Cement Kiln Chemistry 1.

8 Chloride Cycles
Cement Kiln Process Chemistry

Module 1. Cement kiln energy efficiency and productivity.
1.8 Chloride cycles, bypasses and purges.

In the preceding session we talked about the general principles of the alkali cycles, in this
session we will consider the cycles of chlorides in the cement kiln.


Melting Point
NaCl 801C
KCl 776C
Alkali chlorides melt at ~800C or less:

When mixed with sulphates, K
2
SO
4
, Na
2
SO
4
, CaSO
4
,
low temperature eutectic melting takes place below
700C.

As the condensed chlorides are carried into the rotary
kiln with the feed the vapour pressure increases with
temperature until the boiling point is reached:
Boiling Point
NaCl 1440C
KCl 1411C

These boiling points of the alkali chlorides are lower than the burning zone temperature
of a cement kiln. It is therefore very difficult for any chloride to pass through the kiln and
exit in the clinker. Chloride volatility in a cement kiln is very high and the concentration
in the hot meal can be ~100x that in the feed and fuel inputs to the kiln.

Cl volatility =1 Cl %
Clinker
/Cl %
Hot Meal


99% =100% 0.02%/2.00%

In a closed system the outputs in clinker must equal the inputs in the feed and fuel. If the
inputs exceed 0.02% per kg clinker then the concentration in hot meal is likely to exceed
2% (loss free), preheater blockage problems are likely at this Cl concentration in hot
meal.

If the chloride inputs exceed 0.02% then some way is required to break the cycle of
chlorides. One solution to this chloride cycle problem can be the installation of an alkali
bypass or bleed.

Part of the exhaust gases exiting the rotary kiln to the
preheater are extracted before entering the
preheater

and quench cooled to precipitate the volatiles.


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Cement Kiln Chemistry 1.8 Chloride Cycles
The effectiveness of alkali bleeds or bypasses is closely connected with the cycle itself.

A bypass relies on the concentration of recirculating
volatile material being significantly higher than in either
the feed, fuel or clinker.

This means that bleeding out a small proportion of
the recirculating material

is sufficient to take out the equivalent amount that
is entering in the feed and fuel.



The concentration in the recirculating material reduces,
and with it, the tendency for the preheater to block.









The concentration factor of recirculating volatile material is the ratio of the quantity in
the hot meal to the quantities entering the kiln in the feed and the fuel. Concentration
factor is directly related to the volatility of the recirculating species in the burning zone.

Alkali bypasses are effective for highly volatile materials which have high concentration
factors. Chlorides have a very high volatility in the burning zone (>97%), and therefore
high concentration factors. Alkali bypasses are very effective in reducing the cycle of
chlorides, and alleviating chloride based clogging of the preheater.

If the raw materials and/or fuel have chloride content in excess of 0.03% on clinker an
alkali bypass will be required to allow the kiln to operate without preheater clogging
problems. When alternative fuels are burnt in a cement kiln, installation of an alkali
bypass can be necessary to alleviate chloride based preheater clogging problems.

When a bypass is installed on a cement kiln the volatility of the potassium, K
+
, and
sodium, Na
+
, in the feed can be increased by the addition of chloride to the feed as
calcium chloride, CaCl
2
. Potassium and sodium will preferentially combine with chloride
in the hot meal of the kiln and their volatility will be boosted by the volatility of the
chloride. This means that a bypass can be used to produce low alkali clinker from high
alkali raw materials if calcium chloride is added to the kiln feed.

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Cement Kiln Chemistry 1.8 Chloride Cycles

The precalciner kiln process particularly lends itself to the installation and operation of an
alkali bypass.

Less than 50% of the fuel is fired in the main burner of the kiln and the feed is virtually
fully calcined by the time it enters the rotary section of the kiln. Alkalis volatilised in the
rotary kiln are therefore concentrated in the lower volume of kiln exit exhaust gases,
therefore a lower percentage of the kiln exit gases have to be bypassed to break the
chloride cycle.

The operation of an alkali bypass needs to be optimised to minimise the amount of dust
drawn out of the kiln exit with the bypassed gas. 1 gram of dust per kilogram of clinker
produced per percent of bypassed gas should be possible by correct siting of the bypass
take-off. Disposing of the bypassed dust can be a major problem, therefore the less drawn
out of the kiln the better.

Drawing kiln exit gases out of the kiln at 1000C+, and then quenching those gases
inevitably imposes a thermal energy penalty on the process. Each percent of bypass
means ~20 kJ /kg clinker increased fuel consumption on a preheater kiln or 10 kJ /kg
clinker per percent of bypass on a precalciner kiln as the volume of hot kiln inlet exhaust
gases is that much less.

However, if no bypass is installed then that solution is not available to solve chloride
cycle problems.
Because the melting point
of chlorides is so low they
condense higher up the
preheater. This means that
the amount of chloride
exiting the preheater in the
dust to the external cycle
can be as high as the total
inputs in the feed and fuel.
Purging some of the dust
and interrupting the external
cycle can then be sufficient
to avoid preheater
blockages. As much
chloride can be purged in
the dust as is entering the
kiln, of course there is then
the problem of disposing of
the purged dust. This
solution is more applicable
to grate preheater and 1 or 2
stage preheater kilns.
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Cement Kiln Chemistry 1.8 Chloride Cycles
iv
A further solution to chloride problems can be to modify the material flow in the
preheater creating meal curtain at the kiln inlet.


Some cool material
from the upper stages
of the preheater is
diverted to the kiln
inlet.

The chloride in the
gas exiting the kiln
condenses on the cool
meal in the curtain.

The chlorides are
intercepted before
they reach the
preheater and
therefore cannot cause
blockages of the
preheater.



Bypasses and dust purges are much less effective as solutions to sulphate recirculation
problems. Sulphur inputs to cement kilns are significantly higher than chloride inputs.
The volatility of sulphates is not as high as chlorides so the concentration factor is not
high enough for a bypass or dust purge to be effective.

We will consider the chemistry of sulphates in the cement kiln in the next session of the
course.