I want to know about calculation methods of gas and material flow in preheater, pc, kiln, cooler. Further, can you advise why we are getting brown clinker problem in our 2000tpd off-line calciner kiln from which we are recently producing 2500tpd while having 90-91 LSF, 2.26 SM, 1.4 AM and kiln feed residue up to 22 per cent. How we can improve our clinker quality?

The calculation of gas and material flow in the preheater relies on the principles of stoichoimetry (to assess the combustion gas products from the fuel) and the conservation of mass. In principle what goes into the kiln must come out somewhere and will allow you to prepare a mass balance for the kiln. With regard to the brown clinker you are producing this is most probably caused by having reducing burning conditions in the burning zone of the kiln. This is very bad for clinker quality and fuel efficiency of the kiln. Almost certainly the problem is with the kiln burner and might be solved by adjustment. You must be sure that you maintain at least one per cent oxygen at the kiln inlet.

It is known in all cement standards that the Initial Setting Time is 45 minutes minimum and the Final Setting Time is 10 hours maximum. Is there any definite time to be considered between end of initial setting and start of final setting to ensure cement quality and workability.

There is no definite interval between the initial and the final set of cement that denotes good or bad quality or workability. You must bear in mind that the test for setting time is conducted at a constant temperature in the laboratory. This is not the case when the cement is used in practice. In cold countries the setting time extends in practical applications and therefore a short setting time is best in these conditions. In hot countries (such as the Sudan) the setting time is shortened in practice and therefore a longer set in the laboratory conditions would be desirable.

I wish to know if the test of consistency of cement pastes has any significance as regards the quality of cement eg a cement of lower consistency (<25 per cent) is better than a cement of higher consistency (>25 per cent water). Secondly: It is observed that most of international cement standards set limits only to mortar compressive strengths although in practice the main usage of cement is in concrete applications. May I know why is that ?

The standard consistency test certainly has significance regarding the quality of the cement. You need to think in terms of the "concrete making potential" of the cement. In the test the amount of required to produce a paste of standard consistency is determined. This relates directly to the water demand and workability of concrete made from the cement. If more water must be added to achieve the desired consistency or workability then the strength development of the concrete will be impaired and more cement will need to be added to the concrete to compensate for the additional water. The reason for most standards adopting mortar testing of cement is to promote consistency and comparability between tests and laboratories over time. By using a tightly specified grading of testing sand the influences of the aggregates on the strength testing are eliminated or minimised. This means that the strength development potential of the cement is being measured by the standard mortar test.

We are analysing particle size distribution of raw meal, coal mill product and cement mill product through CELAS PSA having facility to determine 0.7 to 400 micron. Q1. What is the best particle distribution (on various micron sizes) of raw mill in terms of best mill performance, burnability and kiln performance and which plant in the world is producing? Q2. What is the best particle distribution(on various micron sizes) of coal mill in terms of best mill and kiln performance? Our coal ash is 28. Q3. What is the best particle distribution( on various micron sizes) of cement mill product to have optimum power consumption, best strength and good performance in concrete.

There is no straightforward answer to your questions. For raw mix the optimum particle size distribution depends on the mineral composition of your raw materials. If you have a high content of quartz then you must have a very tight fine particle size distribution with low content of oversize particles where the quartz will concentrate. On the other hand with a homogenous raw mix with no quartz it is not beneficial to have too small a particle size distribution as this will increase the dust losses from the top stage of the preheater. Similarly with coal there is no hard and fast rule. It is said that the 90 micron residue should not be more than 50 per cent of the volatile content of the coal. Increasing the fineness beyond that is counter-productive. For cement the optimum particle size distribution depends on the cement performance characteristics that your customers have come to expect. Ring formation is caused by the formation of liquids in the kiln at a particular position. These penetrate the refractory lining and cause a localised thickening of the coating. Fe₂O₃ can be a cause of these problems and I am not surprised that raising the alumina modulus has solved the problem. Many cement factories operate with higher alumina modulus than 1.7 and I would recommend keeping the kiln feed mix design with the higher alumina modulus. A snowman is the formation of a large build-up on the first grate of the cooler where the clinker falls from the kiln rather than a build-up in the kiln.

We are facing a problem in bringing the whiteness of material that is close to cement composition in our RandD 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 of FeO 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.

I would be very interested to hear any pointers you might have about where how best to get started on using microscopy as a further tool for examining clinker ? Where training might be obtained ? Anyone you might recommend that I could contact and who could possibly come on site and provide this training ?

An article describing the work we have been doing in Slovakia will appear in the February 2003 issue of International Cement Review. In part this describes how microscopy is used in combination with pattern recognition in hour-to-hour control of the real mineralogy of clinker.

Recently we started production of SRC with 97.5 LSF, 2.35 SM, and 0.75 AM raw meal composition. After that free lime become normal (i.e 1.1 - 1.20). Do you think that with OPC we were operating with high alumina modulus? (Continuation question)

I don't think the high free CaO was associated with the alumina modulus itself. However, the mix of raw materials must have changed in the switch to SRC production. Which raw materials have reduced significantly? Whichever one is the most likely culprit. We can rule out the coal as presumably this remains the same blend as when OPC was produced.

I understand that the cement manufacturing industry is considering the use of copperas (ferrous sulphate) in the manufacture of cement to reduce chromium 6 allergy problems for the end user. My company produces the only trouble free copperas saturator on the market. Our saturator accepts crystal copperas generates a stream of aqueous ferrous sulphate solution for dosing mainly to sewage to remove phosphates or to control odour and septicity in sewers and sewage works. I am interested to understand the mechanisms underlieing the use of copperas for cement production.