How does the content of chromium have influence in quality and performance of grinding balls? Is it better a low content of this metal to improve hardness in balls?

If the grinding balls contain chromium then they are forged, hard grinding balls. If the balls don't contain chrome then they are softer mild steel and the wear rate will be higher. Balls containing chrome cost more but last significantly longer and maintenance of the optimum media loading in the mill is easier. Many cement companies use soft media for raw grinding as the wear on the steel simply increases the iron content of the raw mix slightly. Most cement companies use hard media for cement grinding as the wear rate on soft media when grinding clinker is too high and the mills need continuous recharging.

Our facility operates two Humboldt air-swept ball mills for raw meal production. Combined capacity is 100tph. We would like to optimise the raw meal circuit, in particular, mill loading. Unfortunately, there appears to be very little information on the optimisation of air-swept mills. How can mill throughput be improved, utilising existing equipment? The separator is a mechanical classifier, with adjustable vanes. Current reside set point is 15 per cent on 90 micron. We currently use sonic ears to determine mill loading, with mill and fan current also available. There is also a proposal to implement a grit return weigh system to determine the recirculating load. Would this be worthwhile addition, with any positive process benefits? In summary, we wish to determine the most cost effective means of attaining optimal material levels in the milling circuit, with the intention of automating the operation sometime in the future.

If you are adequately controlling the kiln feed residues to 15 per cent I am not sure what the grit return weigher is going to tell you. Will you be able to remotely adjust the vanes of the mechanical separator from the control room while the mill is running? If not I would suggest periodic sampling and measuring residues around the circuit is all you need to do to determine the rejects rate. What do you mean by "optimise" the raw mills? Do you need more raw mix output? Is there sufficient drying capacity in the mills? I presume they are swept by preheater exhaust gases?

We have a VSK unit with 300tpd production. The raw mix contains limestone, clay, cokebreeze. We are getting continuously high free lime. Please advice the procedure through which we can reduce the free lime.

There are a number of reasons why you might be getting continuously high free lime in your clinker. The most likely ones are: (i) there is too much limestone in your raw mix, (ii) the raw mix is not ground to sufficient fineness, or (iii) the temperature in the kiln is not high enough to finally combine the free lime with the clay minerals. The problem might be a combination of all these factors.

We have started using a new raw material as a silica source. This material contains a significant amount of titanium. Coincidentally, our free lime content has increased. Can the material be the cause? How stable is perovskite (CaTiO₃) as a clinker mineral?

TiO₂ enters into solid solution in the clinker minerals until saturation is reached and then the mineral formed is 3CaO.2TiO₂. Essentially the TiO₂ substitutes for SiO₂ in the clinker minerals therefore it should effectively reduce the lime saturation and so the free CaO content of the clinker. The opposite to the effect you are experiencing. There could be many explanations for that. Is the granulometry of the silica content in the new raw material source the same as in the old source? Are there any other minor components (alkalis, etc.) in the new source which might be affecting the mineralogy and combination?

We are carrying out research on blended cements with a view to increase the utilisation of blast furnace slag/fly ash. In this context, we are exploring use of high energy mills (Jet Mill, Vibratory Mill etc) together with traditional milling devices. I wish to ask:
1. How important is the strength of cement? There is considerable amount of literature on high performance cement/energetically modified cement ie cement with compressive strength in the excess of 90 Mpa and concrete with greater than 145 Mpa.
2. What could be the major limitations in using mills such as Jet Mill in cement industry? We understand these mills consumes lots of energy but it is also true that the efficiency of traditional mills decreases as we approach the target size. Should not the combination work better?
3. I am looking addresses of supplier for lab size jet mill and vibratory mill (2-10 kg/h) for the grinding of cement clinker, BF slag and fly ash.

The strength of the cement is important as the hydration of the cement is responsible for all the early strength development of blended cements. The hydration of the cement minerals also releases the calcium hydroxide which activates the pozzolanic reaction of the fly ash or slag. Jet mills are not used in the cement industry due to the operating costs and the throughput capabilities of the mills. Large cement plants employ mills capable of producing tens por hundreds of cement per hour. I don't think jet mills can reach that capacity? For a laboratory mill my first port of call would be ELE (Engineering Laboratory Equipment).

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 colour changes if air cooled slag was used as a primary raw material.

The process of adding air-cooled slag to the kiln inlet has been patented by TXI under the name Cemstar. The minerals in the slag do not convert back to the free state. They are already intermediate states in the formation of Portland cement clinker and all that is required is the completion of exothermic combination to C₃S. No additional heat is required, also there are no reported changes in the colour of the cement subsequently made from the clinker.

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.

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

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.