We are having a 1.5Mt six-stage Precalciner kiln operating with South African coal having a VM of 28 per cent. Now we thinking to switch over to Indonesian coal with high VM content (up to 42 per cent) and Chinese coal (VM up to 32 per cent). The coal mill is a VRM having hot gases from cooler. Up to what maximum VM we can go without having explosion problems. With high VM coal what are the other changes to be carried out to regarding coal residue so that we do not get problems in the flame shape.

There are well established guidelines for the safety operation of coal grinding equipment. These fall into two broad categories:
(i) explosion prevention, and
(ii) explosion protection.
For prevention the best solution is to ensure that there is less than 12 per cent oxygen in the atmosphere. You cannot do that when cooler exhaust air is used for coal drying. As a general guideline drying with cooler exhaust air is suitable with direct firing systems. Other prevention measures are to ensure there are no fine coal dust accumulations in the system and no possible sources of ignition. These are determined by the design of the coal milling and storage system.

How can the bulk density of cement change? Kindly describe the factors responsible? What are the consequences of bulk density of cement on concrete? How to increase the bulk density of cement? Do you suggest to increase iron ore content in raw meal to increase cement density? What is the range of bulk density and specific gravity for ordinary Portland cement (BS 1997)?

The bulk density of a particular cement can vary due to compaction or due to chemical reactions with the atmosphere or any water the cement comes into contact with. If you specifically want to change the bulk density of freshly ground cement then you would need to adjust the chemical and mineralogical composition. Increased iron oxide would do this but the effects would be very slight. I would also expect problems with excessive coating and ring formation in the kiln. There are certain concrete applications where high density concrete is specified (eg radiation shielding). This is achieved by using heavyweight aggregates such as barium carbonate or sulphate rather than by increasing the bulk density of the cement itself. The bulk density of cement is around 1.5 tonnes per cubic metre.

We operate two fine coal bin of cap. 200 tons each. Due to coal fluctuation we have provided compressed air at 2kg pressure but due to this sometimes we are having problem of fire and sintered formation in bin. We have already tried blower air but with this coal fluctuation problem. Can you suggest an alternate way to avoid this problem.

I would not use air or gas of any kind to agitate the coal in the fine coalbins. I suggest using acoustic cleaners and sounding these at regular intervals to prevent bridging in the bins.

We are manufacturing white cement and facing problem of lump formation as well as coating on the wall of cement silo. We are keeping the temp of cement at inlet of silo at approx 80 degree and have installed a bag filter which runs round the clock to extract moisture from cement. Please suggest how to get rid of the problem and also that wather we can go for a Polymer liner or any other insulating paint to fix on the silo wall to prevent coatings.

At 80 degrees C the cement is still hot enough for dehydration of the gypsum to continue in the silo. You need to cool the cement to less than 70 degrees if you are to be sure to prevent this. The other approach is to ensure that the gypsum dehydration takes place in the cement mill by lifting the mill exit temperature to 115 degrees.

We have to take gypsum out of a rectangular silo and we are not sure if a rotary valve is necessary between the silo and a screw conveyor. At the moment we have problem of ratholing and bridge as well, thanks in advance for any help.

I would suggest installation of an acoustic cleaner and sounding the horn periodically to combat the ratholing problem.

We are facing problem of poor fluidity of cement while extracting cement from silo even with fresh cement stock. We are having only Ordinary Portland Cement grinding with no water cooling facility in mill. Cooling takes place only in O-sepa. Since the last two months we started water spray in mill inlet to control mill outlet product temperature of around 108 to 113 deg C. Product temp. entering to silo is around 85 deg C and at outlet 72 to 75 deg C. Humidity is around 40 to 60 per cent in air because of coastal area. Please highlight some of the possible causes for such phenomenon and also give the method normally applied to determine moisture content in cement. As additional information we are using mineral gypsum 5 to 5.5 per cent in the cement.

I suggest you should raise the mill outlet temperature setpoint of the PID loop controlling the water injection rate. This should allow more water to be driven off the gypsum in the cement mill. At the same time you shouod attempt to maximise the cooling in the O-Sepa.

We are 3100tpd cement plant situated in the Arabian Gulf. We have four cement silos 10,000 tons each at our plant, we are facing the problem of lump formation in silos, which is causing cement extraction delays and is also effecting our market reputation as well. Our cement mill is 136tph rated capacity (OPC) closed circuit with water spray system (made in-house). The humidity reaches 80 to 90 per cent in summers, as it is a coastal area. Till now we have not come to any conclusion about the cause of lump formation in our plant. Can you please suggest us if grinding aid may help in solving the above said problem, how technically, and of course to what extent.

Given your location and the humidity you suffer in the summer, the question you must answer is whether the lump formation is caused by the ambient humidity in the air in the free space in the silo or continued dehydration of gypsum in the silo. I suspect the problem will be continued dehydration of gypsum in the silo. To solve this you must carefully control the temperature of the cement entering the silo. The installation of a cement cooler may be necessary in your climate to bring the cement temperature below 80 degrees centigrade.