We are facing a problem in bringing the whiteness of material that is close to cement composition in our R&D trials. We would like to know the effect of each oxide and cooling method on whiteness of the material.

The principal colouring oxides are those of the first row of the transition metals. In cement the most likely to cause problems are iron, manganese and chromium in that order. The cooling of the clinker is absolutely fundamental to achieving the desired whiteness. You must
(i) burn the clinker as close to the nose ring as possible to eliminate the possibility of oxidation ofFeO to Fe2O3 in the kiln, and
(ii) immediately quench cool the clinker in water.
Air-cooled blast furnace slag can be added to the inlet of the cement kiln in the patented Cemstar process developed by TXI cement in the USA. I know a number of US cement companies are doing this. This is in the manufacture of cement powder. Another use is as an aggregate in the manufacture of concrete.

Please advise us that is it possible to use the combination of Ca(OH)₂ and CO₂ and waste of molasses (in sugar and lump sugar factories) for correction of raw material, instead of CaCO₃?

It is possible to use Ca(OH)₂ as a corrective source of CaO. I do not believe you would want to use CO₂ as this is one of the exhaust products from the cement manufacturing process rather than making a contribution to the final product. I have not heard of the use of molasses in cement manufacturing and can envisage two problems with this material: (i) it is likely to be sticky and present handling problems, and (ii) there will be some organic material in the molasses. This organic material will smoulder and burn in the upper stages of the preheater, or in the chain systems of long kilns and will give rise to CO and hydrocarbon emissions in the exhaust gases. To add this material safely to a cement kiln would require some means to add the molasses to the hotter sections of the process such as the precalciner or the kiln inlet of a preheater kiln or some mid-kiln firing system of a long kiln.

I need to know how and why companies use air cooled blast furnace slag in the manufacture of cement.

Air-cooled blast furnace slag can be added to the inlet of the cement kiln in the patented Cemstar process developed by TXI cement in the USA. I know a number of US cement companies are doing this. This is in the manufacture of cement powder. Another use is as an aggregate in the manufacture of concrete.

We have a problem related with high sand usage to make the consistent raw material mix. Sand addition for silica correction is roughly five per cent into raw material results roughly three per cent quartz content in the raw meal over 45 micron residue. Consequently the overall effect is frequent free lime formation in the clinker due to high quartz content in the raw meal, negatively effecting quality and high heat consumption in burning. In order to tackle with the problem following activities are already done
1. The burner pipe is replaced by a new design burner pipe with high momentum to improve burning and have stronger flame.
2. New coal dosing system is installed to have stable and accurate kiln fuel feed. Also with this system, we started to grind petcoke and other types of coal separately to facilitate burning. However the problem still persists.

I agree with you that the high quartz content above three per cent over 45 micron is the root of the problem. The new burner and the improved coal dosing can only be advantages but they are unlikely to solve this problem of low reactivity of the kiln feed. There are many white cement factories that are operating with very high silica sand additions. These would be the best references. My suggestion of a remedy would be to introduce duplex grinding where the rejects from the raw mill classifer are separately ground and reintroduced to the raw mix. This is a radical solution and requires process modifications to introduce, however it will solve the problem.

Our plant has four cement mills (closed circuit with first generation seprator) - we use about 20 per cent slag , the output is 100tph ,the dimension of every mill is 15.5 x 4.4, the liners in the first chamber are lifting and in second chamber classifier. Are there any ways to increase mill production? How about for a raw mill?

There are numerous ways to increase the production from your cement mills. In the short term use of grinding aids combined with optimisation of the ball charge and drafting would be the best method. Longer term (and at greater cost) you could upgrade the separators to third generation or install a roll press to pre-crush the clinker ahead of the mills. With regard to raw mills you can again use grinding aids or install pre-grinding equipment.

I would like to know the influence of iron ore addition in raw mix ie in burnability, specific heat consumption, etc.

The iron oxide additions in your raw mix increase the melt content at the clinkering temperature in the kiln. The iron oxide forms the mineral C₄AF which is a liquid in the burning zone. This fluxes the combination into the clinker minerals increasing the burnability and reducing the fuel consumption.

I am a Chemical Engineer working in a company in the cement area in South America and the question is: What sort of fluorspar (acid, metallurgic) is used as fluxing agent in clinker process, what percentage of fluorspar and what quantity in the raw mix fed to the kiln. Does it imply savings in heat?

In my experience acid grade fluorspar spar is used as a mineraliser, however there is no reason why other grades should not be used. Again in my experience addition rates have been up to 0.4 per cent to kiln feed, achieving unit fuel consumption savings in excess of 200 kJ/kg clinker.

In our plant, we have a typical problem of high wear rate of mill liners. The basic reason identified is the percentage of free silica in silica (one of the components of raw meal) is very high about 50 per cent and the consumption of this material is also very high (around 14 per cent) due the very high quality of limestone (low grade limestone, clay or shale is not available in near vicinity) and thus making kiln feed free silica around 9 to 10 per cent. We are of the opinion that the recirculation of the O-Sepa is causing a cummulative effect on the percentage of free silica inside the mill, which is the reason why we see that the total life time of the diaphragm plates is continually reducing (from six months to four months to three months). We would like to know, are you aware of any recent developments in design which can provide an arrangements to remove this high density coarse grain material out of the mill to avoid high abrasion of mill liners and diaphragm plates?