CNI Indonesia Laterite Hydrometallurgy Project
Chapter 7 The Sulfuric Acid Plant
7.1 Summary
The hydro-metallurgical plant, which uses the HPAL process, each
year consumes about 1.1 million ton of sulfuric acid. After
comparison, it is decided to use sulfur as the raw material for acid
production. The acid product will be used for leaching. Waste heat
generated from the acid plant will be recovered in the form of steam,
which will be used in the hydro-metallurgical plant as well.
The sulfuric acid plant consists of a number of systems, including
sulfur stockpile and feeding system, sulfur heating system, sulfur
burning and converting system, drying and absorption system,
offgas desulfurization system and stack, fan house, acid storage
tanks, and auxiliaries like waste heat recovery system, cooling
water system, pipe networks, and office building.
The chemical process of acid production: sulfur is burned to
produce sulfur dioxide; sulfur dioxide is oxidized to sulfur trioxide;
sulfur trioxide is absorbed into sulfuric acid, to produce
concentrated sulfuric acid. From sulfur burning to sulfur trioxide
absorption, each step in the process is exothermic, releasing great
amounts of heat. Such heat will be recovered through a waste heat
boiler and economizers, to produce 6.4MPa steam.
7.2 Raw materials
An investigation of the local material supplies shows that solid
sulfur and pyrites can be used as the raw material for acid
production.
7.2.1 Raw sulfur
Raw Sulfur will be supplied as follows:
Specification:
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Color: yellow
Grain size: granule /prill of between 2-10 mm
Chemical compositions see table 7.2.1-1：
Table 7.2.1-1 Chemical compositions of raw sulfur
Description
Content
Unit
Limit
Sulfur content
99.5
% wt/wt
min
Carbon
0.10 (supposed)
% wt/wt
max
Ash
0.05
% wt/wt
max
Acidity as H2SO4 0.20 (supposed)
% wt/wt
max
Free SO2
0.20 (supposed)
% wt/wt
max
Moisture
0.50
% wt/wt
max
Arsenic
0.25 (supposed)
ppm
max
Selenium
1.0 (supposed)
ppm
max
Tellurium
1.0 (supposed)
ppm
max
7.2.2 Pyrite
The estimate S content is based on 35%.
7.3 Technical scheme for sulfuric acid plant
7.3.1 Process for sulfur-based sulfuric acid plant
For sulfur-based sulfuric plant, the process as follows is suggested,
sulfur melting, waste heat recovery, “3+1” double conversion and
double absorption (DCDA). The dilute acid from cleaning section
after contaminated acid handling will be sent to the existing water
treatment plant for further treatment.
The “3+1” four-stage conversion is adopted. The system can be
self-heating balance and excess heat can be used to heat the boiler
feed water. SO3 in the offgas coming from the third and fourth beds
is absorbed by concentrated sulfuric acid in IAT and FAT, and outlet
offgas of FAT will be discharged via tail gas stack. The product acid
will be discharged from circulating tank of absorption section, and
sent to acid storage after cooling by product acid cooler.
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7.3.2 Process for pyrite-based sulfuric acid plant
For pyrite-based sulfuric plant ,the process as follows is suggested,
fluidized bed furnace for direct oxidation roasting, waste heat
recovery, dust collection with dry method, dilute acid scrubbing and
cleaning, as well as DCDA. The dilute acid from cleaning section
after contaminated acid handling will be sent to the existing water
treatment plant for further treatment.
The roasting plant is provided with one roaster and a bin is
equipped in front of the roaster. The pyrite is delivered to the
roaster via belt conveyor, which is equipped with weighing device to
monitor the feeding amount to the roaster. The air is blown into the
roaster via blower. The roasting offgas will be treated by waste heat
boiler, cyclone dust collector, ESP, and then sent to acid system.
The calcine together with the dust from WHB and dust collecting
system will be cooled by cooling drum and then discharged out of
the battery limit.
The offgas from dust collecting system enters the gas cleaning
system and then SO3, dust, As and other impurities in the offgas
are scrubbed by the circulating liquid. After the acid mist is removed
by two-stage WESP, the cleaned offgas enters drying tower and
dried, then will be sent to conversion section via SO2 blower. The
“3+1” four-stage conversion is adopted. The system can be
self-heating balance and excess heat can be used to heat the boiler
feed water. SO3 in the offgas coming from the third and fourth beds
is absorbed by concentrated sulfuric acid in IAT and FAT, and outlet
offgas of FAT will be discharged via tail gas stack. The product acid
will be discharged from circulating tank of absorption section, and
sent to acid storage after cooling by product acid cooler.
7.3.3 Raw material option comparison
Table 7.3.3-1, 7.3.3-2 and 7.3.3-3 show the advantages and
disadvantages of sulfur and pyrite, and their power consumption.
Table 7.3.3-1 Comparison of Sulfur and Pyrite as Raw Material for
Acid Making
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Description
Raw material
Process flow
Investment
Operating costs
Environmental
impact
Sulfur
Stable
composition,
abundant supply.
Short process
flow, stable
production
Relatively low
Low OPEX, see
Table 7-2
No pollution to the
environment
Pyrite
Unstable composition
Long process flow; process
subject to fluctuation as the
feed composition may vary.
High investment, about
1.5-1.8 times as high as the
sulfur option.
High OPEX, see Table 7-3
Producing solid waste and
acidic effluent, causing
secondary pollution
Table 7.3.3-2 Material and power consumption - the sulfur option
(USD)
Consumption per ton of Unit
Price
No.
Item
Unit
acid
price
1
Sulfur
t
0.331
180 59.58
Catalyst and
2
reagents
2.1
Catalyst
L
0.058
5
0.29
Electric
3
power
Kwh
65
0.06
3.9
4
Water
t
3.7
0.6
2.22
-18.7
5
Steam
t
-1.25
15
5
Tota
l
47.24
Table 7.3.3-3 Material and power consumption – the pyrite option
(USD)
Consumption per ton of Unit
No.
Item
Unit
acid
price Price
1
Pyrite
t
1
60
60
Catalyst and
2
reagents
2.1
Catalyst
L
0.058
5
0.29
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3
4
5
Tota
l
Electric
power
Water
Steam
kWh
t
t
110
4
-1
0.06
0.6
15
6.6
2.4
-15
54.2
9
Both options are mature in process and applicable, but in economic
and social terms, the sulfur option outperforms the other. Therefore
in this report the sulfur option is recommended.
7.3.4 Reactions in the sulfur option process
When sulfur is used as the raw material for producing H2SO4, the
process involves the following steps:
(1) Sulfur is burned, according to the reaction: S+O2=SO2;
(2) SO2 is converted to SO3, according to the following reaction:
SO2+1/2O2=SO3
(3) SO3 is absorbed into sulfuric acid solution. SO3 gas combines
with water H2, producing H2SO4, according to the following reaction:
SO3+H2O=H2SO4
7.3.5 Determining the process path
(1) Sulfur processing: as the sulfur material composition is
uncertain for now, 1 stage of filtration will be incorporated to remove
impurities in the raw material;
(2)
Conversion
and
absorption:
the
“3+1”
double-conversion-double-absorption process will be used, with
imported high-efficiency low-pressure reduced vanadium catalyst.
The total SO2 conversion rate will be over 99.82%, and the total
SO3 absorption rate will be above 99.95%. The emission will be up
to
environmental
standard
(SO2≤800mg/Nm3,
acid
3
mist≤40mg/Nm ).
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(3) Heat recovery: the high-temperature heat generated from the
sulfur burning process and the medium-temperature heat from the
conversion of SO2 will be utilized. A waste heat boiler will be
installed and connected to the rear end of the sulfur burning vessel,
and the boiler and some economizers will be involved in the
conversion process. The heat will be recovered in the form of high
pressure saturated steam (280℃, 6.4MPa), which will be sent to
the steam pipe network of the plant.
(4) DCS: Advanced DCS control system will be used for operation
control and monitoring. Main process parameters, such as pressure,
temperature, flow rate, motor status, stream discharge, will be
monitored in a centralized manner, and be regulated automatically
or by human intervention. Such a control system can achieve stable
process operation, improved product quality, remarkably better
hygiene conditions and greatly reduced labor intensity.
7.4 Capacity and product
The smelter will have three high-pressure acid-leaching production
lines. To be aligned with the maintenance and working regime of
the smelter, the acid plant will have two production lines, each with
a capacity of 550000t of 98% sulfuric acid/a. The product acid
quality will meet the requirements for prime product in the Chinese
national standard on industrial sulfuric acid (GB/T 534-2014).
7.5 Process description
This process involves the sulfur stockpile yard and the feeding
system, the sulfur heating system, the sulfur burning and
conversion system, the drying and absorption system, the offgas
desulfurization system and the stack. Of these, the sulfur stockpile
yard, the feeding system, the sulfur heating system, the offgas
desulfurization system and the stack will be shared by the two
production lines. For details of the process flow, see the e1uipment
connection drawing Z1589-0-001-EA21.
7.5.1 Sulfur storage
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Solid sulfur shipped from the wharf will be carried by belt conveyors
to the stockpile yard, which covers about 7500m2. The yard will be
encircled with 4m retaining walls, and will have drainage facilities.
Sulfur in the stockpile yard will be carried by loaders to the feed
hoppers. From here, the sulfur will be conveyed through the feed
belt, the storage hoppers, another feed belt, metal separator, until it
reaches the sulfur melting tank. In the conveyance process, quick
lime, as a neutralizer, will be added. The lime charging device will
be installed on one side of the feed belt.
7.5.2 Sulfur melting
Sulfur feed is transferred by shovel, which is sent to the rapid sulfur
melting tank via the loading hopper, belt conveyor and iron
separator, during which lime is added as neutralizer. There are two
belt conveyance system which will keep sulfur loading and melting
during routine maintenance / cleaning for sulfur tanks.
Sulfur in the sulfur melting tank begins to melt after heated by
steam tubes. The molten coarse sulfur overflows into the filtration
tank, and then delivered into molten sulfur filter by filter pump,
where the impurity solids are removed. It then flows into the sulfur
storage tank via residual pressure which will save intermediate
molten sulfur tank.
Molten sulfur flows by gravity from molten sulfur storage tank into
pure sulfur tank, which is finally sent into the sulfur furnace by pure
sulfur pump.
Steam heating tubes are installed in the sulfur melting tank, filtration
tank, molten sulfur storage tank as well as the pure sulfur tank.
Sulfur in this tanks is kept in molten status by indirect melting or
insulating steam which comes from low temperature heat resource
recovery system in drying & absorbing section.
After condensation of the steam used for heating or insulating of
the various tanks, condensate water is collected and recycled.
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7.5.3 Sulfur combustion, converting and WHB
① Sulfur combustion
Molten sulfur is pressurized by pure sulfur pump and is injected into
sulfur furnace respectively by three sulfur lances.
In sulfur furnace, dry air is mixed with the sprayed fine particles of
liquid sulfur and combust to generate high-temperature combustion
gas, which enters waste heat boiler for heat recovery before goes
into the conversion section.
The air which used for sulfur combust is dried by strong acid in
drying tower, pressurized by main blower, recovered heat by
economizer and the enters into sulfur furnace both from head and
middle part with certain quantity ratio which will decrease sublimed
sulfur.
② Converting
The conversion section usually adopts “3+1” stage conversion
process.
SO2-laden gas (referred to as SO2 gas) from the #1 Waste Heat
Boiler directly enters the first catalyst stage in the converter for the
first oxidization reaction. The resulted SO3 and SO2 laden gas
(referred to as SO3 gas) then enters into the #2 Waste Heat Boiler
for heat exchange / recovery, and the cooled gas enters into the
second catalyst stage in the converter for secondary reaction
Steam produced by waster heat recovery system is sent to the
hydrometallurgy plant.
SO3 gas from the secondary reaction has a temperature gain,
which is then cooled by the hot heat ex-changer and enters into the
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third catalyst stage of the converter for a third reaction.
SO3 gas from the third reaction enters the cold heat ex-changer and
#2 economizer one after another to recovery the heat, and is sent
to the intermediate absorbing tower after cooling.
Gas that is not absorbed in the intermediate absorbing tower is
heated by the cold heat ex-changer and hot heat ex-changer before
it enters into the fourth catalyst stage of the converter for a fourth
reaction.
SO3 gas from the fourth reaction has its heat recovered by the
low-temperature super-heater and #1 economizer before enters the
final absorbing tower. Gas from the tower is then discharged into
the atmosphere through the chimney.
③ Waste heat recovery
The WHB system comprises #1WHB, #2WHB, #1 economizer, #2
economizer. Boiler is used to cool high-temperature gas produced
by the sulfur furnace and first catalyst stage for the purpose of heat
recovery. #1 and #2 heat-tube economizers are used to heat the
boiler makeup water (de- oxygenated water).
De-oxygenated water is sent into the low-temperature section of #1
economizer by the boiler feed-water pump, where it is heated up
before entering #2 economizer. After heated up again in #2
economizer, it returns to the high-temperature section of #1
economizer where it is heated up the third time and then entering
into the boiler drum.
In the boiler drum, the de- oxygenated water mixes with the boiler
water, and enters into boiler proper via down-taker. After being
heated by the high-temperature gas, it returns to the boiler drum.
Steam-water mixture is separated in the boiler drum.
3) Drying & absorbing
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① Air drying
Air is filtered by filter, silenced by sound eliminator, dried by strong
sulfuric acid in the drying tower, pressurized by the air blower, heat
recovered by economizer and then delivered into sulfur furnace.
After moisture absorption, the sulfuric acid flows into the acid
circulating tank from bottom of the tower, which is delivered into the
final absorption tower by the acid pump located on top of the
circulating tank.
② Intermediate absorbing
SO3 gas from the cold heat ex-changer and #2 economizer enters
into the intermediate absorbing tower to have SO3 absorbed by
strong sulfuric acid spray before return to the conversion section.
Sulfuric acid with SO3 being absorbed flows into the circulating acid
tank from bottom of the tower, which is sent into the interpass acid
cooler via the acid pump installed on top of the tank. After cooling
by the circulating water, most of the sulfuric acid enters the
interpass absorption tower, from there some amount of product
acid is led out and sent to the product acid cooler. After cooling by
the circulating water, the product acid is sent to the acid storage
tank.
③ Final absorbing
SO3 gas entered into final absorbing tower from the #1 economizer
is stripped of SO3 by strong sulfuric acid spray before it is
discharged into the atmosphere through the chimney.
Sulfuric acid with SO3 being absorbed flows into the circulating acid
tank from bottom of the tower, which is sent into the acid cooler of
the final absorption tower by the acid pump installed on top of the
tank. After cooling by the circulating water, the acid is pumped into
the drying tower.
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4) Start-up off-gas scrubbing
Off-gas scrubbing system is mainly used during start-up plant,
shut-down plant and emergency status. Sodium hydroxide is
dissolved in the alkali tank, pumped into off-gas tower and sprayed
for absorbing slightly SO2 & SO3 before stack emission.
7.6 Main equipment selection
7.6.1 Principles for equipment selection
Equipment was selected on the basis of acid production (98%
sulfuric acid). Also the characteristics of the production process, the
potential fluctuations in production, and the SO2emission standard
have been considered.
Advanced, energy-saving, and capable equipment was selected,
and equipment using new technologies were chosen as much as
possible, while costs were kept as low as possible.
Considering the characteristics of this project, Chinese products
with good stability, wide adaptability, sound operability, and low
resistance loss were chosen.
7.6.2 Features of main equipment
(1) Converting equipment
The converter is of all-stainless steel structure. Compared with
traditional
carbon-steeled
converters
with
brick
lining,
stainless-steeled converters are lighter and are still strong and rigid
in high temperatures. Such properties can reduce the occurrence of
gas leakage or stream mingling. Besides, stainless-steel converters
are easier to install, and require less maintenance.
The
converter has been so designed that some spare space has been
reserved for each catalyst layer, so that more catalyst can be added
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to improve converting efficiency and bring down SO2 emission.
(2) Drying and absorbing equipment
The drying tower and the absorption tower are steel structures lined
with bricks. The towers have disc-shaped bottoms, and acid is
discharged beneath. Concrete grid will be used to support the
towers, instead of the traditional bulky weight-bearing platform, thus
saving investment. To reduce acid mist generated from the drying
and the absorption towers, to protect the equipment from corrosion
and to ensure that the offgas emission is up to standard, a mesh
demister will be installed at the outlet of the drying tower, and a
high-efficiency candle-shaped fiber demister will be installed at the
absorbing tower outlet. Both demisters will be of 316L for their
casings. The concentrated acid coolers inside the two towers are
Chinese-made anode protection tube-and-shell ones. Such coolers
are easy to manage, efficient and endurable. Also they require only
a small footprint, and are low in capital and operational costs.
7.6.3 Equipment List
See the attached List of the Main Equipment
7.7 Main tech-economic indexes
See Table 7.7-1 for the main tech-economic indexes.
Table 7.7-1 Main Tech-Economic Indexes
No.
1
2
Item
Raw material:
sulfur (moisture
2%, dry-basis
material contains
about 99.8%wt S)
Product: 98%
sulfuric acid
Unit
Series 2
212000
Series 3
312000
t/a
t/h
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CNI Indonesia Laterite Hydrometallurgy Project
High pressure
saturated steam
(6.4MPa 280℃)
3
4
5
6
7
Sulfur utilization
rate
Converting
efficiency
Absorption
efficiency
Desulfurization
efficiency
Offgas discharge
(dry basis)
Of which, SO2
t/h
See
See
thermal
thermal
engineer’s engineer’s
work
work
%
99.82
99.82
%
99.95
99.95
%
90
90
680
680
<40
<40
225
Nm3/h
mg/Nm3
Acid mist
mg/Nm3
Catalyst packing
l/d·t·100%H2SO4
factor
Consumption
Diesel (for furnace
t
heating up)
Low pressure
t/h
steam
Lime powder
t/a
225
1500
14
85
t/a
60
l/t·100%H2SO4
0.1
8
Filtering aid
Catalyst
consumption
Staffing
People
60
60
9
Working regime
d/a
330
330
h/d
24
24
0.1
7.8 Plant layout
All the systems were laid out according to the local terrain and the
needs of the process (the sulfur stockpile yard, the feeding system,
the sulfur heating system, the sulfur burning and converting system,
the drying and absorbing system, the desulfurization system and
the stack). For a plan view of the layout of each system, see
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drawing Z1589-0-001-EA21-8~13; for the overall layout of the
whole plant, see the attached drawing by the GA engineer.
7.9 Existing Issues and Suggestions
For offgas emission standards and stack height, the Owner had
better communicate with the local environmental authorities to
ensure regulatory compliance.
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