Coal briquetting process

A cold coal briquetting process comprises mixing fine coal with polyvinyl alcohol and a hardening agent selected from iron oxide, phosphate rock and bauxite or combination of two or more thereof. The process permits economies in process and capital costs compared to existing briqueting processes, while still producing satisfactory briquettes.

This invention concerns a coal briquetting process, and more especially it 
concerns a process for "cold" briquetting. 
There exist many processes for briquetting fine coals in which the mixture 
fed to the briquetting press is hot, and various binders which melt at the 
operating temperatue, such as pitch are used. In some cases, the coal 
itself, because it cakes during heating to temperature, forms or 
contributes to the binder. There is also interest in "cold" coal 
briquetting processes which require no heating or only heating to about 
100.degree. C. before briquetting without subsequent heating to a higher 
temperature. A binder must be used, therefore, which is effective at low 
temperatures, and many binders have been suggested. A problem faced 
especially by "cold" briquetting processes, is that the binders in the 
product briquetttes may not provide adequate binding of the fine coal at 
or near the combustion temperature, resulting in partial or complete 
disintegration of the briquette and the loss of coal through the grate. 
This is termed "undergrate loss". 
It is an aim of the present invention to provide a cold coal briquetting 
process producing briquettes which have adequate strength, which do not 
show excessive undergrate losses and utilizing an inexpensive binder 
system which is tolerant of variations in the quantities of components and 
is not hazardous. Desirably, the briquettes exhibit good water resistance 
to prevent degradation during open air stocking. 
Binders which have been suggested for cold briquetting processes include 
bitumen, starch and resin combinations, binders based on polyvinyl 
alcohol, e.g. PVA/calcium oxide (see GBP2138442), PA/molasses (Europatent 
0135784), PA/calcium oxide (see GBP2138442), PVA/molasses (Europatent 
0135784), PVA/calcium carbonate (Europatent 0135785), molasses and lime, 
molasses and phosphoric acid (GBP 230306) and binders based on sulphite 
lye, eg. sulphite lye, solium dichromate and sulphuric acid (Europatent 
0127351). We have tested most and considered all of the above processes 
and their product briquettes, and we consider that most exhibit problems 
mitigating against large scale use and acceptability. 
There remains a need in the market for fuel briquettes having acceptable 
burning and strength qualities, despite the many different types of 
smokeless and non-smokeless briquettes commercially available. The better 
smokeless briquettes, for example, are relatively expensive or limited in 
availability. It is an aim of the present invention to provide such fuel 
briquettes by a process which is relatively inexpensive in capital and 
running costs. 
The present invention provides a briquetting process comprising mixing fine 
coal with a binder comprising up to 2% by weight of polyvinyl alcohol 
solids, in aqueous solution, and up to 4%, by weight, of a hardening agent 
for the polyvinyl alcohol selected from iron oxide, phosphate rock and 
bauxite or a combination of two or more thereof, and briquetting the 
coal/binder mixture. The invention also provides briquettes formed using 
such a process. The polyvinyl alcohol may also be in solution acidified by 
an acid such as phosphoric or sulphuric acid. 
It is to be understood that the term briquetting as used herein includes 
all methods of forming an agglomerate of coal, including briquetting using 
conventional ring roll presses, roll presses, die presses and rotary table 
presses as well as extrusion and pelleting, eg using a disc pelletiser. 
The coal may be any bituminous or non-bituminous coal, and the invention is 
applicable to naturally occurring coals having low smoke emissions such as 
anthracite, to coals treated, eg by mild oxidation or pyrolysis, to reduce 
their smoke emissions, to low rank bituminous or non-bituminous coals, and 
to coal blends including caking or coking coal components and/or coke 
breeze. Desirably, the coal is of a particle size below 3mm, and 
anthracite duff is particularly suitable. The coal may be a direct product 
of coal cutting or may be crushed, or may be recovered from a coal 
cleaning process. 
Suitable polyvinyl alcohol materials are commercially available as powders. 
Preferably, those products marketed as "medium viscosity" polyvinyl 
alcohol are used. These are soluble in hot water using 1 part polyvinyl 
alcohol to 5 to 10 parts of water, by weight. 
The inorganic hardening agents are believed to act chemically on the 
polyvinyl alcohol, so that the briquettes increase in strength on storage 
in air more rapidly than would be the case if the effect was solely that 
arising from loss of water. The iron oxide used is preferably ferric 
oxide, especially in the form of haematite, conveniently used as haematite 
ore. It is believed that haematite donates oxygen to the polyvinyl 
alcohol. A particular advantage of the use of haematite is that the ash 
remaining after combustion of the briquette is of increased density and is 
more easy to handle. Bauxite, as the ore of aluminum, is also an oxide and 
is generally found with a proportion of ferric oxide. It will be 
appreciated that all the hardening agents are available in large 
quantities and at low cost. These agents may be used along or in 
combination in total amounts up to 4% by weight. In practice, routine 
testing is carried out to ensure that any individual briquetting mix is 
satisfactory. 
The admixture of coal and binder may be done at a temperature of from 
ambient to 100.degree. C. Conventional mixing equipment may be used. 
The briquettes may comprise additional components such as wattle bark to 
improve combustion properties, or to improve strength, eg green strength 
or strength at combustion temperatures. 
The process of the invention preferably includes a self-hardening stage to 
permit the green briquettes to gain strength. The invention does not 
require the use of carbonisation or hot curing, and thus significient 
capital and processing costs may be avoided. The hardening stage may be 
carried out by cooling and stocking the green briquettes or by maintaining 
them initially at temperatures up to about 100.degree. C. prior to cooling 
and stocking. Stocking may be carried out in the open air or under cover. 
Sensible precautions will avoid unnecessary breakage of briquettes at this 
stage, and care may be required if the ambient temperatures are below 
freezing. 
The present invention will now be described by way of example only.

EXAMPLE 1 
Ninety-four parts by weight of air-dried South Wales anthracite, or nominal 
size below 3 mm, were mixed with a binder comprising 1 part medium 
viscosity polyvinyl alcohol to 5 parts water. After heating with live 
steam to about 100.degree. C. followed by some evaporative cooling air in 
an open screw conveyor, the mixture was briquetted in a pilot double-roll 
press at about 80.degree. C. An initial briquette strength of 5 kg was 
found, which increased during further cooling to 12 kg after one hour and 
to 16 kg after 24 hours. 
EXAMPLE 2 
The procedure described in Example 1 was repeated, except that 1% iron ore 
(haematite) was also incorporarted into the anthracite. An initial 
briquette strength of 6 kg was found, which increased to 15 kg after one 
hour, and to 24 kg after 24 hours. 
EXAMPLE 3 
The procedure described in Example 2 was repeated, except that 0.5% 
orthophosphoric acid was also incorporated into the binder solution. An 
initial briquette strength of 7 kg was found, which increased to 20 kg 
after one hour, and to 38 kg after 24 hours. 
EXAMPLE 4 
The procedure described in Example 1 was repeated, except that the 
anthracite was blended with some coking steam coal and 1% phosphate rock 
was also incorporated into the blend. An initial briquette strength of 5 
kg was found, which increased to 24 kg after one hour, and to 41 kg after 
24 hours. 
The briquettes of examples 1-4 continued to self-harden in air at ambient 
temperature attaining crushing strengths in the range of 110-140 kg after 
7-9 days and 205-260 kg after 21 days. At all stages of hardening the 
briquettes showed satisfactory resistance to water. However, the hot 
strength of the briquetes during combustion was not entirely 
statisfactory. 
EXAMPLE 5 
An anthracite-based blend of coals including pyrolysed and strongly-caking 
coal components was admixed 0.5 parts of "Bitan A"--a wattle bark 
extract--and briquetted with a binder comprising 1 part of 
medium-viscosity polyvinyl alcohol, 0.25 parts of orthophosphoric acid and 
9 parts of water, as outlined in Example 1. The self-hardened briquettes 
showed an improved thermal stability at combustion temperatures.