You are right that cement companies are considering the addition of ferrous sulphate to their cement in order to meet the stricter limits on hexavalent chromium that are being introduced. The application involves adding solid ferrous sulphate to the cement either at the milling or packing stage and relying on this to reduce any hexavalent chromium present in the cement. The industry knows that this solution has its limitations, not least being that the activity of the solid ferrous sulphate will deterioratr with time and this will confer a "shelf-life" to cement treated in this way to meet the directive.

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.

Our clinker chemical composition is as follows: SiO₂= 21 per cent; Ca_O= 64 per cent; Al₂O₃ = 6.0 per cent; Fe₂O₃= 5.2 ; K₂O = 0.4 per cent; SO₃ = 0.55 per cent; MgO =1.0 per cent; Free CaO = 1.0 per cent and the liquid = 34.5 per cent. Liter Wt 1380g/ l. If there is slight reduction in the liquid the clinker becomes dusty though the free CaO is under control. Here it seems the Al₂O₃ comes from limestone does not get into liquid though the calculated liquid is high.If slight reduction in Fe₂O₃ content results in dusty clinker. We add aluminous laterite and hematite ore as fluxes. The question is how to differentiate between the actual liquid and calculated liquid.Does the optical microscopy help in this regard? From your experience give a clear picture on how to reduce the liquid content with improved nodulisation and to have better refractory life in the burning zone. Even the Spinel bricks give only( refra mag-85 of refratecnik) six month's life.

You are correct the theoretical liquid content of your clinker is very high. The litre weight is also high indicating a very dense (almost fused) clinker. Some Al₂O₃ will undoubtedly be taken into solid solution in the alite phase. Optical microscopy can indeed tell you the true liquid content (and the true alite, belite and free CaO content). There are photographic and computer pattern recognition systems available that will do this for you and eliminate the variation between microscopists. With regard to refractory life the liquid content and its properties are important. The viscosity of the flux is a critical consideration. Too fluid a flux can lead to dusting. These inter-relationships are very complex. To answer your questions properly would require a full and detailed process investigation involving microscopic analysis of frequent samples coupled with detailed process data over an extended period.

I wrote you before. Thanks for response. Our problem is high free lime. Here I am giving you the complete analysis of clinker and kiln feed: (data enclosed). We are using South African and Indonesian coal. What may be the cause of high free lime. Other than that kiln feed and clinker analysis, I want to tell you that we have two kilns and free lime increased on both kilns at the same time.

I presume the analysis should read K₂O and Na₂O in the clinker? If so we can discount the K₂O content as the reason for the high free CaO. The K₂O will preferentially combine with the SO3 in clinker and therefore there will be little surplus to combine in the clinker minerals and inhibit the formation of C₃S. The 90 micron residue of the kiln feed is also low at 12% per cent. However the critical factor is the mineral composition of this residue. If there is a high quartz content in these coarse particles that might be the cause of the problems. If not then it is some factor we haven't identified yet. Possibly the kiln operation or the combustion.The fact that the free CaO has risen simultaneously on both kilns suggests that this is a material or chemistry related problem rather than a process equipment problem. What has changed before the rise in free CaO? Raw material sources? A new level in the quarry? Introduction of a new fuel? You should also check the calibration of the equipment (XRF?) used in the quality control of the kiln feed.

We operate one open circuit two chambers ball mill of size 4m dia X 13.5m long for Cement grinding. Capacity:100tpd drive motor: 2x1500 kW (twin drive) Power drawn: 2500 kW (more data on grinding media). >We are maintaining a blaines of 2700 to 2800. The cement is coarser having a residue of 10 per cent over 90 micron sieve and 30 per cent over 45 micron sieve. Increasing the blaines reduces output and increases the power consumption. How to achieve lesser residue keeping the same blaines.

There is no easy way to adjust the particle size distribution of an open circuit mill while maintaining the same Blaine. To do that you need a separator in closed circuit with the mill to send the oversize material for regrinding. You may have some success with grinding aids. Alternatively you could try for a more coarse media grading in the second chamber. However the results of doing that cannot be precisely predicted.

We have cement mill with roll press and separator we are facing problem of low feed. W e have analysed the product and our finding are as below less than 3 micron - 12 per cent is in the final product less than 45 micron -17 per cent is in the separator return I think if we can separate this 17 per cent in the separator than our output will increase. Please suggest how should we increase the velocity across the seprator cage and also suggest how to eliminate less than 3 micron particals from the product.

The problem of the -45 micron material in the separator return is known as the "bypass" and is the proportion of fine material that reports to the returns even though it is ground fine enough. Adjustment of the separator speed is unlikely to dramatically improve this. My suggestion would be to conduct trials varying the air flow through the separator and assess which changes improve the performance. The super fine material in the finished product is most likely associated with the ball change grading in the finish grinding ball mill. Reducing the separator bypass may assist in reducing this. Otherwise you will have to tip and regrade the grinding media in the mill and conduct trials as you recharge the mill.

I sent you various mails regarding free lime in clinker. Your response was always very positive. Now I am sending you some data of clinker analysis. Please send your comments (data attached to original file - not available here).

Thanks for the extra data. The K₂O contents between the OPC and SRC are very similar, so again that does not seem to explain the high free CaO on OPC production. Do you know the reason why the clinker LSF is 6 per cent lower than the kiln feed when SRC is being produced, and only 1 per cent lower when OPC is being produced? Presumably the coal feed is the same when producing both clinkers? Are you operating with a bypass? Is the dust loading on the electrostatic precipitator the same when producing bioth clinkers? How are you handling the precipitator dust?

Our ball mill is closed circuit. Its design capacity is 70tph, but we can run 85tph. However, we run it now 80tph, ie total feed is 80tph. We add limestone feed with clinker & gypsum. The limestone percantage is increasing & deceasing based on LOI. The Blaine is: 3300 to 34700 cm²/g Residue (return): 12-13 per cent SO3: 2.6-3.0. LOI: 0 to 2.5. When mill is in steady condition our seperator speed is 76 and sepex damper position within 59 to 61. But now my new terget is that Blaine is same i.e. within 3300 to 3400 cm²/g but residue to be within 8-9 per cent. Is it possible to control the blaine with respect to the requisite residue levels?

To control the residue separately to the Blaine you need to vary the recirculating load of the cement mill. To maintain the same Blaine and reduce the residue you will need to operate with a higher recirculating load.