Home / Coating, rings and balls - Dr Michael Clark, Whitehopleman
Coating, rings and balls - Dr Michael Clark, Whitehopleman Dr Clark reviews the well worn topic of the formation of coating, rings and clinker balls in cement kilns. A recent flurry of enquiries prompts Dr Clark to review the literature on the topics to provide a reappraisal of the possible causes and, of course, their solution when they cause interruptions in kiln operation. Dr Clark would welcome contributions from readers who are encountering such problems. This month’s Technical Forum concerns the related topics of the formation of coating, rings and clinker balls in cement kilns. This is not a new topic. We have had many questions about these topics over the years, but during the last few months there have been a new flurry of questions, particularly relating to the formation of large clinker balls. Over the years the standard answer to these questions has been that all these phenomena are related to the properties of the liquid phase in the charge in a cement kiln. The final conversion of kiln feed raw meal into cement clinker is a sintering reaction because the principle reactants, the lime liberated from calcium carbonate and the dicalcium silicate initially formed from the combination of that lime with silica, and the final product the tricalcium silicate remain solids throughout the conversion process. However, this solid to solid conversion is greatly enhanced by the presence of 20 to 30% liquid phase formed from the calcium alumina ferrite and tricalcium aluminate phases plus the alkalis and sulphates and up to 2% of the magnesia present in the clinker. While the C2S and CaO reactants and the C3S product are solids, they each dissolve to some extent in the liquid. The C3S combination reaction proceeds towards the phase equilibrium between the reactants and product in both the solid and liquid phases at the temperature at the particular position in the kiln. The liquid flux plays a vital transport role in dissolving and bringing together the solid reactants to allow the solid product to be formed and crystallise from the melt. The effectiveness of the liquid flux in accelerating the reaction between C2S and CaO to form C3S depends on: (i) the amount of liquid at a given temperature, (ii) the viscosity of the liquid and (iii) the surface tension of the liquid. In the quaternary CaO-SiO2-Al2O3-Fe2O3 (CSAF) system the minimum eutectic temperature, when melting will take place and liquid be formed, is at 1338°C. In the presence of MgO and the alkalis, Na2O and K2O, as is the case in an industrial raw mix, the minimum eutectic temperature, when melting will take place and liquid be formed, is at 1280°C. The amount of liquid then increases as the temperature rises and more of the minerals melt. The first consideration is the amount of this liquid phase, or flux, that is present in the different zones of a cement kiln. There are well established equations, based on the
oxide content of the clinker, that allow the flux content at different temperatures to be estimated: Flux1338°C = 6.1x%Fe2O3 + %MgO + %Na2O + %K2O Flux1400°C = 2.95x% Al2O3 + 2.2x%Fe2O3 + %MgO + %Na2O + %K2O Flux1450°C = 3.00x% Al2O3 + 2.25x%Fe2O3 + %MgO + %Na2O + %K2O These formulae can be used up to a maximum MgO content of 2%, arising from the maximum 5 to 6% solubility of MgO in the flux, and a typical flux content for cement clinker of 30%. The formula for the flux present at 1338°C applies to raw mixes of alumina modulus greater than 1.38. This then leads to the concept of the “clinkering range” of a particular mix, or clinker composition. The “clinkering range” is the temperature range starting from the temperature when sufficient liquid is formed to cause shrinkage and nodule formation, and ending when so much liquid is formed that serious balling of the mix takes place. Mixes or clinkers with a wide “clinkering range” create less operational problems such a ball or ring formation, than those with a narrow “clinkering range”. High Fe2O3 mixes tend to have a narrower clinkering range. With a narrow “clinkering range” any fluctuations in temperature can cause ball or ring formation to occur and fluctuations in temperature are inevitable with an industrial cement kiln.
Unfortunately it is not possible to define the exact “clinkering range” for a particular mix or kiln. The minimum flux content for shrinkage and nodule formation and the maximum flux content to avoid ball and ring formation are also dependent on the viscosity and surface tension of the liquid flux. The viscosity and surface tension of the flux are dependent on the relative proportions of Fe2O3 and Al2O3 in the clinker. The viscosity of the flux is important as its effectiveness depends on “wetting” the surface of the solid C2S in the kiln. A mobile, low viscosity flux is more effective in wetting the C2S and promoting reaction with lime via diffusion across the solid state phase boundary. Terrier, Endell and Hendrick have conducted some studies into the viscosity of the flux and found that viscosity is significantly increased with rising SiO2 content and to a lesser extent Al2O3. All these considerations point to the ferric oxide, Fe2O3, being a more effective flux than the alumina, Al2O3. The alumina does not contribute to the flux at the lower temperatures and also increases the viscosity of the flux. Both factors should be detrimental to the fluxing of the kiln, increasing the temperature required for clinker formation and fuel consumption of the kiln. However, cement companies in fact experience the opposite effect, with a fall in the alumina content at the expense of ferric oxide resulting in lower kiln outputs and higher fuel consumption. Dr Stanislav Chromy, the renowned clinker mineralogist from the Czech Republic has a ready explanation for this. Due to the lower molecular weight of alumina in comparison
with ferric oxide, liquid phases rich in alumina are much less dense than those richer in ferric oxide. The lower density means these fluxes occupy a much greater volume in the kiln and are much more effective in wetting the clinker and promoting C3S formation. Mass percentage and viscosity are not the overriding considerations. Volume percentage is! Just as “wetting” of the C2S surface is important to promote the conversion of the C2S into C3S so the “wetting” of the refractory lining by the flux is an important contributor to coating formation on the lining in a cement kiln. Until there is sufficient liquid phase present to “wet” the lining there will be no coating formation. This explains why coating is only present in the upper transition and burning zone of cement kilns. This then is the chemistry underlying the formation of coating in the rotary section of cement kilns. Why then does this coating sometimes locally thicken into rings to constrict the flow of exhaust gases through the kiln and the passage of the clinker down the kiln? Why does the kiln feed sometimes ball up into clinker boulders which can be more than 2m in diameter? In terms of the formation of rings our usual answer in the Tech Forum is that it is caused by a local cooling of the charge resulting in the viscosity of the flux changing at a particular point in the kiln. This local cooling might be external through the kiln shell or brought about by a phase change at a particular point. We have not offered any explanations for the formation of clinker balls other than that they are often associated with heavy sulphur recirculation in the kiln and this affects the quantity and properties of the flux. At this stage we should perhaps highlight that sulphate, SO3, is not included in the equations for calculation of the amount of flux present at different temperatures in a cement kiln! Alkali sulphates certainly melt contributing the flux and affecting its properties. Viscosity and surface tension are reduced making the formation of clinker nodules more difficult and leading to a finer clinker size distribution with large proportions of clinker dust. That provides no explanation as to why sometimes this causes the opposite effect of the formation of large clinker balls! These short comings in our standard answers to such enquiries have prompted a reappraisal of the topics of coating, ring and clinker ball formation. That reappraisal began with a literature search to see what has been published on the topic. Searching back through 15 years of International Cement Review back-issues reveals that remarkably little has been published on the topics. The following is a review of the literature starting with the topic of coating formation. The first point to make is that there are different types of coating that form in a cement kiln system. There are coatings that form on the impellor of the preheater induced draft fan. There are coatings that build-up in the preheaters and kiln inlet chamber of cement kilns. There are “snowmen” that build-up on the first grate of a clinker cooler. There are “rhino horns” that build up on the top of the main burner of the kiln. These are not the subject of this discussion. Here we are talking about the clinker-like coatings that form on the refractory lining in the rotary section of the kiln. These coatings on the refractory lining of a cement kiln are desirable because they protect the refractory lining from thermal shocks, abrasion by the kiln charge and
chemical infiltration by the gases in the cement kiln. The uneven surface of the coating assists in transferring heat into the kiln charge by causing the charge to climb and tumble with the rotation of the kiln. The coatings also thickens the effective refractory wall in the kiln, increases the insulation of the kiln shell from the high temperatures of the process and reduces radiation losses from the kiln and therefore overall thermal energy consumption. The formation of this stable, desirable coating is dependent on the kiln feed and clinker properties, the refractory lining material and the operating regime in the kiln. The principle means of coating formation is by the clinker flux wetting and penetrating into the refractory lining to the depth in the lining where the flux stiffens and freezes. The wetted surface of the lining then glues solid particles to the lining building up into a coating. Clearly the amount and properties of the flux at different points in the kiln will affect its ability to form coating by this mechanism. Increasing alumina to iron oxide ratio makes formation of coating more difficult, as does increasing silica to alumina ratio. The optimum alumina modulus for stable coating formation is reckoned to be between 1.8 and 2.3. The optimum silica to alumina modulus is reckoned to be between 2.5 and 3.3. No doubt the type of refractory lining has an effect of the ease and stability of the coating formed. Magnesite chrome based lining provided for good, stable coating formation but these are increasingly not being installed due to the potential for hexavalent chromium formation, contamination of the clinker and the problems in disposal of the spent refractory lining. Magnesia spinel linings form coating less easily and of a more unstable nature. The most stable coating is formed on dolomite burning zone linings, but these require long uninterrupted campaigns due to atmospheric hydration and destruction of the lining if the kiln is stopped after the lining has been brought into service. Finally a stable operating regime is critical for maintaining a stable coating on the refractory lining in the kiln. Any changes in the feed and fuel delivery rates, burnability of the kiln feed, or the combustion of the fuel in the main burner of the kiln will affect the temperature profile and the amount and properties of the flux at different points in the kiln. With these changes in the amount and properties of the flux the coating will build in different places and might be destroyed after it has formed. Stable operation is critical for maintaining a stable coating on the refractory lining and obtaining long campaigns of service from the refractory lining. So formation of a coating is a good thing and dependent on the amount and properties of the flux at different points in the cement rotary kiln. Why does this coating sometimes grow to a thickness of more than 0.5m, becoming a ring and presenting problems for the operation of the kiln? In a dynamic, high temperature process such as a cement kiln there is a balance between the accretion, or growth, of the coating at a particular point and the erosion, or destruction, of the coating at a particular point. What are the conditions that cause the accretive, or growth, tendencies to far outweigh the erosion, or destruction, tendencies at a particular point and for the coating to grow into a ring? What does a literature search tell us? In the September 2002 issue of ICR Linda Hills’ article stated that: “Changes in burnability and/or clinker formation can have important practical impact on what
happens in the kiln, the mill, and the product. If the mix is hard to burn, the operator will be obliged to increase the burning zone temperature to achieve the desired free lime level. Hard burning will tend to cause low clinker porosity, large crystals of alite, and often contributes to generation of dust and/or large clinker balls, instead of good, nodular clinker.” In the July 2006 issue of ICR Holcim’s alternative switch article stated that: “….. previously experienced problems with sulphur rings in the kiln and excessive sulphur content in the raw meal had been eliminated, thanks to the narrow flame produced by the Duoflex burner, which prevents the flame from impinging upon the kiln charge.” In the March 2007 issue of ICR Argos’s optimising coolers article stated that: “Installing a new static front end or a complete grate using the original casing should be analysed in detail from a process point of view. When the cooler throat is small or the cooler roof is low, static grates reduce the available area for gases going into the kiln, this is compensated either with higher gas velocities or restrictions of the gas flow into the kiln. The first situation usually generates clinker dust which is carried back from the cooler to the kiln causing rings and flame deformation. With the second method, reducing conditions are very probable with the known consequences.” In the September 2007 issue of ICR Ashgrove’s article on their Montana cooler investment stated that: “The plant has also seen an improvement in the handling of snowmen and large chunks of coating within the new cooler. Running a high secondary air temperature usually resulted in unstable kiln operations, with fine clinker and the formation of rings in the kiln and snowmen in the cooler. The original cooler could not address these issues, requiring plant personnel to frequently shoot out the build-ups. As the plant has become comfortable with the new cooler and its ability to handle the chunks coming from the kiln and the air blasters’ ability to prevent the formation of snowmen on the inlet of the cooler, shooting in the cooler is no longer necessary.” In the September 2008 issue of ICR Stewart Service’s article on petcoke related refractory problems stated that: “The results of flame fluctuations can be readily seen in kilns as multiple sulphur rings, sulphur balls, burnt-out brick in the No 2 tyre neighbourhood and nose ring brick problems.” There are certainly clues in these articles but no definitive analysis of cause and effect. Kiln feed chemistry, sulphur recirculation, kiln main burner adjustment and cooler operation all have an effect on ring and ball formation. But which one in a particular instance? Before we can answer the question of why the accretive, or growth, tendencies far outweigh the erosion, or destruction, tendencies at a particular point for the coating to grow into a ring we also have to recognize that different types of coatings and rings occur at different positions in the rotary section of a cement kiln. In the calcining zone of a long kiln, or suspension preheater kiln, rings can form due to the formation of the spurrite, 2C2S.CaCO3, intermediate mineral. This mineral is reported to have a monoclinic, needle-like structure causing the material to bind together. It’s formation requires that partially combined C2S be present in combination with uncalcined calcium carbonate, CaCO3. For this reason it is not normally encountered with
modern, precalciner kilns where the CaCO3 is almost fully calcined to CaO in the precalciner before entering the rotary section of the kiln. However, there is an analogue of the spurrite mineral, sulphate spurrite, 2C2S.CaSO4, where the calcium carbonate, CaCO3 is replaced by calcium sulphate, CaSO4. This mineral can certainly be present at the inlet of modern, precalciner kilns where there is an excess of sulphate over alkalis in the hot meal. Such a situation might arise from reductive burning at some point in the kiln system and attendant heavy recirculation of sulphate. This is thought to be a common cause of kiln inlet rings in some modern, precalciner kilns. Such spurrite and sulphate spurrite based rings are particularly problematic. They form too far back from the kiln outlet to be effectively shot out by an industrial gun firing through the kiln hood. Sometimes they can be addressed by CO2 charges fired through the shell of the kiln is ports for the firing of such charges are provided. As they form so close to the kiln inlet they can dam the calcining feed, causing it to spill back through the kiln inlet seals presenting a significant hazard to personnel and equipment. The kiln has to be frequently stopped to cool down and physically remove the ring. The only satisfactory solution to such ring problems is to adjust the process to prevent the sulphate building up in excess of the alkalis in the hot meal. This might involve changing the raw material or fuel inputs and most importantly avoiding reducing burning conditions at any point in the kiln. Further down the kiln any coating and rings are formed by sintered material, i.e. all the CO2 has driven off the calcium carbonate and the liberated lime has begun to combine with the acidic SiO2, Al2O3 and Fe2O3. Once the temperature is high enough for significant flux formation then coating begins to form on the refractory lining, but any local cooling can cause the flux to freeze and the coating to grow into a ring at that point. At the beginning of the burning zone where the flux is first forming the refractory is successively cooled as the kiln rotates and the lining is exposed to the hot kiln atmosphere and then cooled as it turns into the cooler charge. The flux adhering to the lining softens as it is exposed to the kiln atmosphere and then stiffens as it is cooled by turning into the charge. The weight of the charge causes solid particles to adhere to the stiffening flux layer and a ring can rapidly grow. Such rings can usually be combated by minor adjustments to the kiln main burner, slightly shifting the burning zone and the position in the kiln where flux is first formed, with this shifting causing the ring to break away and reform at a different point. More problematic are longer, middle rings caused by recirculation of clinker dust in the combustion product gases from the main burner. Dusty clinker leads to a heavy dust burden in the secondary combustion air drawn into kiln and flame from the cooler. At the temperatures in the flame the flux in the clinker particles re melts and is carried back to the point in the kiln where the flux stiffens again and sticks to the refractory lining. An elongated ring builds up in layers with a fine structure showing the curvature of the kiln. These rings can be shot out with an industrial gun but their elongated nature means that many rounds and significant downtime can be required. The solution to problem is to eliminate the dust recirculation from the cooler. This is often best achieved by adjustment of the main burner to produce a short, dense flame. Long, lazy flames tend to produce dusty clinker due to higher sulphate recirculation and/or slow heating and cooling of the clinker.
The final rings we should mention are kiln outlet or nose ring rings. The secondary combustion air drawn into the kiln leads to cooling of the clinker falling over the nose ring. This can cause stiffening of the flux in the clinker and its pronounced adhesion to the refractory lining at the nose ring. This tends to dam the combined clinker in the kiln at high temperature and can lead to clinker quality problems. These kiln outlet rings can be combated by pushing the burner into the kiln creating a clinker cooling zone within the kiln itself. Adjustment of the insertion distance of the burner into the kiln will significantly push the burning zone up the kiln and can lengthen the burning zone requiring other adjustments to maintain a short, intense flame. Finally we come to the question of clinker ball formation. Linda Hills talked of “Hard burning will tend to cause low clinker porosity, large crystals of alite, and often contributes to generation of dust and/or large clinker balls, instead of good, nodular clinker.” Stewart Services talked of: “flame fluctuations can be readily seen in kilns as multiple sulphur rings, sulphur balls……”. Back in December 1997 Floyd Hamilton talked about “a long flame, directed along the axis of the kiln, causing the formation of a material ball. Some of these growing to 6 to 8 feet in diameter, and burning with a shorter flame preventing their formation unless caused by alkalis, sulphur and chloride.” Again clues, but no definitive cause and effect. Elsewhere it is stated that balls tend to form where there is already a tendency to form meal or sinter rings. There needs to be a core, or seed formed and then the build-up into a ball is caused by the weight of the ball itself as it turns with the rotation of the kiln, compressing material and sticking it to the surface. Those of us who made snowmen or had snow ball fights in our youth will understand how that mechanism would work. This is where the Tech Forum needs to be thrown open for contributions from regular readers encountering such problems with clinker ball formation (or indeed excessive coating and rings). I know that clinker ball formation can certainly be associated with excessive sulphur recirculation caused by reducing burning at one or other point in the kiln. These sulphur balls sinter to a hard shell as they pass through the kiln, but contain unsintered material when broken open. Anecdotally and also from distant experience I know that changes in the MgO content of the kiln feed and clinker can lead to clinker ball formation. These MgO related balls are hard and fully sintered right through to their core. Has anyone got any contributions they can make? If so send them to
[email protected]. Also anecdotally these issues with clinker balls tend to come and go. To my mind this means that some kilns are susceptible to clinker ball formation. There is an underlying “latch” condition that makes the kiln susceptible. Then at certain times a “trigger” condition arises which causes clinker balls to start to be formed. Any thoughts on what those “latch” and “trigger” conditions might be would be of major interest. Tech Image:
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