AU636847B2 - Process for the production of activated carbon - Google Patents

Description

AUSTRALIA

PATENTS ACT 1952 COMPLETE SPECIFICATION 68Form Form

(ORIGINAL)

FOR OFFICE USE Short Title: Int. Cl: Application Number: Lodged: Complete Specification-Lodged: Accepted: Lapsed: Published: Priority: Related Art: 0 b DIVIDED FROM AUSTRALIAN APPLICATION 52234/86 ft

S

TO BE COMPLEE3D BY APPLICANT Name of Applicant: Address of Applicant: CRA SERVICES LIMITED 55 COLLINS STREET MELBOURNE VICTORIA

AUSTRALIA

S. S 0 THE UNIVERSITY OF MELBOURNE Address of Applicant: Actual Inventor: Address for Service:

PARKVILLE

VICTORIA

AUSTRALIA

Kaye Frances HARVEY Reginald Basil JOHNS Alan Stuart BUCHANAN David Anthony CAIN Theodore Vincent VERHEYEN GRIFFITH HACK CO., 601 St. Kilda Road, Melbourne, Victoria 3004, Australia.

Complete Specification for the invention entitled: PROCESS FOR THE PRODUCTION OF ACTIVATED CARBON The following statement is a full description of this invention including the best method of performing it known to me:- 2

V..

PROCESS FOR THE PRODUCTION OF ACTIVATED CARBON This invention relates to the production of high surface area activated carbon from brown coal.

In a general aspect the invention provides a process which involves pyrolysis of upgraded brown coal, followed by activation to produce the desired activated carbon.

A suitable upgraded brown coal may be provided by the densification hardening process of our Australian patent 561586 (24294/84), and the patent of addition thereto No. 588565 (52'90/86)) o PROCESS FOR THE PRODUCTION OF thereto No. 588565 (52590/86) 3 n 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 Dunkel GmbH Co., KG IKA-Werk Biengen.

The abovementioned densification 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 15 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 of coals or by 20 appropriate additives to meet various quality 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 25 which is part of the thermal activation process for

OS

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 The low ash content of many Victorian brown coals is also 30 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, of more expensive starting materials, high ranking coals which are naturally strong and coherent, or of 0 NH ,/t 4 coconut shell or of peat. Brown coal is not used. The cost of the materials used, or of certain processing requirements in the prior art, is generally much higher than that of brown coal upgraded by our aforementioned process. We have also found that in the present process, activation temperatures some 50-100 0 C below those used in conventional processes can be employed successfully.

In accordance with one aspect of the invention, starting materials are first obtained by upgrading brown coals by the abovementioned densification hardening process or alternatively by a process which may comprise kneading (shearing) only part of the coal, followed by brief blending of the remainder of the coal in the plastic mass so created.

S In accordance with another aspect of the invention, selected brown coals are upgraded by the abovementioned process. A starting pellet size of 2-3mm S* diameter is generally suitable.

The freshly extruded pellets are then permitted .o 20 to dry at or near ambient temperature, either in a still atmosphere or with some small movement of air (of the order of 0.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 25 a further two to three days the pellets will be i approaching equilibrium water content (10-15%) and maximum strength. The latter varies greatly with the o..

nature of the coal and the additives used during the preparation.

30 The dry pellets are then ready for pyrolysis 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 350-500 0 C in an inert gas 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 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 in the temperature range 700-800 0 C. 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 95°C) 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 iner,t atmosphere.

The properties of granular activated carbons s. are commonly assessed in terms of nitrogen adsorption 25 surface area, iodine adsorption from aqueous medium and resistance to attrition by tumbling in an aqueous phase.

e Table 1 illustrates a wide range of values of such properties obtained on samples prepared by the process of this invention.

S.

-6- Table 1 Comparative Surface Area Measurements on Activated Carbons Sample (Latrobe Valley coals) (4 hrs activation at 800 0

C)

unless otherwise stated Surface area BET nitrogen Iodine number adsorption m 2/g mg/g Loy Yang, medium-dark 2 50-500,tk 150- 2 50 19m sample, with low 1 2 number sample, with high 1 2 number sample prepared from coal kneaded for 1 hour 00 0 00 Se SO 0 *0 00 00 0

SO

05 0S S S @0 00 00 S S

S

@505 00 0@ S 0 05 0 06

S.

00 000

SS

S S 00 0 OS S SOS 764 1049 Loy Yang, medium-dark Mg(OH) 2 whole granules Morwe II C92 (Maryvale) -7 The pH stablished 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 Sample Coal Activated Carbon S. S @0 66

S

*0 00 @6 0 .5 0@ 90 S 0 @0 00 S

S

0 Loy Yang, pale-light Loy Yang, medium-dark 27% Na0H and 57% HCHO 5% Mg(OH) 2 S S 0* S 0S S

SS

00

S

@55

S.

S S @0 0 SOS SOS 0 Loy Yang, dark 10% Fe 203 Morwell Maddingley 5% Mg(OH) 2 20 0.1% NaOH 0.47% Na 2CO3 9.6 10.4 9.4 8.9 8.3 11.0 11.2 11.8 10.6 11.2 8- 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 enabling efficient recovery to be made from a heterogeneous system.

In yet another aspect of the invention the total energy expended in attritioning the coal may be reduced by only kneading a portion of the coal and blending the unkneaded remainder with it.

Means are available, as disclosed in our co-pending Australian Application PG 9107 for substantially improving the compressive strength and attrition resistance of dried densified coal granules with appropriate additives. The degree of improvement 15 possible varies with the origin of the coal and with its natural pH.

In general, small effects produced by additives S. are not reflected in marked improvements in crush strength or total surface area of the activated carbons.

20 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 .00' illustrated in the following non-limiting examples.

00 EXAMPLE 1. Improvements Attained in the Strength of Activated Carbons with Suitable Additives A range of activated carbon samples was prepared using the following detailed procedure.

200 g of bed-moist coal was kneaded for five hours in a low-speed kneading machine to yield a wet plastic mass which was then extruded through a 3 mm diameter nozzle attached to a hand operated screw extrusion device. In those cases where an additive was required, this was added at the commencement of the 9 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 20°C for one week to develop maximum strength and achieve moisture equilibrium with the atmosphere (usually in the range 10-15%).

The pellets were next pyrolysed at 400-450°C and the water and low molecular weight organic compounds evolved were removed using a stream of inert gas (usually nitrogen). The activation procedure consisted of erosion in the steam-charged nitrogen stream at a temperature in the vicinity of 750 C. The steam partial pressure was established by saturation of the nitrogen stream on passing through water maintained at 95 C. Activation was 15 continued for a period of four hours and the pellets were cooled in the inert atmosphere.

S

The pellets were tested by determining their compressive strengths in the initial dried state and after activation, and also by measuring uptakes of iodine 20 from 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 o 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 numerically approximately the same as the surface areas (in m2/g) determined by BET nitrogen 30 adsorption procedures.

0 0 Table 3 sets out measurements made on the samples prepared from a variety of brown coals of Victorian origin, viz., Loy Yang, Morwell and Maddingley.

10 Table 3 Coal pH Additive Compressive Strengr-" MPa Dried After steam granules actixva tion 4 hrs Iodine Weirght nUmbe r Loss mglg Loy Yang Borehole (1276) b 00 00 I 00 go 4 Pale-ilight Medium-dark 3.2 Dark Medium-dark, Morwell 5.4 Maddingley 7.1 2%NaOH+5%HCHO 5%Hexamine

(NH

4

OH+

HCBO)

0 .47*/Na 2 C0 3 832 901 841 690 649 840 786 568 0000 Al.

00 0 0.

*e 0 0 @0 The abovE6 table illustrates the range of 20 con4pressiVe strengths (l0-60MPa) 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.

25 Dual purpose additives, as shown in Table 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 throu -h very markad improvements in strength of the steam activated granules (Loy Yang Medium-dark). The mpoentin strength is uisually 00 0 0 00 0 0.0000 A 0 11 accompanied by a decrease in surface area (as shown by reduced iodine adsorption), but this may be tolerable in circumstances where unusually strong granules are required.

Suitable additives not only improve the compressive 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 resistance of d relatively weak carbon derived from Loy Yang medium-dark coal.

Table 4 i Comparative Attrition Tests on Activated Carbon do 0.

5g sample, 30 ml water, in 250 ml cylinder rotated end for end at 15 r.p.m. for 8 hours. Product sieved.

Sample Imm 1-0.21mm 0.21-0.075mm 0.075mm 0 (1000 l m) (210-1000 .ml (75-210 km) Loy\Yang, medi' -,dark i w I number 90.1 0.2 0.1 9.6 20 high-i 2 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 *s 1:1, kneaded: unkneaded 73.5 1.4 0.2 24.9 1:1, kneaded: unkneaded 0.5% NH 4 OH 80.9 1.3 0.3 17.5 1.0% NH 4 OH 89.5 1.0 0.2 9.4 12 EXAMPLE 2. Improvements Resulting from Partial Attritioninq 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 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 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 S 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 parts, one of which was kneaded in a sigma-kneader for five hours, the other reserved till the end of this period and then briefly blended (2 minutes) in the plastic mass in the kneader.

Extrusion and subsequent treatment of the gran Ces followed the.procedure set out in Example 1. Measurements made on these granules are given in Table 0S S S -13 Table Sample Compressive strength MPa Iodine number Wt loss Dried After steam mg/g granules activation 4 hrs Loy Yang, pale light 1:1 kneaded: 7 29 832 unkneaded 1:1 kneaded: 13 30 619 62 unkneaded 0.5% NH 4 0H 1:1 kneaded: 36 58 634 15 unkneaded 1% NH OH 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 fragments of raw coal, with their high porosity even after pyrolysis, provide better access of solutes to the internal surface of the granules. The effect is illustrated in Table 6 below 6 below.

14 Table 6 Loy Yang, pale-light coal of iodine absorbed after 1 hr 2hr 3hr 4hr Sample 0@ C r Ce OC C

SO

CC

S. C S

S.

C

Fully kneaded 27.0 35.8 41.8 46.7 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 0550 *6 5* 0

S.

S

6*

C.

S

S00 Loy Yang, medium-dark coal Kneading time hr Weight loss on activation Iodine number mg/g Compressive strength MPa S 1 60 764 41 2 64 860 58 63 901 47 15 EXAMPLE 3. Improvements 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 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 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 e.

experiments on various Victorian brown coals are set out in Table 8.

t *5 16 Table 8 GoalI Lithotype Steam partial Temp. of Time of Iodine pressure activation activation number (temp. of 0Chr of steam gener- product ation 0 C) mg /g Loy Yang (ii) Loy Yang (iii) Loy Yang pale-light dark da rk OS S 00 *o 0@ 0 0S

SS

S. 0 @0 S0 06

S

S.

00 @0 @0 S

S

800 800 800 800 710 800 16 80 800 (iv) Morwel1 3 540 3 1073 3 562 3 1473 4 837 (64.1%.

Wt loss) 4 1049 (65.9.

Wt loss) 4 690 (89.6% Wt loss) 4 786 (69.6%.

Wt loss) 1 759 4 998 3 270 6 630 k 350 1 572 4 507 *of0 S06 4 0 Lay Yang medium-dark (vi) Morwell (vii) Maddingley 5% M(R 800 800 800 800 800 800 800 Temp.pH 2 0, 0 C 75 mmHg 289.1 633.9 760 i -17 Items 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 75 0 °C to that corresponding to 760 at 100°C increased t:ie 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 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.

15 Items (iii) and (iv) in Table 8 indicate that activation may be achieved at temperatures above about 680 0

C

with most satisfactory results at about 800°C. Below 680°C activation times become inconveniently long.

Within limits, the surface area attained is related to the percentage loss of weight during activation. Too great a loss of weight may lead to reduced areas as the structure of the carbon is eroded away.

t" 'Items (vi) and (vii) in Table 8 indicate that increasing times of exposure to the' activating gas in the range 1-6 hours increase surface areas markedly. The effects, however, are dependent on the origin of the coal.

Loy Yang coal, which produiced 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 prolonged activation.

18 EXAMPLE 4. Magnetic Activated Carbon Recovery of activated carbon granules (or fragments thereof) from heterogeneous systems, such as suspensions of minerals in water, will be facilitated if the carbon responds 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 details set out in Example 1 above, using Loy Yang dark lithotype coal. During the kneading of the coal 10% by weight 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.

There are no serious adverse effects, from a functional point o* of view, due to the presence of the reduced iron phases, on s* either the strength or the attainable surface area of the 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 significant part of the iron with effective solvents for the metal or oxides. The magnetic properties remained intact after such extractions.

The properties are shown in Table 9.

S*

Sooo 19 Table 9 Coal Wt loss on Compressive strength MPa Iodine activation before activation after activation number Smg/g L y Yang, dark 60% 20 67 841 Ly Yang, dark 59% 22 33 545 10% Fe 10 Both preparations (with and without Fe 2 0 3 gain Sstrength on activation, the latter rather less than the former. Erosion during activation is somewhat reduced by the presence of the iron and this is reflected in a rather lower surface area; however, more severe activation conditions could be used to generate a higher surface if required.

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 application 20 52234/86 and the entire disclosure in the specification of

C.

that application it by this reference incorporated into the present specification.

0

C

Claims (7)

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:- pyrolysing the said upgraded brown coal at a temperature of 350 to 500 0 C in an inert gas stream; activating the pyrolysed product of step by treatment with steam added to the inert gas stream at a temperature of 700 to 800 0 C; and cooling the activated carbon product of step (b) in an inert atmosphere.

2. Process according to Claim 1 in which the inert S. 20 gas in step comprises nitrogen.

3. Process according to Claim 2 in which the pyrolysis step is carried out in a stream of nitrogen and the activation step is effected in the presence of steam added to the nitrogen stream.

4. Process for the production of activated carbon from brown coal which comprises the following steps:- upgrading brown coal by subjecting it to shearing forces, compacting the mass so produced and drying the mass to form a hard, relatively dense o**o product; pyrolysing the product of step at a temperature of 350 to 500 0 C in an inert gas (c) (d) stream; activating the pyrolysed product of step by treatment with steam added to the inert gas stream at a temperature of 700 to 800 0 C; and cooling the activated carbon product of step (c) in an inert atmosphere. Process for the production of activated carbon from brown coal which comprises the following steps:- 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; pyrolysing the product of step in a stream of nitrogen at a temperature from 350 to 500oC; activating the pyrolysed product of step by treatment with steam added to the stream of nitrogen at a temperature of 700 to 8000C; and cooling the activated carbon product of step (c) in an inert atmosphere. 9* a.

S

6. from (a) 25 (b) Process for the production of activated carbon brown coal which comprises the following steps:- 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; pyrolysing the product of step at a temperature of 350 to 500oc in an inert gas stream; activating the pyrolysed product of step by treatment with steam added to the inert gas stream at a temperature of 700 to 800°C; and 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:- 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; pyrolysing the product of step in a stream of nitrogen at a temperature from 350 to 500 0 C; activating the pyrolysed product of step by treatment with steam added to the stream of nitrogen at a temperature of 700 to 800°C; and cooling the activated carbon product of step (c) in an inert atmosphere. DATED THIS 3RD DAY OF MARCH 1993 THE UNIVERSITY OF MELBOURNE and CRA SERVICES LIMITED By their Patent Attorneys: GRIFFITH HACK CO Fellows Institute of Patent Attorneys of Australia.