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Ask Dr Clark a question
Ask Dr Clark
The idea of the “Ask Dr Clark” feature is that lots of people in the cement industry around the world periodically ask Dr Clark for his advice on issues they are currently facing. Through “Ask Dr Clark” anyone is able to ask Dr Clark’s advice on their current issues.
Q:
calorific value
Among the following fuels which one has d higher calorific value;
1 coal
2 petcoke
3 natural gas
4 AGO
5 LPFO
6 HFO
and others
A:
Calorific value is the heat content per unit weight or volume of the fuel. Heavy fuel oil and LPFO would have similar calorific values. Petcoke will be lower and coal lower still in terms of MJ/kg. Natural gas calorific value would be in heat content per unit volume rather than weight therefore not directly comparable. I do not know what AGO is.
Q:
Differential pressures
1. what is differential pressure and can we control it?
2. why do we measures differential pressures for
a. Mills
b. Bag house and other equipments in cement industries?
A:
Differential pressure is the difference between the pressure at the inlet and outlet of an item of equipment. It is related to the flow of gas through the equipment and the resistance to that flow presented by the equipment. We monitor the differential pressure to monitor the flow and the change in the resistance to that flow, i.e by blinding of baghouse are build-up of material within the equipment.
Q:
Cement mill seperator
Hi sir
We have a closed circuit cement mill and documentary of the seperator is SKS-Z-3500/290/4*3200 .
The seperator included of dynamic and static separator The static seperator have 100 blades (50blades in up and50blade in down).
The rotation of the seperator is between 43 to 170 and it work always in 115-120 RPM and the blaine is 2900-3100 in opc.
The efficiency of the seperator is between 65-70% .(over screen 90mic)
AT first:
1.The efficiency of this seperator is good or Bad?
2.How I can improve the efficiency By the change of the distance between the blades in statitcs seperator?(in now The distance is 8-16 cm)
Best regard
M.Bahrampour
A:
65-70% separator efficiency is mid-range, neither good not bad. How are you assessing separator efficiency? You will have to conduct trials with different setting of the separator blades and remeasuring the separator efficiency.
A:
Workability is a combination of the ease of placing concrete in a mould and the consistency of the concrete, i.e. the tendency of the concrete to segregate into its constituent fractions. It is related with the quantity and the reactivity of the C3A component of the clinker and the amount and mineral form of the calcium sulphate in the cement. Besides slump you can get some indication of workability from the flow of standard mortar measured on a flow table and the water demand of the mortar to achieve a certain flow. This is one of the most difficult quality parameters to control requiring that the gypsum addition to cement be matched to the amount and reactivity of the C3A in clinker and the temperature regime in the cement be closely controlled.
Q:
What is the difference between fineness in blaine and in residue? Can we correlate both together?
A:
You can expect there to be an inverse relationship between Blaine and residue. As Blaine rises so residue at a particular sieve size should fall. The relationship will not be perfect as other factors such as the cement mill configuration and clinker grindability will affect both Blaine and residue. That is why both should ideally be monitored and controlled.
A:
That depends on the mass of the exhaust gas whose temperature you want to reduce. You need to develop a mass and energy balance for the gas conditioning tower.
A:
Yes, I would expect this synthetic gypsum to affect the quality of the cement produced. The hydration of cement produces calcium hydroxide as one of the reaction products. The acidity of the gypsum will react with that calcium hydroxide.
A:
You can still use the clinker with a controlled addition rate to the cement mill. Perhaps 10 or 15% maximum. You should control the fineness of grinding to a residue target rather than a surface area target.
Q:
Concerning my last question on the reduction of kiln diameter.
Dear Dr. Clark,
Can you be more specific about your comments on mechanical complexities?
You said the rollers, riding tyre and the kiln inlet housing would be for the wider diameter. What does it mean?
And if we reduce the diameter of kiln feed end, does the load of kiln drive system will be lowered under the same production rate?
Thank you.
A:
I don't know how many support roller stations there are, or the extent of the wider diameter section of the kiln. My thoughts were that one of the support rollers and riding tyres would be for the wider section, in which case mechanical changes would be needed if you go to a narrower diameter. I don't expect there would be any increased load on the kiln drive motor.
A:
If the temperature in the preheater bottom stage or precalciner is higher then the degree of calcination of the feed would be expected to be higher, therefore the loss on ignition would be expected to be lower.
Q:
Kiln shell and tyre temperature
Hi sir
How we can calculate alowed degree diffrential between the tyre and kilnshell?and how much it should be?
Best regard
M.Bahrampour
A:
I do not know of any universal method to calculate the temperature differential between the kiln shell and riding tyre. This calculation would involve many considerations such as the design of the tyre, tyre pads and shell. The ambient conditions in terms of temperature and ventilation. Considerations of heat conduction, convection and radiation. The difference between the temperatures will also be a function of the time of the measurement during the kiln operating campaign. During the initial warm-up of the kiln the differential will be the greatest. The tyre heats up much more slowly than the shell due to its greater mass and being reliant on heat transfer from the shell. This is the danger time when the differential expansion of the shell and tyre might case a constriction of the shell within the tyre. This must absolutely be avoided. The clearance between shell and tyre must be closely monitored during warm-up. if the gap begins to close then the rate of heating of the kiln or the tyre must be brought into line. If necessary sullpementary heating should be applied to the tyre.
Q:
high free CaO
Dear Dr. Clark
Is it possible to have high temperatures in the kiln and also high f.CaO?
A:
Yes. If the lime saturation of the clinker is greater than 100% then there will be free CaO irrespective of the temperature in the kiln. Also if there are coarse particles of calcium carbonate or silica in the kiln feed. There could also be other more complex reasons such as reducing conditions due to the kiln main burner, but the most likely is too high lime saturation.
Q:
Concerning the kiln shell diameter...
Dear Dr.Clark,
The kiln was Polysius AT type with 5 staged-preheater and a planetary cooler. The capacity was 3300 t/d and the length of the kiln shell was 61 meters. We increased the kiln capacity to 4300t/d in 1999 by several modifications of a new SLC calciner(MFC) and a IKN clinker cooler. There were no changes with kiln shell.
Concerning the diameter of kiln shell,
The first section of (0 - 9.4)m from kiln feed end is 5.0m of diameter, the second section of (9.4 - 12.4)m is conical type and the diameter reduced from 5.0 to 4.8 m. the third section of (12.4 -61.0)m is 4.8 m of diameter.
We're planning to replace the kiln shell of the first(5.0m diameter) and the second conical section because of some damages(deformation). At this time, how about replacing those sections with same diameter(4.8m) as the third section ? Can you expect any negative impact when we do that ?
Best Regards.
A:
I would not expect any negative process impact. The volume loading of the kiln would remain within the normal range for SLC kilns with IKN cooler. There will be mechanical complexities: the rollers and riding tyre will be for the wider diameter. The kiln inlet housing and seal will be for the wider diameter.
Q:
Relationship between Fineness (Blaines) and TPH
Sir,
I want to ask precisely if there is any formula to Calculate the relationship between fineness and Ton per hour of Cement production. All I assumed is the Higher the Blaine's or Fineness the likelihood of the drop in production ( Is this theory correct?).
What techniques can be applied to Increase the fineness without corresponding reduction in Production Output?.
I will appreciate your prompt response to these questions.
Thank you .
A:
Yes, there is an inverse relationship between increasing Blaine and the mill throughput in tonnes per hour. To achieve greater fineness you need to complete more grinding work per unit of product so the output of the mill goes down. The best way to increase fineness with no loss of output would be use grinding aids. If these are already being used then some precrushing of the feed to the mill in a hammer mill or roll press. The amount of grinding work and the throughput rate of the mill are related to the change in fineness that must be achieved. If you have a finer feed to the mill then less grinding work is required.
A:
A V-separator relies on the feed falling through gases ascending through the separator. You do not want the ascending gases to rise through the feed chute so you need some sort of air seal.
A:
The particle size distribution will be wider with open circuit grinding than with closed circuit grinding at equivalent Blaine. The separator has the effect of sending coarse grains back to the mill for regrinding. This means there will also be less super-fines at the equivalent Blaine. Whether the strength will be higher or lower depends on the Blaine. At low Blaine (3000 cm2/g) I expect that the strength from an open circuit mill would be as high or even higher as a closed circuit mill. At higher Blaine (4000 cm2/g) I expect that the strength from a closed circuit mill would be higher.
Q:
Intensity of kiln feed
Hi sir
How we can get the analyze of material , from intensity of kiln feed?
Best regard
A:
I presume you mean the intensify of the fluorescent X-radiation from a kiln feed sample in X-ray fluorescense spectrometer. These are calibrated so that the SiO2, Al2O3, Fe3O3, CaO, etc.content of the kiln feed is correlated with the X-ray intensity.
A:
The efficiency of the preheater top stage is calculated by dividing (1 - the dust loss from the preheater) by the kiln feed rate to the preheater.
A:
You will change the injection velocity of the coal. Better to control the fineness of grinding of the coal to ensure there are no large particles that can cause choking of the coal channel.
Q:
why the cement stick on the silo wall and the bulk trucks wall
why the cement stick on the silo wall and the bulk trucks wall.. our cement analysis is
SiO2 Al2O3 Fe2O3 CaO MgO SO3 K2O Na2O
20.49 5.04 3.67 64.34 2.05 2.48 0.13 0.23
LSF SM AM C3S C2S C3A C4AF
97.95 2.35 1.38 55.01 11.92 7.17 11.14
Sieve Blain F.l
9 2950 1
A:
The analysis of your cement is probably irrelevant. The alkalis are not high. The problem is most likely delayed hydration of the gypsum in the cement. I will post my article "Hornblower's first command". Read that for further explanation.
Q:
effect of high L.O.I cement of concrete
Sir,
We got one customer in need of cement of class 42.5N, the customer also wants the L.O.I of the cement to be less than 5%, our limestone cement (Class 42.5N) have L.O.I of 7.5%. what do you think is the effect of the high L.O.I value to concrete strength.
A:
There will be no effect if your cement complies with class 42.5N. The EN197 standard makes the composition of the cement irrelevant. It is performance based, i.e. you are guaranteeing that it will meet 42.5N.
Q:
cenent application in sub-zero temp.
If some % (<2%) of free CaO can help to produce in heat of hydration when cement is used in below 0C ?
A:
Yes, free CaO will hydrate quickly and boost the heat of hydration. However, that alone would not be sufficient to make the use of the cement assured in sub-zero temperatures.
A:
There could be many reasons: the warming up procedure; interruptions in the warming up procedure; the application of fuel and temperature control during the warming up procedure; the quality of the bricks installation; the quality of the bricks; the chemistry of the feed; etc. It is impossible to say without a full analysis of the causes of the failure.
A:
I presume this a white cement kiln? That is where the clinker is likely to be quenched in water. Certainly the quenching can affect the density of the clinker. The clinker structure is often shattered by the quenching and some prehydration will occur. Both these will affect the density of the clinker.
Q:
Effect of RawMeal residue !!
Hello sir,
We are facing frequent kiln flushing and dusty kiln condition in white cement plant, So we are trying to find the problem..
Could you pls explain me the effect of residue in this problem sir?
We are maintaining a residue of 8 to 9% on 90mic..
or how can we over come this problem sir?
Thanks in advance
A:
White cement kiln feed is difficult to combine due to the lack of iron oxide to form flux in the kiln. The higher the 90 micron residue the harder it becomes to combine the clinker. Therefore it is common practice to grind the kiln feed to a low residue of less than 5% on a 90 micron sieve. Finer grinding might solve your problem. An alternative would be to add a fluxing/mineralising agent such as calcium fluoride.
A:
I presume you mean the coarse rejects that fall from the table of a vertical roller mill. The only way to control these rejects is by adjustments of the nozzle ring and dam ring height.
Q:
Dear sir plz tell me main chemical reactions in kiln and also how they took place.thax.faisal ejaz
A:
The main chemical reactions are the calcination of the calcium carbonate and the subsequent combination of the liberated free lime with the acid oxides to form the clinker minerals. Temperature in excess of 850 degrees C is required for the calcination to take place at rapid rate. Over 1400 degrees C with the formation of liquid phase is required for the final combination to form C3S.
A:
Because the differential pressure across the mill is a function of the gas flow through the mill and the amount of material being lifted through the mill. These two are also related. The differential pressure gives an indication of the loading on the mill.
A:
There is no simple formula to calculate flame length that I am aware of.
A:
You cannot readily calculate the clinker bed height in the cooler. There are too many unknowns.
A:
Dust loss is fundamentally an inverse function of the top stage cyclone collection efficiency. Therefore you cannot calculate the collection efficiency without considering the dust loss. You should be able to estimate the dust loss from the indicated kiln feed rate and the changes in the kiln feed and clinker inventories over time.
Q:
What is effect of C.F. intermediate bin material on process and quality of clinker?
Sir, we divert the raw mill discharge air slide bag filter material to intermediate bin of C.F. silo, this cause any effect on the quality of kiln feed. We add our ESP and GCT material to the CF silo intermediate bin. Due to addition of raw mill bag filter material to the intermediate bin cause any serious effect on quality of kiln feed. This is feasible as process point of view?
A:
There should be no problem provided that this is done all the time. The boundaries of the mass balance are the feeders to the raw mill and the clinker discharge from the cooler. What is happening with recirculating ESP, GCT and raw mill airslide bag filter dust is irrelevant, provided that the same procedures are followed all of the time.
A:
A plume is a cloud of material appearing as the exhaust gases from the process exit to atmosphere. Most commonly it is a cloud (or plume) or water droplets as the water vapour in the exhaust gases condenses on meeting the cold, atmospheric air. Occasionally it can be a cloud (or plume) of crystallising salts, usually ammonia based. This also sometimes referred to as "blue haze".
Q:
WHAT IS FLUSHING PHENOMENON IN CLINKER KILN,
Dear Mr,
Pls help me to understand about flushing and which cases happend it?
What is disadvantage of this phenomenton?
Thanks much!
A:
Flushing in a cement kiln is when uncombined material rushes down the kiln into the burning zone and the cooler. Sufficient thermal energy must be applied through the main burner to maintain the full combintion of the clinker in the kiln. If not the material can flush through.
A:
LOI is loss on ignition. A sample is placed in a furnace at 1000 degrees C and the loss in weight is measured. Any combined water will be lost and also any CO2 remaining in uncalcined calcium carbonate. So the LOI of the kiln feed reduces as the feed passess down the preheater tower and into the kiln as first combined water is lost and then CO2 is lost as calcination takes place.
A:
Whitehopleman are specialists in cement rather than lime. In a cement kiln an ash ring would be dealt with by adjustments of the main burner. That depends on the type of burner and the adjustment possibilities. The coarsest adjustment would be to move the burner in to, or out of the kiln slightly.
Q:
Dear Sir, please give me idea about how we reduce the rejection in vertical(atox)coal mill Thanks
A:
The only adjustments you can make are the height of the dam ring or the length of the raw coal delivery chute into the mill.
A:
Rhino horn is the accumulation of clinker dust on top of the main burner of the kiln.
Q:
Info: How to calculat the actual coal consumption for clinker production as during grinding of coal meal dust coming with hot dust from preheater get mixed with fine coal.Booking the coal as per flow meter reading of fine coal will lead to stock imbalance
How dust qty mixed with coal could be worked out. Is it correct to calculate the qty of dust on the basis of C.V difference of raw coal & fine coal.
A:
Certainly dust will be entrained into the fine coal. That is why the calorific value and ash content of the fine coal should always be used when assessing the energy consumption of the kiln. There is no straight-forward way of measuring the amount of dust in the fine coal as some volatiles will also be lost during drying. The only way is adjust the amount of coal burnt to match the inventory of the coal.
A:
Oxygen enrichment of the combustion air leads to reduction in the volume of nitrogen and also a hotter flame.
A:
Many cement companies use cement sacks including a plastic membrane between the layers of paper. Sometimes lumps are formed due to the presence of syngenite in the cement. This can only be reduced by control of the alkali cycle in the cement kiln.
A:
The dust load in the gas will affect the power drawn by a fan. The volume and pressure for a particular rotational speed should remain constant.
A:
Because the top stage cyclone of the preheater acts as a separator. The coarsest particles pass down the preheater to the kiln. The finest particles are not collected and become the dust. Normally the finest particles are preferentially limestone and therefore the LSF of the dust is higher than the raw mill product. As the kiln feed is the combination of the raw mill product and the dust it's LSF is higher than the raw mill product.
A:
It is possible, although it would be an unusual thing to do. You will increase the thermal load on the refractory lining in the burning zone as the cross-sectional area of the kiln will be reduced. At the same clinker output the degree of filling and specific loading of the kiln woould increase. Of course the clinker output might reduce.
A:
Normally the length of the first chamber of ball mill is designed such that around 10kW of the mill motor is drawn by the media in the first chamber of the mill. Normally about 1/3 of the total mill length.
A:
Certainly the cement milling temperature can affect the loss on ignition as the degree of gypsum dehydration will be affected by the milling temperature. Recarbonation can occur to some extent by long-term storage of the clinker.
A:
The coal mill inlet temperature is already high at 500 degrees C. I suggest that you increase the volume of hot gas drawn through the coal mill to improve the drying capacity.
Q:
Our kilns is 4.75 dia x 75 m length , we face so many a times coating ring formation problem at 22 to 28 meter .
Clinker analysis in general , SM 2.34 , AM 1.28 , LSF 89-90 Liquid 26 +/- 0.5 , MgO 1.05 , SO3 0.45 - 0.55. K2O 0.34 and Na2O .09 We use mostly South Africa A grade 100 % in kiln and A Grade B grade coal 40:60 % in calciner . though we keep liquid in close watch we face heavy ring or coating formation in burning zone .How can we come out of problem ?
A:
The alumina modulus is low. Raising that should reduce the coating tendency. Beyond that there are many other considerations such as the ash content of the fuel, how finely the fuel is ground, the settings of the kiln main burner, etc.
Q:
Hi Dr. Clark I'm an Engineering student currently developing a proyect in a cement company and I was wondering if you could help me. I need to make a heat and mass balance in a VRM.
The mill is an Atox 30 with a hot gas generator with Fuel Oil Nr 6 as fuel. Raw material feed is up to 100 ton/h and between 6% and 12% humidity. I really need some help with heat and mass balances from the heat generator to the chimney. Thanks
A:
The first step is to construct the mass balance. You need to calculate the gas flow through the mill from the amount of fuel oil burnt and it's ultimate analysis. Once you have the mass balance then you calculate the energy balance by multiplying the mass flows by their specific heats at the operating temperatures and the energy absorbed to dry the raw material.
