 to 800°C; and (c) cooling the activated carbon product of step (b) in an inert atmosphere.

Description

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PROCESS FOR THE PRODUCTION OF
., _ . . _ SACTIVATED CARBON
This invention relates to the production of high surface area activated carbon Erom brown coal.
In a general aspect the invention provides a process which involves pyrolysis of upgraded brown coal, 10 Eollowed by activation to produce the desired activated carbon.
A suitable upgraded brown coal may be provided by the densification hardening process of Canadian Patent No.
1,221,837 and Canadian Patent Application Serial No. 500,721 15 filed January 30, 1986 including a process in which the , .

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coal is subjected to shearing and extruding in a continuous manner, ~or example in a Sigma Knetmaschine HKS 50 manufactured by Janke & Dunkel GmbH ~ Co., KG
IKA-Week Biengen.
The abovementioned densificatic,n hardening process provides a means of converting raw soft brown coals to comparatively hard attrition resistant solids.
Briefly stated, the said process application involves subjecting brown coal to shearing forces to produce a wet plastic mass which is compacted, for example by extrusion into pellets, and subsequently dried to form a hard, relatively dense product which may be produced in any desired granular size and configuration. Such solid granules or pellets composed of upgraded brown coal are most suitable starting materials for the production of high surface area activated carbons by the process of this invention. The properties of the starting pellets may be varied widely by suitable choice o coals or by appropriate additives to meet various quality ;~0 requirements.
While the raw brown coal is comparatively high in oxygen and hydrogen, the ratios of these elements to carbon may be considerably changed during the pyrolysis which is part of the thermal activation process for producing activated carbons. At this stage chemical elimination of water results in a final product which is comparatively high in carbon (of the order of 85~). The low ash content o~ many Victorian brown coals is also advantageous in enabling the final activated carbons to be relatively low in inorganic elements.
In existing processes, activated carbon is produced by pyrolysis, followed by steam activation, o~
more expensive starting materials, e.g., high ranking coals which are naturally strong and coherent, or of Nfl _ 4 _ ~ ~56~

coconut shell or o~ peat. Brown coal is not ~sed. The cost o~ the materials used, or of certain processing requirements in the prior art, is generally much highee than that of brown coal upgraded by our aforementioned 5 process. We have also found that in the present process, activation temperatures some 50-100C below those used in conventional processes can be employed successfully.
In accordance wi~h one aspect of the invention, there is provided a process for the production of activa~ed 10 carbon from upgraded brown coal which has been produced by s~bjecting brown coal to shearing forces, compacting the mass so produced, and drying the mass to form a hard, relatively dense product; said process comprising the following steps: (a~ pyrolysing the said upgraded brown 15 coal; (b) activating the pyrolysed product of step (a) by treatment with steam at elevated temperature; and (c) cooling the activated carbon product o~ step (b) in an inert atmosphere.
In accordance with another aspect of the 20 invention, selected brown coals are upgraded by the abovementioned process. A starting pellet size oE 2-3mm diameter is generally suitable.
The freshly extruded pellets aee then permitted to dry at or near ambient temperature, either in a still 25 atmosphere or with some small movement of air (of the order of O.lm/sec) to increase the rate of drying. After 24 hours some 80~ of the contained moisture will have been eliminated by evaporation to the atmosphere. After a further two to three days the pellets will be 30 approaching equilibrium water content ~10-15%) and maximum strength. The latter varies greatly with the nature of the coal and the additives used during the preparation.
The dry pellets are then ready for pyrolysis 35 and activation for the production of the desired surface area appropriate to the intended field of application.
According to a further aspect of the present invention, the first process stage consists of pyrolysis at temperatures in the range of 350-500C in an inert gas 3L2561~

stream (usually nitrogen) to eliminate residual water, chemically evolved water, low molecular weight organic components (mostly phenols), and gases such as carbon dioxide, carbon monoxide and hydrogen. Negligible 5 quantities of tars are produced.
After this pyrolysis treatment, further heating of the pellets to higher temperatures generates only the gases hydrogen and carbon monoxide.
Activation consists of steam treatment, usually 10 in the temperature range 700-800C. For this purpose, steam is added or injected into the nitrogen stream at a controlled rate through water maintained at a temperature below the boiling point (usually 95C) so as to secure the desired partial pressure of water in the nitrogen.
15 The mixed gas stream now has the capacity to erode selectively parts of the pyrolysed pellets so as to produce very large internal surfaces and a microporous structure. The water gas reaction may be involved and possibly other processes also. Steam treatment is 20 continued for several hours until the required internal surface has been developed. The carbon is cooled in the inert atmosphere.
The properties of granular activated carbons are commonly assessed in terms of nitrogen adsorption 25 surface area, iodine adsorption fro~ aqueous medium and resistance to attrition by tumbling in an aqueous phase.
Table 1 illustrates a wide range of values of such properties obtained on samples prepared by the process of this in~ention.

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Table 1 Comparative Surface Area Measurements on _ctivated Carbons Surface area Sample ~Latrobe Valley coals) BET nitrogen lodine number 5 (4 hrs activation at 800C)adsorption unless otherwise stated m /g mg/g Loy Yang, medium-dark 250-500 ~m 578 652 150-250~Lm 573 652 sample, with Low I2 number 459 461 sample, wl~h hlgh I2 number oO0 901 samplc preparad ~rom coal kneaded for 1 hour 679 764 Loy Yang, medium-dark +5% Mg(OH)2, whole granules 668 456 ~orwell 728 786 C92 (Maryvale) 403 538 NH
1.

~25684 !3 The pH established by an activated carbon when it is placed in an aqueous medium is of some importance in determining the extent of adsorption of metal ions by the carbon. ~Refer Table 2).

Table 2 pH Measurements on Coals and Activated Carbons Dispersed_in Water pH

SampleCoal Ac~ivated Carbon Loy Yang, pale-light 3.8 9.6 Loy Yang, medium-dark3.0 9.6 ~ 2% NaOII and 5% HCHO tO.4 + 5% Mg(OH)23.0 9.4 15 Loy Yang, dark3.2 8.9 + 10% Fe203 3.2 8.3 Morwell S.4 11.0 Maddingley 7.1 11.2 + 5% Mg(OH)2 ll.o + 0.1% NaOH 10.6 + 0.4% Na2C03 11.2 NH
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A variant of the process of this invention involves a procedure which produces a magnetic activated carbon, i.e. one in which the granules respond to an applied magnetic field, so enablinq e~ficient recovery to 5 be made from a heteroqeneous system.
In yet another aspect of the invention the total energy expended in attritioning the coal may be r~duced by only kneading a portion of the coal and blending the unkneaded remainder with it.
Means are available, as disclosed in co-pending Canadian Patent Application Serial No. 500,235 filed January 23, 1986 for substantially improving the compressive strength and attrition resistance of dried densified coal granules with appropriate additives. The degree of 15 improvement possible varies with the origin of the coal and with its natural pH.
In general, small effects produced by additives are not reflected in marked improvements in crush strength or total surface area of the activated carbons.
However, where significant effects have been achieved by suitable additives, these are carried through to the final,products to give significant improvements.
Preferred embodiments of the invention are illustrated in the following non-limiting examples.

XAMPLE 1. Improvements Attained in the Strength of Activa ed Carbons with Suitable Additives_ A range of activated carbon samples was prepared using the following detailed procedure.
200 g o bed-moist coal was kneaded for five 30 hours in a low-speed kneading machine to yield a wet plastic mass which was then extruded thro~gh a 3 mm diameter nozzle attached to a hand operated screw extrusion device. In those cases where an additive was requieed, this was added at the commencement of the Nll .:' ', ~2~i6~

attritioning period. The addition was calculated in weight percent based upon the dry weight of coal. The extruded pellets (cut into suitable lengths) were permitted to dry in still air at 20C for one week to 5 develop maximum strength and achieve moisture equilibrium with the atmosphere (usually in the range 10-15~).
The pellets were next pyrolysed at 400-450C
and the water and low molecular weight organic compounds evolved were removed using a stream of inert gas (usually 10 nitrogen). The activation procedure consisted of erosion in the steam-charged nitrogen stream at a temperature in the vicinity of 750C. The steam partial pressure was established by saturation of the nitrogen stream on passing through water maintained at 95C. ~ctivation was 15 continued for a period of four hours and the pellets were cooled in the inert atmosphere.
The pellets were tested by determining their compressive strengths in the initial dried state and after activationl and also by measuring uptakes of iodine 20 rom aqueous medium (the iodine number). The iodine number is the number of milligrams of iodine adsorbed from a 0.05 N aqueous iodine solution by one gram of carbon when the iodine content of the residual filtrate is not less than 0.02 N. For the measurement, the carbon 25 is finely ground, a sieved fraction taken and contact with the iodine solution is limited to a fixed short time interval. The values of the iodine number are reported as being numeeically approximately the same as the surface areas (in m /g) determined by BET nitrogen 30 adsorption procedures.
Table 3 sets out measurements made on the samples prepared from a variety of brown coals of Victorian oriqin, viz., Loy Yang, Morwell and Maddingley.
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Table 3 Coal pH Additive Co~pressive Strength MPa Iodine Weight Dried After steam Rumber loss granules activation mg/g ZO
4 hrs . _ . . _ . _ . . . _ Loy Yang Borehole (1276) Pale-light - 10 56 832 63 10 Mediu~-dark 3.2 - 12 47 901 63 Dark - 20 67 841 60 Medium-dark 2%NaOH~5%~CH0 60 76 690 59 5%11exa~ine 29 75 649 S9 i (NH40H~
lS ~IC~10) 0.4%Na2co3 13 60 840 62 Morwell 5.4 ~ 57 49 786 70 .
Maddingley 7.1 - 29 10 568 94 The above table illustrates the range of 20 compressive strenqths (10-60MPa) found in dried granules which have not been modified with any additive. While the weaker (acidic) coals show considerable improvement in strength on pyrolysis and steam activation, the stronger coals tend to lose strength after these treatments.
Dual purpose additives, as shown in ~able 3 (i.e.
additives to raise the pH and to provide additional bridge-bonding species), not only improve the strength of the dried granules but also carry through very marked improvements in strength of the steam activated granules ~Loy !
30 Yang Medium-dark) The improvement in strength is usually !

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]. 1 accompanied by a decrease in surface area (as shown by reduced iodine adsorption), but this may be tolerable in circumstances where unusually st~ong granules are required.
Suitable additives not only improve the compressive 5 strength of activated carbons (refer Table 3) but also can very considerably enhance the attrition resistance of activated carbon granules, as is demonstrated by the data in Table 4. In this case the single additive ammonium hydroxide has achieved a remarkable improvement in the attrition 10 resistance of a relatively weak carbon derived from LQY Yang medium-dark coal.

Table 4 Comparative Attrition Tests on Activated Carbon 5g sample, 30 ml water, in 250 ml cylinder rotated end for end at 40 15 r.p.m. for 8 hours. Product sieved.

Sample lmm 1-0.21mm 0.21-0.075mm 0.075mm (1000 ~m) ~210-lOOO~lm) (75-210~m) ~75~m) Loy Yang, medium-dark (a) low I2 number 90.1 0.2 0.1 9.6 20 (b) high I2 number 93.7 0.5 0.1 5.8 Loy Yang, pale-light 93.3 0.5 0.2 5.9 Loy Yang, medium-dark ~a) 1:1, kneaded:
unkneaded 73.5 1.4 0.2 24.9 25 (b) 1:1, kneaded:
unkneaded 0.5% NH40H 80.9 1.3 0.3 17.5 1.0% NH40H 89.5 1.0 0.2 9.4 NH

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EXAMPLE 2. Im rovements Resultinq Erom Partial At _ ti~ning P _ As well as modifications introduced by way of additives, it is also possible to vary the properties of dried granules and the activated carbons prepared from them 5 by varying the attritioning conditions. As stated, such variation may consist of extended kneading of only part of the coal, followed by brief blending of the remainder of the coal in the plastic mass so created. Alternatively the kneading time of the whole mass of the coal may be varied 10 considerably while still producing satisfactory final products. In either case the energy expended in attritioning the coal may be advantageously reduced together with other useful results (as detailed below).
The following procedure was used to study the 15 effects of kneading only part of the coal: 200 g of bed-moist coal, which had been reduced in size by one passage through a hammer mill, was divided into two equal par.s, one of which was kneaded in a sigma-kneader for five hours, the other reserved till the end of this period and then briefly 20 blended (2 minutes) in the plastic mass in the kneader.
Extrusion and subsequent treatment of the granules followed the procedure set out in Example 1. Measurements made on these granules are given in TaùLe 5.

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~2~8~.15 Table 5 Sample Compressive strength MPa Iodine number Wt loss Dried After steam mg/g %
granules activation S 4 hrs :

Loy Yang, pale light (a)l:l kneaded: 7 29 ô32 65 unkneaded (b)l:l kneaded: 13 30 619 62 unkneaded -~ 0.5% NH40H
(c)l:l kneaded: 36 58 634 60 unkneaded ~ 1% N114011 . _ It is apparent that activated carbons of adequate strength with high surface areas can be obtained by kneading 20 only part of the original coal, and using this kneaded part as a bonding phase for the remainder.
The activated carbons prepared in this way have the considerable advantage of adsorbing solutes more rapidly than do those prepared from fully kneaded coal. Apparently the 25 fragments of raw coal, with their high porosity even after pyrolysis, provide better access of solutes to the internal l surface of the granules. The effect is illustrated in Table i 6 below.

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Table 6 Loy_Yang, pale-light coal % of iodine absorbed after Sample 1 hr 2hr 3hr 4hr .

(a) Fully kneaded 27.0 35.8 41.8 46.7 (b) 1:1 kneaded: 32.4 44.0 52.7 58.8 unkneaded The time of kneading of the coal may also be reduced considerably (from 5 hrs to 1 hr) without serious loss of either compressive strength or total surface area of the resulting activated carbons, as indicated by the data in Table 7.

Table 7 Loy Yang, medium-dark coal . ......... _ . _ :
Kneading time Weight loss % Iodine number Compressive ;
hr on activation mg/g strength MPa 20 _ _ ~
I
1 60 764 41 li

2 64 860 58 63 ~01 47 .

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~2568~3 EXAMPI.E 3. Im~ ements Resulting from Proper Activation As indicated in the general description of the process, activation consists of treatment of the dried and pyrolysed granules with high temperature steam which is a component of 5 an otherwise inert atmosphere. Important variables in the activation procedure are partial pressure of water vapour in the activating atmosphere, the temperature at which activation is conducted and the time of treatment.
To investigate these variables, dried granules were 10 prepared using the procedures detailed in Examples 1 and 2.
After initial pyrolysis the granules were activated under varying conditions. The final products were evaluated in terms of iodine number and in some instances by measurement of weight loss during activation, since this can vary a good 15 deal with the severity of the conditions. The results of the experiments on various Victorian brown coals are set out in Table ~.

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Table 8 Coal Lithotype Steam partial Temp. of Time of Iodine pressure activation activation number (temp. of C hr of steam gener- product ation C) mg/g . _ (i) Loy Yang pale-light 75 800 3 540 (ii) Loy Yang dark 7S 800 3 562 (iii) Loy Yang dark 95 710 4 837 (64.1%
Wt loss) (65.9%
Wt loss) (iv) Morwell 95 680 4 690 (89.6%
Wt loss) ~69.6%
Wt loss) Z5(v) Loy Yang medium-dark 9S 800 1 759 (vi) Morwell 95 ôO0 3 270 (vii) Maddingley 95 800 ~ 350 + 5% Mg(OH)2 95 800 1 572 ,.",~ . , ._. _ Temp. C 75 95 100 pH20, mmHg289.1633 9760 NH

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Items ~i) and (ii) in Table 8 demonstrate the importance of steam partial pressure in achieving effective activation. Increasing the relative partial pressure (see box at foot of Table 8) from that corresponding to 289 to 5 75C to that corresponding to 760 at 100C increased the surface area of the activated sample 2-2.5 times.
It could therefore be advantageous to use the highest practicable steam partial pressure consistent with adequate control of the process. If boiling water is used to 10 provide the steam, activation will proceed very rapidly and may lead to severe degradation of the granules. The process is therefore likely to be most satisfactory if the inert gas stream has a relatively high partial pressure of steam but is not saturated with this component.
Items (iii) and (iv) in Table 8 indicate that activation may be achieved at temperatures above about 6B0C
with most satisfactory results at about ~00C. aelow 680C
activation times become inconveniently long.
Within limits, the surface area attained is related 20 to the percentage loss Oe weight during activation. Too great a loss of weight may lead to reduced areas as the structure of the carbon is eroded away.
Items (v), (vij and (vii) in Table 8 indicate that increasing times of exposure to the activating gas in the 25 range 1-6 hours increase surface areas markedly. The effects, however, are dependent on the origin of the coal.
Loy Yang coal, which produced granules which are relatively weak at the outset, achieves a high surface area in a relatively short time with reduced effects after prolonged 30 exposure. Morwell coal, which forms strong dried granules, gives low surface areas for short exposures but responds well to longer exposures. Maddingley coal appears to reach a maximum surface area after activation for about one hour, with a decline in surface area on prolon~ed activation.

Nll ~2561~d8 EXAMPLE 4. Magnetic Activated Carbon Recovery of activated cacbon granules (or ~ragments thereof) from heterogeneous systems, such as suspensions of minerals in water, will be facilitated if the carbon responds S to an applied magnetic field. An activated carbon with this property was prepared from brown coal by incorporation of fine ferric oxide during the preparation of the granules.
The preparative procedure followed the cletails set out in Example 1 above, using Loy Yang dark lithotype coal. During 10 the kneading of the coal 10% by wei~ht of fine ferric oxide was added to the wet plastic mass and thoroughly mixed. The granules so prepared dry in the normal way, and during pyrolysis and activation sufficient reduction of the ferric oxide takes place to make the granules strongly responsive to 15 a magnetic field. Reduction is probably due to the hydrogen and carbon monoxide evolved during the heating of the coal.
'rhere are no serious adverse eEfects, from a functional point oE view, due to the presence of the reduced iron phases, on either the strength or the attainable surface area oE the 20 granules. The magnetic properties of the activated carbons are not destroyed by immersion in aqueous medium and indeed it has proved extremely difficult to extract any signiEicant part of the iron with effective solvents for the metal or oxides. The magnetic properties remained intact aEter such 25 extractions.
The properties are shown in Table 9.

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Table 9 . . .

Coal Wt loss on Compressive stren~th MPa Iodine activation before activation after activation number mglg . . _ . . . _ . . .

Loy Yang, dark 60% 20 67 841 Loy Yang, dark S9% 2~ 33 545 + 10% Fe203 10 Both preparations (with and without Fe203) gain strength on activation, the latter rather less than the former. Erosion during activation is somewhat reduced by the presence oE the iron and this is reflected in a rather lower surface area; however, more severe activation conditions 15 could be used to generate a higher surEace if required.
It will be clearly understood that the invention in its general aspects is not limited to the specific details referred to hereinabove.

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Claims (9)

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. Process for the production of activated carbon from upgraded brown coal which has been produced by subjecting brown coal to shearing forces, compacting the mass so produced, and drying the mass to form a hard, relatively dense product; said process comprising the following steps:-(a) pyrolysing the said upgraded brown coal;
(b) activating the pyrolysed product of step (a) by treatment with steam at elevated temperature; and (c) cooling the activated carbon product of step (b) in an inert atmosphere.

2. Process according to Claim 1 in which the pyrolysis step (a) is carried out at a temperature of 350 to 500°C in an inert gas stream.

3. Process according to Claim 2 in which the inert gas comprises nitrogen.

4. Process according to any one of Claims 1, 2 and 3 in which the activation step (b) is carried out a temperature of 700° to 800°C.

5. Process according to Claim 3 in which the pyrolysis step (a) is carried out in a stream of nitrogen and the activation step (b) is effected in the presence of steam added to the nitrogen stream.

6. Process for the production of activated carbon from brown coal which comprises the following steps:-(a) upgrading brown coal by subjecting it to shearing forces, compacting the mass so produced and drying the mass to form a hard, relatively dense product;
(b) pyrolysing the product of step (a);
(c) activating the pyrolysed product of step (b) by treatment with steam at elevated temperature;
(d) cooling the activated carbon product of step (c) in an inert atmosphere.

7. Process for the production of activated carbon from brown coal which comprises the following steps:-(a) upgrading brown coal by subjecting it to shearing forces, compacting the mass so produced and drying the mass to form a hard, relatively dense product;
(b) pyrolysing the product of step (a) in a stream of nitrogen at a temperature from 350 to 500°C;
(c) activating the pyrolysed product of step (b) by treatment with steam added to the stream of nitrogen at a temperature of 700 to 800°C;
(d) cooling the activated carbon product of step (c) in an inert atmosphere.

8. Process for the production of activated carbon from brown coal which comprises the following steps:-(a) upgrading a quantity of brown coal by subjecting a part of it to shearing forces, then blending the treated part with the remainder of the said quantity to produce a plastic mass, compacting the mass so produced and drying the mass to form a hard, relatively dense product;
(b) pyrolysing the product of step (a);
(c) activating the pyrolysed product of step (b) by treatment with steam at elevated temperature;
(d) cooling the activated carbon product of step (c) in an inert atmosphere.

9. Process for the production of activated carbon from brown coal which comprises the following steps:-(a) upgrading a quantity of brown coal by subjecting a part of it to shearing forces, then blending the treated part with the remainder of the said quantity to produce a plastic mass, compacting the mass so produced and drying the mass to form a hard, relatively dense product;
(b) pyrolysing the product of step (a) in a stream of nitrogen at a temperature from 350 to 500°C;

(c) activated the pyrolysed product of step (a) by treatment with steam added to the stream of nitrogen at a temperature of 700 to 800°C;
(d) cooling the activated carbon product of step (b) in an inert atmosphere.