Chemical Engineering June 2016 - 36

Technology Profile
Precipitated Calcium Carbonate from Limestone
By Intratec Solutions
C
alcium carbonate (CaCO3) is
a mineral that occurs naturally
in chalks, limestones
and marbles. Its largest use
is in the pulp-and-paper industry as a
filler and pigment. Other calcium carbonate
applications include its use in
building construction, plant and crop
fertilizers, food additives, water and
sewage treatment, ceramics, pharmaceuticals
and cosmetics.
The process
The following paragraphs describe a
carbonization process for precipitated
calcium carbonate (PCC) production
from limestone. Figure 1 presents a
simplified process flow diagram, showing
the main pieces of equipment.
Calcination. Initially, the limestone
from a quarry is stored in bins. Typical
extracted limestone contains about
75 wt.% of CaCO3. Mined limestone
is crushed in a jaw crusher and calcined
(burned) in a vertical-shaft lime
kiln at about 1,000°C. In the kiln, the
calcium carbonate present in the
limestone decomposes into calcium
oxide (CaO; also known as lime or
quicklime) and carbon dioxide, which
is captured and treated for reuse in
the process.
Slaking. The CaO is fed to a stirred
slaker tank, where it is hydrated (slaked)
with water to form a calcium-hydroxide
slurry called slaked lime or slake. Before
carbonation, the impurities (silica
and other oxides) originating from the
limestone are separated from the slake,
producing a pure slaked lime.
Carbonation. The pure slaked lime
is then fed to a set of atmospheric
stirred reactors, where it is diluted and
Limestone
from quarry
1
4
2
5
Heavies
Fuel
Figure 1. The process flow diagram shows precipitated calcium carbonate (PCC) production from limestone
36
ChemiCal engineering www.Chemengonline.Com june 2016
6
7
8
PCC bags
3
n Raw material n Process n Main product
Pulp-and
paper-mill
Carbon dioxide
present in stack
gas emissions
Lime or limestone
PCC plant
PCC
Figure 2. This diagram shows how a PCC plant could be a satellite plant for a pulp-and-paper mill
subsequently reacted with the recovered
carbon dioxide from the lime kiln.
The carbonation reaction is regulated
by the equilibrium of the solution: as
calcium ions are converted to calcium
carbonate and precipitated out of solution,
more calcium hydroxide dissolves
to equalize the concentration
of calcium ions.
Drying, grinding and finishing. The
slurry from the carbonators is sent to
a filter press, which removes most of
the water. Recovered water is recycled
for reuse in the slaking step. The
PCC from the filter is dried with hot air,
de-agglomerated in grinders, packed
and stored.
Economic performance
An economic evaluation of the process
described was conducted
based on data from the first quarter,
2014. The scope of this analysis assumes
a PCC plant located near a
limestone quarry.
The total capital investment required
to construct a plant with the
capacity to produce 50,000 metric
tons per year of PCC in the U.S. is
estimated at about $21 million. The
capital investment presented includes
fixed capital, working capital
and additional capital requirements.
The production costs (including
Process water
costs associated with plant operation,
product sales, administration,
R&D activities and depreciation) are
about $260/ton of PCC produced.
Process integration
When PCC plants are integrated with
pulp-and-paper mills (Figure 2), they
are called PCC satellite plants. These
plants use the stack gas emissions
from the lime kiln in the paper mill,
mainly because of its higher carbon
dioxide content. PCC satellite plants
commonly use lime as a main raw
material, rather than limestone. This
integration between the two processes
has a major effect on the emission
profile of the pulp-and-paper mill,
since greenhouse gas emissions are
significantly reduced.
This column is based on " Calcium
Carbonate Production from Limestone:
Cost Analysis, " a report published
by Intratec. It can be found at:
www.intratec.us/analysis/calciumcarbonate-production-cost.
n
Edited
by Scott Jenkins
Editor's Note: The content for this column is supplied by
Intratec Solutions LLC (Houston; www.intratec.us) and edited
by Chemical Engineering. The analyses and models presented
are prepared on the basis of publicly available and nonconfidential
information. The content represents the opinions
of Intratec only. More information about the methodology for
preparing analysis can be found, along with terms of use, at
www.intratec.us/che.
1. Limestone storage & crushing
2. Lime kiln
3. Carbon dioxide recovery area
4. Slaker
5. Carbonator
6. Water removal area
7. Grinder
8. PCC storage & packing

http://www.intratec.us/analysis/calcium http://www.intratec.us http://www.intratec.us/che http://www.Chemengonline.Com

Chemical Engineering June 2016

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