I have to ask you a question with regards to the chlorides and kiln-preheater operations. The Cl in our limestone is approx 0.003-0.007. However, I have to go down a bench in the quarry and the chlorides down there are like 0.027 based on one of the samples (the high chlorides as a result of the sea bed interface). My quarry is actually being exhausted and I am forced to go down there to buy some time till town and country planning approve new quarry lands. What we have is a VA design with a pair of 1st Stage Cyclone, one 2nd stage cyclone and three shaft stages. They went with this design as it lends itself to the least possibility of plugging. This leads me to the question....How forgiving is a shaft stage preheater?

I can't tell you whether your shaft preheater will forgive the higher chloride input....no-one can and that's why no-one writes it in a manual! It depends on may factors including the other kiln chemistry parameters and the way you operate the kiln. However, I appreciate you need some confidence to go forward. I suggest going ahead but make preparations to break the external alkali cycle of the kiln. That will involve taking some of the kiln dust out of the kiln. Hopefully that will be sufficient to allow the kiln to operate smoothly with the raw materials from the lower level.

We are interested to start a mini slag cement plant in India. We are thinking to use air cooled blast furnace slag instead of ground granulated blast furnace slag.Is it OK or not?

I'm afraid its not OK to use air cooled blast furnace slag instead of granulated blast furnace slag. Air cooled slag has virtually no hydraulic properties compared with granulated slag. The air cooled slag would be better used as a supplementary raw material in cement manufacture.

What is lime used for in cement ? Many people tell me different answers. What is the ratio of lime to cement and when to use in a particular job ?

The simple answer is "around one per cent", but the exact response may rather depend what you mean by "lime" and what class of cement you are talking about! Portland cement is ground from a clinker produced by heat treatment of a raw mix that consists mainly of finely-ground limestone and shale. On heating, the limestone forms lime which, in due course, conbines with other substances to form calcium silicates and aluminates and there is only one per cent or so residual uncombined lime remaining. Cement sets and gains strength through the reaction of these substances with water. So, some people would say that a lot of lime goes into cement: if you analyse the clinker and do not take account of the fact that the lime is combined as aluminates and silicates, then about 65 per cent of ordinary Portland cement is "lime". Now when it comes to making mortar, we have a different story. A long time ago, lime/sand mortars were used (about 25% lime in the mix); then, to get more rapid strength development and higher final strength, people started to add cement. (Just using cement and sand leaves you with a lot af sand and not much cement and a harsh mix that is difficult to work with a trowel - and if you add more cement to overcome this then the mortar becomes too strong for general use.) So, some traditional mortar mixes are 2 parts of cement plus 1 of lime and 9 of sand ( or 2 - 2 - 12, or 2 - 6- 24 are also OK). You can buy masonry cements instead: in North America these are typically made from about 50% Portland cement interground with an inert material such as limestone and a plasticiser to aid workability and you can mix 1 part with 3 parts of sand to get a mortar roughly equivalent to one of the traditional mixes. Now, if someone says "lime" instead of "limestone" - as is common in some regions of the world - you need to change our first answer quite a bit! Concrete mix design is quite another field. Concrete is basically a mixture of aggregate - crushed stones or gravel - in which the voids have been filled by a mortar mix (sand and cement) that yields suitable flow properties when fresh and, in due course, appropriate strength. (Add too much water to get it to flow OK and you lose out on ultimate strength; add too little water and the mix will not flow into all the corners where it is needed!) Most builders' merchants, do-it-yourself outlets and suppliers of cement will give you rules of thumb as to what mixes to use for different jobs.

What should be the standard sieve sizes for limestone feeding to Vertical roller mill. We were earlier using Limestone having bond index of 8-9kWh/short tonne. Now we are having limestone with bond index 11-14kWh/ short tonne. Wether any reduction in mill feed size will improve the output of the rawmill. How to calculate finess modulus of crushed limestone and what is the optimum value for VRM.

Yes, reducing the size of the feed to the raw mill should result in an increase in output. Bond's equations use the size grading where 80 per cent of the feed is passing to characterise the size of the feed to the mill. The optimum size of mill feed is related to the nip angle between the rollers and table. 60mm is quoted as the maximum size.

We regulate our raw material pile at 105-110 lsf. However the lsf of the raw mill's sample becomes 85-90. How can we have such a large difference?

There might be a problem in the sampling station. More likely you have a problem of segregation of the material. This is most likely when you are reclaiming the end-cones of the pile or in the intermediate feed bin between the pile and the raw mill. One solution is to add the end-cone to the next pile rather than sending it to the raw mill. The intermediate bin should be kept at a high level by setting the reclaimer start signal to a high bin level.

We are about to lose our traditional sand supplies for our raw mix. Now we are using 78-80 per cent limestone, 15-17 per cent clay, 1-2 per cent iron ore and 2-4 per cent sand. Now we are thinking to use low grade limestone and no sand. Our management says to use iron dust through kiln firing system in place of using in the raw mill grinding. Mix design is iron dust 0.5 per cent limestone 70 per cent, low grade limestone 18.5 per cent and clay 11 per cent. My point is any raw mix is made to be through raw mill grinding and only fuel through firing system. Let me know your opinion and suggestion.

1. As regards the enquiry, I have no experience of anyone firing iron ore through the burner. I presume they plan to use an insufflation pipe and not simply feed through the coal mill as this could lead to a number of problems. Many plants around the world insufflate kiln dust successfully without major quality problems but care has to be taken to ensure the dust can be properly absorbed into the raw mix and, of course, the chemistry of the dust is very similar to the kiln feed chemistry. I assume the plant is wet process and would suggest looking at back end insufflation as a possible alternative to ensure thorough mixing of the iron ore. If it is dry process then there is some comparable experience in Malaysia at the Rawang plant where they introduce their secondary raw material - oil shale - to get the benefit of the fuel value and thus rely on the precalciner and kiln to do the raw meal blending. To insufflate iron ore as a separate component will, in my view, be far from ideal and the following problems may be encountered: - the kiln feed mix would be deficient in flux and hence difficult to burn and less able to absorb the iron - there would be the risk of chemical reduction of the iron ore in the flame to ferrous state which can increase the basicity of the mix by iron replacing calcium and hence artificially raising the LSF making the mix difficult to burn - there would be the possibility of increased coating and build-up 2. Many cement plants add high ballast and high ash fuels through the firing system. However, I do not know of any plant that is specifically doing that with iron additive. I also see no reason to do that. There are potential problems of in homogeneity in the clinker. Also you need the iron to form flux in the transition zone, not at the end of the burning zone.

What kind of changes are required in raw mix to increase the granulation with 50 per cent pet coke total CV 4.3 per cent, LSF 93, SM-2.1 per cent, AM-1.0 per cent, in kiln feed?

The granulation problems with pet coke firing arise because of the low viscosity of the flux caused by the sulphur in the pet coke. Therefore raising the silica and alumina modulii should help to improve the granulation.

If air cooled slag is used as a raw material for kiln , replacing uncalcined calcium carbonate, will the calcium silicates, aluminates, etc. revert back to free state, so they can reform clinker crystals of cement design proportions? Will the existing sicates go to C₂S state with the increased temperature and time. In other words could this slag combined with correction proportions of silica, iron, alumina, calcium be used as a primary raw material for a kiln feed? If there is a reversion of the state of the compound will there be a requirement for additional heat (as opposed to the exothermic heat released to compound formation) ? Would there be any big color changes if air cooled slag was used as a primary raw material.