KC 1.8 Chloride Cycles.pdf

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.

Alkali chlorides melt at ~800°C or less: When mixed with sulphates, K2SO4, Na2SO4, CaSO4, low temperature eutectic melting takes place below 700°C. 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:

NaCl KCl

Melting Point 801°C 776°C

NaCl KCl

Boiling Point 1440°C 1411°C

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, CaCl2. 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 1000°C+, 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

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.

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