Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention reIates -to a fluidised bed'method of
processing phosphogypsum into sulphur di'oxide and calcium oxide.
Phosphogypsum is a waste material of the production '
of phosphoric acid by sulphuric acid digestion of,apatite and
phosphorite, the phosphoric acid generally being concentrated
subse~uently to form phosphoric acid or used in the production
of complex fertilisers. The phosphogypsum waste material con-
sists largely of calcium sulphate in the form of gypsum and
phosphoric acid and is produced in an amount of from 1.3 to
,
10 1.6 tons for each ton of apati-te and phosphorite,employed.
Phosphogypsum may be~ used~in the production of a ~ ,
variety of products, including sulphuric acid and calcium oxide,
sulphuric acid and cement,'plaster of Paris, building materials
,and filler compositions. ' ~ -
;; ~Sulphur dioxlde and calci~n'oxide have been produced;~
by the thermal dissociation of phosphogypsum which is carried ~,'` '
out under reducing conditions in,rotating furnaces., This type
of processing takes a relatively long time, generally taking
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'`from 2 to 4 hours and suffers from a number of basic shortcomings,~
including poor utilisation of the volume of the furnace, low
efficiency, high operational expenses and capital costs, low
~sulphur dioxide concentration gene~ally only being from 5 to
.
7%, and poor quality'of lime obtained owing to interaction of
' the calcium oxide produced with silica at the elevated tempera~
tures employed. ~ ~'
Improvement in utilisation of the furnace and eEfi-
ciency of the process can be obtained if fluidising conditions
are employed; this leads to a multifold increase in the rate of
dissociation~ However, the phosphogypsum obtained in the phos- -
~0 phoric acid is polydisperse and possesses a high moisture con-
tent and is not suitable for use directly in fluidised bed
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processing. It is necessary that it be subjected to a preli-
minary drying and granulation t~ suitable particle size for
~luidised bed processing. In handling particulate phospho-
gypsum, care must be taken to ensure that sintering thereof
does not take place at the temperature at which it is subjec~
ted to decomposition, which is optimally 1200 to 1220C. A
slight decrease in the processing temperature below 1200C has ~;
been found to reduce sharply the degree and rate of dissociation
making the process largely impracticable. A rise in tempe-
rature much above 1200C increases the degree of dissociation
but also increases the danger of aggregation of the particles :
as a result of the sinteriny of the starting material.
It is an object of the presentinvention to provide a
method for processing phosphogypsum into calcium oxide .
and sulphur dioxide, which offers the possibility to carry out
the thermal dissociation in a fluidised bed, to form granules
featuring good physical and mechanical properties and to in-
crease the temperature of phosphogypsum sinteriny.
In accordance with the invention, there is provided
a method of fluidised bed processing of phosphogypsum into sul-
phur dioxide and calcium oxide, which comprises the steps of: .
a) preliminary drying humid phosphogypsum;
b) mixing the preliminary dried phosphogypsum with
- 0.01 to 6% by weight of granulation-binding additives, in aque-
ous solution, and optionally with l to 15% by weight of~carbon-
containing materials and with 2 to 25% by weight of sulphur-
containing raw mate.rials, the weight percentages being based
on the weight of the phosph~gypsum, the blnding additives being
. selected rom the group consisting of carbamide, carboxymethyl
cellulose, ammonium nitrate, calcium nitrate, polyethylene
oxide and the distillation fluids from soda production,
f~
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1~98~33
cl g~anula~ing the mixture;
d~ subjecting the granules to a thermal treatment
at 100~700C for 10~90 minutes;
e) screening the thermally treated granules to iso~
; l late a fraction having 1--3 mm size range; and
f) subjecting the isolated fraction to a the~mal
dissociation in fluidised bed, in the presence of propane-
butane gas mixture as a reducer.
When carrying out the method of the invention, the ~ ~
10 ~ starting phosphogypsum is generally dried at a temperature of ~ ;;
from 90 to 150C for a period usùally of from 1 to 5 hours.
To the dried phosphogypsum is then added the binder~additives ~;
in an amount of from 0.01 to 6~ by weight of the dried phospho~
gypsum. In accordance with a preferred embodiment of the inven~
:: ~ . , . . :~ :
tion~ carbon-containing raw materials can be incorporated in
the granulate in an amount of from l to 15~ by weight of the
dry phosphogypsum,the carbon-containing raw material preferably
bein~ coke, anthracite, coal or powdered coke. Sulphur-con- ~ ~
taining mineral can also be incorporated in the granulate, ~ ~-
preferably in an amount of erom 2 to 25~ by weight of the phos-
phogypsum. Elemental sulphur or pyrites can be used for this~
pu~pose. In both cases, ~he carbon-or sulphur-containing raw
materials may be mixed with the phosphogypsum separately from
the binding additives.
Granulation of the mixture ~hus produced may be
e~fected in a drum, conveyer, plate or Blounget granulator
the mixture preferably having a moisture content of 15 to 30%
by weight. The gra~ules obtained are then submitted to thermal
treatment usually at from 100 to 700C, usually for from 10 to
90 minutes, to achieve hardening thereof. In order to obtain
granules having a particle size of from 1 to 4 mm which are
most suitable for use in the fluidised bed production of lime
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and sulphur dioxide~ the granules ~re subsequentl~ sieved.
The use o~ the carbon~containing and sulphur~contai-
ning raw s.tarting materials in the production of the phospho-
gypsum granules has been found to favour the processing of
phosphogypsum into calcium oxide and sulphur dioxide by
- ` thermal dissociation.
The carbcn-containing raw material, the carbon ~
generally being in solid form, serves to accelerate and in-
crease the degree of thermal dissociation of phosphogypsum
by shifting the decomposition reaction to the right as a re-
sult of freed oxygen being combined with the carbon to form
carbon monoxide as follows: -`
Ca SO4 ~ C ->CaO + SO2+ CO ~;
The carbon-containing raw material in the granules
enables the required slightly reducing atmosphere in the
reaction space to be obtained as a result of the interaction
of the free carbon with carbon dioxide from the combustion of
- fuel for the heating.up of the reaction space, in accordance~`
with Boudoirs reaction which, at the optimum reaction tempe-
rature, that is at about 1200C, iS almost completely shifted
to the right.
The addition of.the carbon-containing raw material
as reducing agent enables full.use to be made of the fuel heat
as it undergoes complete combustion, while the availability
of reducing components in~the gaseous phase is ensured by the
solid carbon-containing reducing agent.
The thermal dissociation of phosphogypsum is an endo-
thermic process and the purpose of the sulphur is to compen-
sate for the heat absorbed in the thermal dissociation as the
sulphur itself undergoes combustion. The presence o~ sulphur
leads. to the production of a gaseous phase having a high
concentration of sulphur dioxide which renders it particularly
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favourable ~o~ processing into sulphuric acid,
The following Examples illustrate this invention.
EXAMPLE 1 ' ~ '
~ .
1 kg of phosphogypsum which had been dried for 3 hours-
at 105C was introduced into a drum-granulator. As the drum
~a~ was rotated at a speed of 41 r.p.m., 240 ml of water and 40 g
of urea were added to the contents of the drum. Granulation
was continued for 8 minutes, whereupon the granules obtained
were heat treated for 65 minutes at 120C, then sieved and the ;
fraction sized 1 to 3 mm was collected. This useful fraction
co,nstituted 85.9% of the granulation product. The granulate
possessed a compressive strength o~ 3~0 kg~cm2 and a resistance
to rubbing of 62.85~.
.
The granules were then subjected to thermal dissocia-
tion at 1250C in a pilot installation for a fluidised bed ~'~
process. The installation was 60 mm in diameter and I000 mm
, in height. The rate of flow of gas through the reactor was '
'- 0.32 m/sec and the rate of supply of granules thereto was 750 g
per hour. A reducing atmosphere was provided in the reactor by
2C supply of propane-butane gas. The calcium sulphate underwent
98.7% dissociation into su,lphur dioxide and calcium oxide and ~ ;~
the' dissociation of the entire amount of phosphogypsum granules
was completed within the space of several minutes.
EX~MPLE 2
1 kg of phosphogypsum, dried for 3 hours at 105C
wa5 placed in a drum granulator. As the drum underwent rotation
at a speed of 38 r. p. m., 24'0 ml of water and 10 y of carboxy-
methyl cellulose were added to the contents of the granulator.
,Granualtion was contlnued for 4 minutes whereupon the granules
obtained were heat treated for 45 minutes at 120C, sieved and
the fraction havlng a compressive strength of 15 kg~cm2, a
dynamic strength of 81% and a resistance to rubbing of 99.7%.
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The granules ~ere then sub~ected to thermal dissociation at
1220C in a pi.lot installa-tion for carrying out a fluidised bed
process. The rate of gas flow (propane-butane gas used as
reducing agent) was 0.36 m/sec and the rate of supply of yra-
nules was 600 g per hour. The degree of dissociation of cal-
cium sulphate obtained in the process which took several minu-
tes to complete was 96.8%.
EXAMPLE 3
~ .
1 kg of phosphogypsum was dried for 3 hours at 105C
and plac0d in a drum granulator. While continuously rotating
the drum at a rate of 40 r.p.m., 240 ml of water and 30 g of
ammonium nitrate were added to the contents thereof. The gra-
nulation process was allowed to proceed for 7 minutes, where-
upon the granules obtained were subjected to a heat treatment
at 120C for 65 minutes. The granu]es were then sieved and
the fraction 1 to 3 mm in size was retained for further use.
This fraction amounted to 75~ by wei.ght of t.he granulation
product and the granules possessed a compressive strength of
6.51 Xg/cm2 and a resistance to rubbing of 70%. The granules
wer~ then subjected to thermal dissociation under fluidised bed
conditions at temperatures ranging from 1200 -to 1350C.
EXAMPLE 4
1 kg of phosphogypsum was dried for 2.5 hours at
110C and then placed in a drum granulator. Continuous rota~
tion of the drum at a rate of 45 r.p.m. was then allowed to
take place and during the rotation of the druml 255 ml of water
and 48 g calcium nitrate were added to the contents thereof.
The granulation process took 10 minutes and the granules ob-
tained were then heated at 350C for 40 minutes. The granu-
lation product was then sieved and the ~rac-tion of si~e 1 to
3 mm was retained for further use. This fraction corresponded
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to 75~6 by ~ei~ht of the g~anulation product. The granules
possessed a compressive strength of 19.5 kg/cm2 and a resis-
tence to rubbing of 84%. The granules thus obtained were
subjected to thermal dissociation under fluidised bed condi-
tions at temperatures ranging from 1200 -to 1350C.
EXAMPLE 5
.
1 kg of phosphogypsum was dried for 2 hours at 110C
and was then introduced into a drum granulator. ~1.5 g of poly-
ethylene oxide in the form of a fract~on, 0.1 mm in size,
10 were added. The contents of the granulator were uniformly
mixed and were then mi~ed with 225 ml of water as the drum
underwent continuous rotation at 48 r.p.m.. Granulation took
11 minutes and the granules were heat treated at 6`50C for
15 minutes, then sieved and a fraction 1 to 3 mm in size was
retained for urther use. The higher temperature of the
heat treatment resulted in an increase in the strength o~
the granules. The granules were then submitted to thermal
:
dissociation under 1uidised bed'conditions at temperatures
,ranging from 1200 to 1350C.
EXAMPLE 6 ' ~ ~ ,
0.5 kg of phosphogypsum were dried for 3 hours at
105C, mixed with 40 g of fine coke particles and then intro-
duced tino a drum granulator. As the granulator underwent
rotation at 40 r.p.m. 120 ml of water together with 15 g of
urea were added to the contents thereof. The granulation ;~
took 9 minutes to complete. The granules were then submitted
to a treatment at 120C over the course of 60 minutes and the
fraction of size 1 to 3 mm thus obtaine,d was retained or
further use. This fraction represented 86,5% of the granulate
and possessed a static strength of 2.5 kg/cm2 and a resistance
to rubbing of 62.7%, The granules obtained were then subjected
to thermal decomposition in a fluidised bed at a temperature
28~
of 120Q to 1250C.
EXAMPLE 7
O,5 kg of phosphogypsum were dried for 3 hours at
105C and then introduced into a drum granulator. As the gra-
nulator underwent continuous rotation at 40 r.p.m. 120 ml (133 g)
of a distillation fluid obtained from soda production were added
by pulverization~ The granulation process lasted 8 minutes~
The granules obtained were then subjec-ted to a heat -treatment at
120C for 50 minutes and then sieved. The raction of parti-
cle size 1 to 3 mm was retained for further use. This fraction
represented 88~ of the granulate and possessed a stati~ strength ~ ;
of 4 kg/cm and an abrasive resistance of 62.7~. The granules
obtained were then subjected to thermal decomposition in a
fluidised bed at a temperature of 1200 to 1250C.
EXAMPLE 8
0~5 kg of phosphogypsum were dried for 3 hours at
- 150C and then mixed with 80 g of sulphur and placed in a drum
granulator. As the drum underwent continuous rotation at
40 r.p.m., 120 ml of water containing 20 g of urea were added.
The granulation process lasted 8 minutes. The ~ranules ob-
tained were then subjected to a heat treatment at 120C
for 65 minutes and then sieved. The fraction of particle size
1 to 3 mm was retained for further use. The 1 to 3 mm parti-
cles were used in the thermal decomposition of the phosphogyp-
sum thereof under fluidised bed conditions at a temperature of
1200 to 1250C.
Reference is next made to the following Table which
sets out the thermal stability characteristics of phosphogypsum
granules and admixtures thereof with fuel substances as
aforesaid.
In summary, the method of this invention enables
granules to be obtained which are suitable for use in the
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fluidised bed process.ing of their phospho~ypsum content to
produce calcium oxide and sulphur dioxide~ As can be seen
from the Table, the method allows phosphogypsum to be processed
into calcium oxide and sulphur dioxide without the occurrence
,-;of any sintering of the working mass at temperatures as high
as 1300C. The possibilities o~ working at temperatures of
higher then 1200C enabled the process to be carried out with
greater throughput unaer shorter reaction times so that there is :
obtained a high quality lime as well as a gaseous~product
10having a hlgher sulphur dioxide concentratlon then hitherto:. :
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