Production of carbon from coal granules prepared in a fluid energy mill

Granular active carbon having satisfactory adsorbency and hardness characteristics and having a high proportion of its porosity in the below 2000 A range is produced from granules prepared from coal fluid energy milled until not more than 1% of the particles have diameters above 38 microns. The coal has a low ash and high carbon content and may have a British Standard Swelling number from 2 to 7 although it is preferred for the volatile matter content of the coal to be not more than 40% on a dry ash free basis. The granules may be produced by compacting milled particles having a temperature of from 50.degree. C to 250.degree. C between unheated pressure rolls.

This invention relates to a process for the production of carbon. 
Active carbon is widely used as an adsorbent in gaseous and liquid systems. 
In at least some applications, it has been considered desirable for the 
active carbon to be in granular form. Granular active carbon, for example, 
lends itself to use in adsorption columns and like apparatus in which it 
may be desired to pass a gas or liquid through a bed of carbon. It is 
important that granular active carbon be able to stand up to the materials 
handling techniques used in such applications and in processes for 
regeneration of carbon after it has lost some or all of its adsorptive 
capacity in use. Granular active carbon should also have a high adsorptive 
capacity per unit bulk so that transport costs and capital equipment costs 
can be kept to a minimum. 
U.S. Pat. No. 3,483,134 has, as a purpose, the preparation of an activated 
carbon having such properties. The process disclosed in U.S. Pat. No. 
3,483,134 comprises pulverising bright banded bituminous coal under impact 
in the presence of a stream of air until the particles are of the 
following size range: at least 60% by weight through 200 mesh, at least 
25% by weight through 325 mesh, introducing into the finely divided coal 
between 1% and 10%, by weight of the dry coal, of a cereal which will 
break down at a temperature between 220.degree. F. and the temperature of 
heat treating, regulating the moisture content of the finely divided coal 
to between 2.5% and 11% on the dry weight of the coal, moulding the finely 
divided coal into briquettes in a closed mould under a pressure as 
follows: where the volatile in the coal is above 38% by weight, in excess 
of 5000 psi, where the volatile in the coal is below 38% by weight, in 
excess of 10000 psi., heat treating the coal in thus moulded form at a 
temperature between 250.degree. F. and 800.degree. F. and not in excess of 
the temperature of agglomeration of the coal for a time of at least 4 
hours, the coal and the cereal reacting exothermically during heat 
treatment, breaking down the briquettes into granules and activating the 
carbon granules. 
U.S. Pat. No. 3,483,134 discloses that certain features of the above 
described process are particularly important to attain the desired result. 
One feature is to use a bright banded bituminous coal which consists mainly 
of vitrain and clarain and which has a low ash content and a "high" 
volatiles content. `High` is defined as being from 28 to 46% and most 
desirably from 39 to 42% on a moisture free basis by weight. A further 
feature is the incorporation into the coal, prior to reaction under heat, 
of potential chemical energy capability by shattering the macromolecules 
and forming free valencies. In order to do this it is not enough to grind 
by one of the accepted methods which will produce finely divided 
particles. Instead it is necessary to shatter the particles by impact and 
therefore it is essential that an impacting mill be employed. The 
preferred mill is a Raymond mill through which air is circulated. The air 
cools the particles, notwithstanding that much heat is generated by the 
impact, and blows the ground products out of the mill. The cereal additive 
performs a very important function in accelerating the heat treatment 
which, without the additive, would require at least 9 hours. To heat treat 
the coal even much less effectively without such an additive would require 
a considerably longer period of heat treatment than that necessary when 
carrying out the disclosed invention. The moulding must be carried out in 
a closed chamber mould and the resulting tablets are preferably next 
broken down to produce granules for the heat treatment described above. 
The present invention relates to the production of a readily heat treated 
and activated carbon granules from coal without the use of many special 
features designed to improve the susceptibility of the granules to heat 
treatment and activation which is a feature of the prior art. 
This invention comprises a process for the production of active carbon 
comprising milling coal in a fluid energy mill under a non-oxidising 
atmosphere until not more than 1% by weight of the particles so produced 
have diameters about 38 microns, compacting the particles to produce 
agglomerates, granulating the agglomerates and converting the resulting 
granules into active carbon. 
Finely divided coal may combust spontaneously in contact with air as may 
the granules made therefrom, particularly if hot. It is, therefore, 
necessary to practice normal safety precautions associated with the 
handling of combustible materials such as the prevention of contact with 
air where necessary throughout the operation of this invention. 
While it is contemplated that the fluid energy milling step of the present 
invention my be conducted in the presence of any non-oxidising atmosphere 
suitable to prevent combustion, for example nitrogen or an inert gas, it 
has been found particularly advantageous to mill under an atmosphere of 
superheated steam. The temperature of the superheated steam is preferably 
from 150.degree. to 400.degree. C. and most preferably from 250.degree. to 
350.degree. C. The fluid energy mill used may be of any of those 
commercially available. Examples of suitable mills which operate by means 
of peripheral jets about the diameter of a grinding chamber are the 
Micronizer, the Jet Pulverizer, the Reductionizer or the Jet-O-Mizer. An 
example of a suitable mill operating by the action of two opposing jets of 
fluid is the Majac. (The words Micronizer, Jet Pulverizer, Reductionizer, 
Jet-O-Mixer and Majac are Trade Marks). 
The fluid energy mill may be controlled to produce the degree of fineness 
required. Preferably the mill is controlled so that less than 10%, and 
particularly preferably not more than 1%, by weight of the particles, have 
diameters below 1 micron. Particularly preferably from 90 to 99% by weight 
of the particles of the coal are ground to a particle diameter of not 
below 1 micron and not above 10 microns. The particle distribution of the 
milled coal may be monitored by the use of British Standard Sieves except 
for the finest sizes which may be monitored by an electronic device such 
as a Coulter (Trade Mark) Counter. 
After the coal has been ground it is compacted to produce agglomerates. 
However, it is particularly advantageous to avoid the use of binders and 
to form the agglomerates utilising a medium to highly caking coal, under 
the influence merely of heat and pressure. This may be accomplished by 
compacting the fluid energy milled particles by means of pressure rolls. 
Pressure rolls are a very suitable means of applying the pressure. If 
unheated pressure rolls are of relatively simple construction and low 
capital cost. Such rolls may comprise at least one pair of rollers 
arranged to rotate about their axes in opposite senses and with their 
circumferences descending towards a line of contact between them. The 
rollers may be urged together by means of hydraulic rams. To avoid uneven 
pressure the rollers must be of great rigidity and therefore usually 
consist of solid cylinders up to a foot or more in diameter and up to 
several feet in length. It is not easily practicable to heat these rollers 
evenly over their rolling surfaces without considerable complexity of 
construction. 
Where the coal particles are hot it has been found possible to use unheated 
pressure rolls. It is, therefore, a further and advantageous feature of 
this invention to recover the milled particles from a hot gaseous effluent 
from the fluid energy mill and to pass the recovered particles through 
unheated pressure rolls. Preferably the temperature of the particles is at 
least 50.degree. C. and not more than 250.degree. C. while being passed 
through the pressure rolls. Preferably the operation of the rollers is 
controlled to exert a pressure of from at least 580 Kg and preferably from 
580 Kg to 714 Kg, per cm of roll length, between the rolls. Higher forces 
may be used if desired although without any marked improvement in the 
properties of the final product. 
The agglomerated coal, whether in tablet form from a tabletting press, or 
as briquettes, or as flakes, is converted to granules which are preferably 
particles having largest diameters not less than 0.25 mm and not greater 
than 3.0 mm. A preferred apparatus for use in granulation is one which 
will produce brittle fractures resulting in minimal dust formation, such 
as a suitable cutter. 
In common with many other processes for the production of active carbon 
using coal as a raw material it is preferred that the coal used should 
have a low-ash content, which may preferably be up to 6% on a dry weight 
basis, and a high fixed carbon content, which is preferably at least 50%, 
and if desired up to 65%, on a dry weight basis. A particular feature of 
the present invention however, is that it is possible to utilise coal of 
only moderate or even weakly caking properties as shown by its British 
Standard Swelling Number in contrast to the high caking coals usually 
required for such processes. Highly caking coals are, of course, also 
utilisable. Preferably the coal used has a BS Swelling Number of from 2 to 
7. However, it may not be desirable to use coal having very high swelling 
numbers because of their high content of volatile matter. The heat 
treatment removes volatile matter and tends to produce a low density 
carbon having some undesirably large pores which may contribute to 
weakness in the carbon granules if the volatile matter content of the coal 
has been high. It may therefore be preferred to use coal having a volatile 
matter of up to 40%, for example from 25 to 40% on a dry ash-free weight 
basis. The following British coals are typical of the range of coals 
suitable for use in the present invention: 
__________________________________________________________________________ 
WOLSTANTON 
WOOLLEY MARKHAM (NEWCASTLE- 
BERSHAM CLIPSTONE 
(BARNSLEY) 
(DONCASTER) 
UNDER-LYNE) 
(WREXHAM) 
(MANSFIELD) 
__________________________________________________________________________ 
H.sub.2 O % wt. 
9.0 10 9.3 8.7 10.0 
Swelling No. 
(BS) 61/2 2 7 7 6 
Volatile 
Matter % 
34.9 36.0 34.7 37.0 37.3 
(Wt. dry 
ash-free) 
Fixed Carbon 
% wt. 59.7 59.0 56.6 53.8 53.1 
Ash % wt. 
3.5 3.5 4.0 5.5 5.2 
Sulphur % 
wt. 1.5 1.5 1.33 1.0 1.0 
__________________________________________________________________________ 
The granules produced by the present invention are converted into active 
carbon by a three stage treatment comprising oxidation, carbonisation, 
comprising progressively raising the temperature of the oxidised granules 
to reduce the volatile matter content thereof, and activation, comprising 
heating at an elevated temperature in the presence of an activating gas. 
The oxidation process may be conducted in a rotary kiln or in a multiple 
hearth furnace but is most effectively conducted in a fluidised bed 
utilising air as the fluidising medium. When using a fluidised bed the 
oxidation of the granules is preferably conducted at a temperature of from 
150.degree. to 250.degree. C., for example from 180.degree. to 220.degree. 
C., for a period of, preferably, from 30 minutes to 3 hours. This 
treatment renders the coal non-swelling and non-caking and prevents if 
from fusing during subsequent processing. The oxidation of highly caking 
coal can be difficult but it has been found that even highly caking coals 
can be oxidised in the above temperature range when it is in the form of 
granules manufactured according to the present invention. As a 
demonstration of this a sample of the Clipstone coal described above was 
fluid energy milled using superheated steam according to this invention, 
was agglomerated using unheated pressure rolls, was reduced to 1.0 mm 
granules, and was oxidised in a fluidising bed at 200.degree. C. After 1 
hour of oxidation the coal had been rendered non-swelling and non caking. 
The carbonisation stage is preferably accomplished by progressively raising 
the temperature of the oxidised granules to at least 800.degree. C. and 
preferably to 950.degree. C., suitably in a multiple hearth furnace or a 
rotary kiln. Preferably, in the operation of this invention the 
temperature is increased relatively slowly, that is to say, preferably, at 
a rate not above 150.degree. C. per minute but, particularly preferably, 
not above 100.degree. C. per minute. There is no lower limit to the 
heating rate although the use of a very slow rate will affect the 
economics of the process. We find a particularly suitable heating rate is 
50.degree. C. per minute. This allows the uniform release of volatile 
matter so as to create a uniform pore structure throughout the granules. 
Preferably the temperature of the oxidised granules is raised from 
400.degree. to 950.degree. C. at a rate not above 100.degree. C. per 
minute. 
The activation stage may comprise heating at an elevated temperature in the 
presence of an activating gas comprising steam, or carbon dioxide, or 
admixtures of steam and carbon dioxide. A suitable activating gas may be 
obtained from the stoichiometric combustion of fuel oil or gas. Nitrogen 
may be used in addition to the activating gas if required. The activation 
step may be conducted either in a fixed bed or in a fluidised bed although 
the latter is preferred since it involves a lesser residence time. The 
activation step may be conducted in the temperature range of 600.degree. 
to 1000.degree. C. and, preferably, in the range of from 850.degree. to 
950.degree. C. Temperatures above 1000.degree. C. lead to increased 
graphitisation and to a progressive reduction in the volume of the 
micropores. When conducting a fluidised bed activation the activating gas 
may be used as the fluidising medium. The ratio of activating/fluidising 
gas to the weight of coal in the bed is an important parameter and may be 
optimised to suit both a particular residence time and to control, to an 
extent, the pore size distribution in the final product. Preferably from 
0.2 to 2.0 kg of activating gas is used per kg of original coal feed per 
hour of activation. By original coal feed we mean the weight of coal 
milled. It is a particularly advantageous feature of this invention to 
fluid energy mill the coal in a steam atmosphere in that the effluent 
steam from the fluid energy mill may be used in the activation process if 
its temperature is boosted to a suitable level. The duration of activation 
step is preferably from 1 to 3 hours when a fluidised bed is used. Where a 
fixed bed is used, the duration of the activation step may be from 2 to 6 
hours. 
The present invention may produce granules which are particularly easily 
heat treated and activated and which result in a dense active carbon which 
is resistant to mechanical degradation, which shows a high degree of 
activity as an absorbent and which contains few or no macropores above 
10,000 angstroms while retaining a high proportion of porosity in the 0 to 
2,000 angstrom range. 
The apparent bulk density of a sample of the material taken after each 
stage of a process according to the invention was as follows: 
After compaction: 1.35 g/cc 
After oxidation: 1.35 g/cc 
After carbonisation: 0.90 g/cc 
After activation: 0.66 g/cc 
The method used to measure apparent bulk density is as follows: 
A weighed amount of carbon (w.sub.s) is introduced into a calibrated 
pyknometer (Vol. Vp) at constant temperature and mercury is added (density 
d.sub.R) to the reference mark. The weight (w.sub.F) of the pyknometer and 
contents is then noted. Care must be taken to ensure the carbon particles 
are surrounded by mercury and excess air is not entrained. The apparent 
density if found from the expression 
EQU dRWs/V.sub.p d.sub.R + w.sub.p + w.sub.s - w.sub.F 
(where w.sub.p = wt. of pyknometer).

The plant contains a conventional coal mill 6 having hardened steel 
crushing surfaces and capable of grinding the coal into particles not more 
than about 2 mm in diameter. The coal mill 6 is arranged to be serviced 
with coal by a mild steel elevator 1, a rubber belt conveyor 2, the two 
storage hoppers 3, a further rubber belt conveyor 4 and a pocket belt 
elevator 5. A Micronizer fluid energy mill 7 is arranged to receive the 
coal particles from the coal mill. The Micronizer is arranged to operate 
using superheated steam from a waste heat boiler 8. 
A cyclone 9 is provided to separate the coal particles in the effluent from 
the fluid energy mill and the unheated pressure rolls 10 are positioned so 
as to receive these particles direct from the cyclone. The pressure rolls 
are made of mild steel. The rotary cutters 11, positioned so as to receive 
the flakes issuing from the pressure rolls have hardened steel blades and 
are capable of cutting the flakes into granules. The fines resulting from 
the action of the rotary cutters may be separated by means of a screen 12 
for recycle to the fluid energy mill and the granules passed via the surge 
hopper 13 and the pneumatic elevator 14 to the hopper 15 for storage while 
awaiting oxidation. 
The means for oxidising the granules to render them non-caking comprises 
fluidised bed batch reactor 16 lined with carbon steel and provided 
internally with water sprays for temperature control. The oxidiser is 
arranged to operate using air as the fluidising gas provided through the 
inlet 17 and the product may be removed by means of duct 18. The granules 
are introduced into the oxidiser by means of screw feeder 19. The oxidised 
granules may be removed via the water cooled screw conveyor 20 to the 
surge hopper 21 and elevated in a pneumatic elevator 22 to hopper 23 for 
storage while awaiting carbonisation. The effluent fluidising gas from the 
oxidiser is stripped of entrained fines in the cyclone 24 whence they are 
recycled to the pressure rolls. From this point until after the carbonised 
granules are cooled the plant is sealed to prevent ingress of air which 
could result in spontaneous combustion. The gases from cyclone 24 are 
vented. 
Carbonisation is conducted in the rotary kiln 25 which is arranged to be 
operated at an oxygen concentration of not more than 2% to prevent 
combustion. The granules are passed into the kiln via the rubber belt 
conveyor 26 and the screw feeder 27. The volatile matter from the rotary 
kiln is removed to fuel the waste heat reboiler 8 and the carbonised 
granules are passed to the cooler 27 to prevent combustion and are 
retained in the storage bin 28 awaiting activation. 
The activator comprises fluidised bed vessel 30 arranged for batch 
operation. The fluidising gas is a mixture of steam recovered from the 
fluid energy mill by means of the cyclone 9 and combustion gases from an 
oil burner 31 operating without excess air. The carbonised granules are 
passed into the activator by means of pneumatic conveyor 32, hopper 33 and 
screw feeder 34. The activated granules are passed to cooler 35 before 
being passed to screen 36 by means of the screw feeder 37 and the 
pneumatic elevator 38. Screen 36 separates appropriate size fractions as 
required. The waste gases from the activator are passed through cyclone 39 
whence they are passed to waste heat reboiler 8 the fines being recovered. 
The hardness of the granulated active carbon produced by the present 
invention may be measured by the following method. The basic method is 
that disclosed in the Encyclopaedia of Industrial Chemical Analysis edited 
by Snell & Ettre 1969, Vol. 8 pages 139 onwards, wherein a sample of 
carbon is retained in a sieve pan with steel balls of different diameters 
and is subjected to mechanical degradation for 20 minutes using a RO-tap 
sieve shaker. The valve of hardness is obtained by measuring the weight 
fractions of both initial and tested samples, calculating the weight mean 
particle diameter for each, and expressing the ratio of final and initial 
weight mean diameters as a percentage. This test was modified to give a 
result in which the fraction of tested material greater than 105 microns 
is expressed as a percentage of the total sample weight. 
To determine the pore size distribution of the active carbon product 
mercury penetration porosimetry may be used to determine the quantity of 
pores greater than 400 angstroms diameter and, to determine the 
distribution of finer pore sizes, a suitable gas adsorption apparatus of 
the kind used to measure BET adsorption isotherms may be used. 
The effectiveness of active carbon as an adsorbent may be judged by 
reference to its Iodine Number and/or its Methylene Blue Number. 
The Iodine Number of a carbon adsorbent may be determined as follows. 
Prepare standard 0.1 N solutions of iodine and of sodium thiosulphate. 
Place 1 g of carbon ground to 325 mesh (Tyler) in a stoppered flask. Add 
10 ml 5% Hcl to the flask, swirl, boil for 1 minute and cool to room 
temperature. Add 100 ml of the iodine solution, stopper, shaker for 30 
seconds and filter on a Whatman No. 12 filter paper discarding the first 
portion of the filtrate. Pipette off 50 ml of the filtrate and titrate 
with the sodium thiosulphate solution using starch as an indicator. The 
titre should be between 4 and 16. If outside these limits the sample 
weight should be adjusted accordingly. The filtrate normality is 
##EQU1## 
Depending on the filtrate normality a factor is selected as follows: 
______________________________________ 
Normality Factor 
______________________________________ 
0.008 1.16 
0.01 1.12 
0.012 1.09 
0.014 1.06 
0.016 1.04 
0.018 1.02 
0.20 1.00 
0.22 0.98 
0.24 0.97 
0.26 0.96 
0.28 0.95 
0.30 0.94 
0.32 0.93 
______________________________________ 
The Iodine Number is F .times. (mg of I.sub.2 at outset - mg I.sub.2 
determined by titration). 
The Methylene Blue Number of a carbon adsorbent may be determined as 
follows: 
Place 1 g of carbon ground to 325 mesh (Tyler) in a flask Add 0.1% aqueous 
methylene blue solution from a burette, 1 ml at a time, stirring for one 
minute after each addition. At the end of each 1 minute interval observe 
the residual colour of the carbon suspension by spotting on a plate or 
filter paper. If any blue colour remains the end point has been reached. 
Results are recorded as milligrams of methylene blue adsorbed per 1 gram 
of carbon. 
The invention is illustrated by the following example 1 in which the 
granules produced according to a preferred embodiment of the invention are 
processed on a laboratory scale into granular active carbon the properties 
of which are examined by the tests described above. Examples 2 and 3 are 
not according to the invention and are inserted for comparative purposes. 
Example 4 is according to the invention. 
EXAMPLE 1 
The Coal used was the Woolley Coal described previously. The coal as 
received was dried and crushed to below 1 mm size and fed to a fluid 
energy mill (Micronizer) operating under the following conditions: 
______________________________________ 
Micronizer diameter 
5.0 cm (6 nozzles - 1/2 mm 
diameter) 
Feed Rate 3 kg/hr 
Feed Size less than 1 mm 
Ring Pressure 
25 psi 
Injection Pressure 
150 psi 
Steam Temp. 300.degree. C 
Product Temp. 
190.degree. C 
Product Size .gtoreq.38 micron 0% 
&lt;38 micron but &gt;10 micron 1% 
3-10 micron 91% 
&lt;3 micron 8% 
______________________________________ 
The product was fed directly on to unheated plain faced 6 inch diameter 
rolls operating at 2 rpm. A force of 3500 lb/linear inch of roll was 
applied to the rolls. The product was obtained as flakes, any unbonded 
coal being returned to the fluid energy mill and recycled. The flakes were 
then broken to produce granules in the size range of 0.3 to 3.0 mm. The 
granules were rendered non-agglomerating by heat treatment comprising 
oxidation in air using a 10 cm diameter fluidised bed reactor. Oxidation 
was completed in the 10 cm diameter reactor in 1 hour operating at 
200.degree. C. with a preheated air flow of two times that required to 
just fluidise the particles. Carbonisation of the oxidised granules was 
achieved by heating in rotary kiln at a rate of 50.degree. C./minute over 
the range 400.degree.-900.degree. C. which served to devolatilise the coal 
and increase product hardness. The carbonised granules were activated in a 
fluidised bed reactor using a fluidising gas velocity twice that required 
just to fluidise the particles and a flow of steam at a rate of 0.8 kg 
steam per kg of original coal per hour of activation as the fluidising 
gas. After 2 hours at 950.degree. C. an activated carbon of suitable pore 
size distribution was obtained. The final hardness of the product was 83% 
and no macropores in excess of 30,000 Angstrom units diameter were 
observed. The adsorption properties of the product were as follows: 
Iodine No.1300mg I.sub.2 /g carbon 
Methylene Blue No. 351 mg Methylene Blue/g Carbon 
The pore size distribution of the product was as follows (as % of total 
porosity) 
______________________________________ 
&gt;30,000 Angst. None 
30,000 - 20,000 1.5 % 
20,000 - 10,000 9 % 
10,000 - 2,000 22 % 
2,000 - 1,000 5 % 
1,000 - 400 10 % 
400 - 0 52.5 % 
______________________________________ 
In comparison the following commercial activated carbons were found to have 
the following properties: 
______________________________________ 
Methylene Blue 
Hardness % 
Iodine No. 
No. 
______________________________________ 
Chemivron 
Filtrasorb 200 
65 1000 169 
Westvaco WV-W 
70 850 261 
______________________________________ 
EXAMPLES 2 TO 4 
Markham Coal as identified in the table on page 8 of the specification was 
pulverised in a Mikropulveriser (Trade Mark) impact mill and sieved on a 
45 micron sieve. The Coulter Counter (Trade Mark) analysis of the material 
passing through the sieve showed: 
below 38 microns: 65% 
below 10 microns: 10% 
This material was pelleted cold in an unheated ram press without binders at 
a pressure of 700 Kg/sq.cm. The resulting pellets were cut to a 1-4 mm 
size and oxidised with air in a fluidised bed at a temperature of 
200.degree. C. for 1 hour. The oxidised coal was carbonised and activated 
as disclosed in Example 1 and the resulting active carbon product had the 
properties described in the following Table in the column headed Example 
2. The Example was repeated with the sole modification that the ram press 
operated at a temperature of 200.degree. C. The properties of the 
resulting active carbon product are described in the column of the 
following Table headed Example 3. 
The same coal was fluid energy milled as disclosed in Example 1 to give a 
product of which 100% of the particles were below 38 microns in diameter 
and of which more than 90% of the particles were from 3 to 10 microns in 
diameter. The milled coal was then treated as in Example 2 and the 
properties of the active carbon product are described in the column of the 
following Table headed Example 4. 
TABLE 
______________________________________ 
Example 
2 3 4 
______________________________________ 
Iodine No. 735 405 960 
Methylene Blue No. 125 -- 169 
Hardness % 46 -- 48 
Pore size distribution % 
&gt;40,000 A 2 -- 0 
&gt;20,000 - 40,000 A 7 -- 1 
2,000 - 20,000 A 27 -- 22.5 
1,000 - &lt;2,000 A 4 -- 3 
400 - &lt;1,000 A 4 -- 1.5 
&lt;400 A 27 -- 44 
out of a total porosity % 
of 71 -- 72 
______________________________________