The silica will inevitably build up in the recirculating load of the mill as this is the hardest component of the raw mix. The limestone is preferentially ground leaving the silica in the rejects. Unfortunately I don't know of any means to avoid this.

Can I have the details about "patented TXI Cemstar addition of air cooled slag to cement kilns". Do you have any information of any cement plant using Blast furnace slag as raw material for rawmix. What are the basic requirements and the precaution we must take to use GBFS as a rawmix component. Will it affect productivity of the mill as I heard that slag is very difficult to grind.

The TXI Cemstar process involves the addition of air cooled slag to the inlet of the rotary kiln with the aim of boosting the output by up to 20 per cent. The distinction between "air cooled slag" and "granulated blast furnace slag" is important. I know of no factories using granulated slag as a raw mix addition while a number in the USA are using air cooled slag and the Cemstar process. This air cooled slag is not added to the raw mill, but added directly to the kiln inlet via a hopper and appropriate feeding device. This is the basis of the TXI patent from the Cemstar process.

Can we get the good quality clinker by using phosphogypsum as a raw material? Is any plant producing sulphuric acid and cement clinker as a by-product from waste phosphogypsum? If yes, kindly give the contact address of that plant1.

In Lea you can find reference to co-production of sulphuric acid and cement. Gypsum, clay, coke, iron oxide and sand are burnt together. The coke dissociates the gypsum producing lime and SO₂. The lime combines with theother materials to produce clinker and the SO₂ is captured to produce sulphuric acid. There would potentially be a problem with phosphogypsum due to the high P₂O₅ content of the resulting clinker. Sorry, I don't know of any plants using this technology.

We have got two sulphate resistant cement types (CEM I and a CEM II using fly ash), and our clinker has got: C₃S: 57-64 per cent, C₂S: 10-18 per cent,C₃A: 0-1 per cent, free CaO less than 1 per cent. Our fly ash has: SiO₂: 48 per cent, Al₂O₃: 19 per cent, Fe₂O₃: 10 per cent, CaO: 15-18 per cent (free:2-4 per cent), MgO: 4 per cent, SO₃:2 per cent.How much (in percentage terms do you recommend to use of fly ash? How will it effect sulphate resistance? How should the fly ash be ground and what is the most important limit in being able to use the fly ash (I mean e.g. CaO, free CaO, Al₂O₃, residue on a 90 micron sieve or other else)?

1. The percent fly ash addition depends on the cement standards in your country and also on the expectations of your customers. You must expect a drop in early strength and the maximum fly ash will be determined by the reduction in early strength the market will accept.
2. Addition of fly ash should increase the sulphate resistance of concrete made with the cement.
3. It is often best to add the fly ash to the separator of the cement mill circuit. The material that is fine enough already will pass straight to the finished product without grinding. This will preserve the spherical shapes of the ash and promote workability of the cement and reduce water demand.
4. The most important limit is the reduction in early compressive strength.

We have a problem in determining which residue is correct, either the raw meal or the kiln feed. In fact we do get about 13 per cent residue on 90 micron for raw meal and when it comes down to kiln feed it is about two per cent higher. Could you be kind enough to explain why this phenomenon takes place and what actions could we attempt to resolve the differences.

The kiln feed normally contains the dust from the precipitator in addition to the raw meal and therefore the two samples are quite distinct. The kiln feed is normally finer than the raw meal but this depends on the exact circuit used on the particular factory. Your situation is different and might be explained by drying the raw materials in the preheater exhaust prior to precipitation. In terms of quality control the raw meal is the residue you should be controlling. The kiln feed sample is within the external dust recirculation circuit. If you send more details of the circuit I can explain better.

The conversion factor raw meal to clinker, obviously depends on the raw meal's LOI. Which temperature do you recommend to make the test of LOI to have the best estimation of this factor?

It is normal to measure the LOI at 1000 degrees C. However the higher the better up to the sintering temperature of 1450 degrees if your furnace will reach those temperatures. In practice the conversion factor is also affected by the dust loss from the kiln.

While our plant was designed in 1998, the necessity of preblending stacker and reclaimer system was not incorporated based on the ground that run of mine limestone quality is very consistent(CaO standard deviation 0.60 per cent) and silica, iron or alumina oxide are very less in the range of 0.6,0.1,and 0.3 per cent respectively. Our raw meal is four-component limestone, silica rock (75 per cent purity as SiO₂),iron ore (55 per cent purity as Fe₂O₃) and bauxite (48 per cent purity as Al₂O₃). Now since the beginning of this year from the mines receipt limestone CaO is reduced from 54 to 51.5 per cent and at the same time silica, alumina and iron oxide increased to the level of 3.5 to 4.00, 0.8 to 1.1,and 0.25 to 0.35 per cent respectively. The standard deviation for LSF on three random eight-hourly samples we are getting in the range 40 to 90. I would appreciate to have feedback from you which is the level of variation in run of mine limestone making it necessary for installation of preblending stacker and reclaimer system for a one million tonne dry process cement plant.

I do not believe a stacker/reclaimer should be necessary in your situation. With a good software system proportioning the materials into the mill you should be able to achieve less than two per cent standard deviation in kiln feed and clinker LSF. You have another big problem with your mix because it is heterogeneous and therefore of very low reactivity. Do you have problems with high fuel consumption or excursions of high free lime in the clinker. I would recommend strict control of the mineral composition of the clinker and possibly the introduction of Duplex grinding.

We've been faced with a problem that the collecting efficiency of the electrostatic precipitators lowers remarkably since having started to use sewage sludge as the secondary material. I'm not sure where the cause is from. Is there any possibility of causing such problem by using sewage sludge, and which kind of components that sewage contains can affect the performance of ESP? As the reference, we've been experienced in the same problem when we used TPA sludge(Telephtalic Acid) two years ago.

I have not heard of this phenomenon before. The resistivity of the dust exiting the preheater must be changed when burning these sludges. I hope all the sewage sludge is being burnt in the kiln and the problem is not caused by organic residues entering the ESP. If so then the problem can only be associated with volatiles derived from the sludge entering the ESP with the dust. The most likely source is chlorides from the sludge. You should also check carefully that the problem is not caused by mercury from the sludge. If so then you have a big problem.