A typical rotary kiln used in iron ore pellet production usually has a length between 30-50 m, and a diameter of 5-7.5 m (normally not more than 7.2 m, as difficulties with the refractory lining may occur at larger diameters), and is fired by coal or natural gas. The highest temperatures in the process are achieved in the kiln, up to about 1400C. The refractory lining in the kiln normally comprises bricks based on Al2O3 and SiO2. There are also kilns lined with castables.
The Rotary Cement Kiln Saxena, J.p.||
Rotary kilns were originally developed in the late 19th century for Portland cement production, and the cement industry is still the largest user (Boateng, 2008). To improve energy efficiency in cement plants, a pre-heater in the form of a Lepol grate was used for the first time in 1927 (invented by Otto Lellep, marketed by Polysius), and it is from this system that the grate-kiln for iron ore pelletizing originated (Trescot et al., 2000). Today, rotary kilns have been adopted for processing several different metal ores (besides iron ore), e.g. nickel (Tsuji and Tachino, 2012) and titanium (Folmo and Rierson, 1992), as well as for direct reduction of iron ore (Tsweleng, 2013).
Coal always contains inclusions of mineral matter that remain as fly-ash after combustion (Reid, 1984). Disintegrated pellets can, together with fly-ash from the coal burned to heat the kiln, form accretions on the lining, sometimes as ring-forms in the kiln (Jiang et al., 2009; Xu et al., 2009). This phenomenon is also common in lime kilns (Potgieter and Wirth, 1996) and cement kilns (Recio Dominguez et al., 2010). This material can also be deposited as stalagmite structures in the kiln (Figure 5) or as accretions in the transfer chute.
Lining problems in rotary kilns in Wuhan, China have been reported by Xu et al. (2009). These kilns also had problems with rapid accumulation of deposits on the lining that were hard to remove. One of the main reasons for this was the use of burner coal with a high fly-ash content and low ash melting temperature. Another important factor was the compressional strength of the iron ore pellets, which was observed to depend on the qualities of the ore and bentonite, mixing method, and moisture content during mixing. Pressure and air flow were observed to be important parameters in one of the plants, since these dictate where the fly ash falls, and also affect the extent of pellet disintegration, contributing to deposits on the lining.
The refractory lining in the grate and in the cooler seldom causes failures that lead to urgent shutdowns, as are caused by kiln failures. Upon heating, the lining in a rotary kiln expands with temperature in proportion to the coefficient of thermal expansion (CTE) of the refractory (Shubin, 2001a).
Even during steady-state conditions, the lining is exposed to temperature oscillations as it rotates (Shubin, 2001a). In rotary kilns used for iron ore pellet production the lining can be assumed to be exposed to different temperatures during each revolution of the kiln. When the pellet bed covers the lining, it is exposed to less radiation from the flame, but is exposed to heat from the energy liberated from the oxidation of the pellets (when using magnetite). If the kiln rotates at approximately 2 r/min (which is common for rotary kilns in this application) it revolves 3000 times a day, with a temperature oscillation during each revolution. The temperature oscillation range varies from kiln to kiln, depending on operating conditions, but the temperature variation can be as high as 100C in the lining, down to a depth of 30 to 40 mm beneath the hot face (Shubin, 2001a). This cycle gives rise to thermal fatigue. Kingery (1955) showed that thermal expansion hysteresis is associated with microcracks in ceramic materials.
A study carried out in a grate-kiln plant in Kiruna, Sweden (Stjernberg et al., 2012), showed that migration of potassium through the hot face of the lining caused the formation of feldspathoid minerals, leading to spallation. Moreover, haematite was found to migrate into the lining. This phenomenon was also found in rotary kilns for pellet production in China (Zhang et al., 2009). Although these are relatively slow phenomena, they contribute to the overall degradation of the lining. Zhu et al. (2003) reported lining problems in a rotary kiln in Qian'an, China. Urgent production stops caused by fallouts of bricks occurred only a few months apart. Different types of brickwork and lining material (e.g. chamotte, high alumina, and phosphate-bonded alumina) were tested in the kiln to avoid fallouts and rapid deterioration of the lining. In 2000 a mullite brick of an increased size was tested, which was still in service two years later.
The riding rings (tyres) of rotary kilns are subjected to static and dynamic stresses caused by mechanical forces and temperature gradients, of which only the stresses caused by mechanical forces can be influenced by the dimensions of the ring. The initiation of a crack can be caused by either the static strength or the fatigue strength being exceeded. Hertzian pressures between the ring and the rollers reach their maximum beneath the surface, and consequently cracks are usually not visible until they reach an advanced stage. Riding ring cracks are in general not a consequence of poor dimensioning, but of unfavourable running conditions and/or material defects (Bowen and Saxer, 1985).
Riding cracks do not occur as frequently as some of other issues stated here. However, failures of the riding ring that necessitate replacement and the associated actions are time-consuming. A replacement of the riding ring involves cutting of the kiln on both sides of the ring, a heavy lift, and repair welding. It is therefore important that the riding ring satisfies the requirements of high rigidity or stiffness, high surface durability, and high static and fatigue strength, to achieve the longest possible lifetime (Bowen and Saxer, 1985). Figures 7a and 7b are from a riding ring replacement at LKAB's grate-kiln plant in Svappavaara, Sweden, in 2009.
Gearboxes, sliding and rolling bearings, kiln girth gears, pinions etc. require continuous maintenance. In addition to the slow operating speeds of many of these parts, there are thermal, alignment, and cleanliness issues that need to be considered. Safe operation relies on a hydrodynamic oil film to avoid metal-to-metal contact (Singhal, 2008). Use of inadequate lubricants may decrease the service life of these mechanical parts markedly. Hankes (2013) reviewed the selection and application of lubricants for rotary kiln girth gears and pinions. He stressed the importance of not only using a correct lubricant, but also of using it correctly. Monitoring is the key to avoiding catastrophic tooth damage.
Lovas (2003) reported the performance of two identical cement kilns: one that ran without problems; the other that was plagued with drive-related failures. On the problem kiln, the pinion had to be replaced three times and the gear realigned three times over a five-year period. During this time the identical gear and pinions on the comparison kiln remained as good as new. Analysis of the problem showed that the uphill face of the thrust tyres comprised several noticeable discontinuities. When the tyre was cast there were probably voids in the cast, which were repair-welded and machined. The welded portions of the tyre were much harder than the surrounding areas, and high spots were developed at these locations. As the kiln revolved, these high spots on the side of the tyre created a sharp impact load towards the discharge end. This pressure caused pitting and wear of the pinion and gear. The problem was resolved by resurfacing the thrust tyre and eliminating the vertical load between the thrust rollers and tyres.
In 2001 the Minntac division of US Steel introduced a ported kiln in their plant (Trescot et al., 2004). It was a well-proven design that had been in use for more than ten years at the Tinfos direct reduction kiln for ilmenite in Norway. However, this was the first ported grate-kiln plant for iron ore pelletizing. This system injects air under the bed of pellets in the rotary kiln through slots in the joints between the specially designed refractory bricks. This design results in more rapid oxidation of the pellets. The company noticed several benefits. As the magnetite oxidizes more rapidly, a lower kiln temperature can be used. With more energy liberated in the kiln, the heat load in the annular cooler is reduced, and therefore a higher tonnage can be produced. An improved pellet quality was also observed.
In the Tinfos direct reduction kiln for ilmenite in Norway, the lining consists of a monolithic castable applied by shotcreting (Folmo and Rierson, 1992). The air slots present for cooling complicate the installation of bricks, and shotcreting is a quick method. This could be the method of choice for the linings of rotary kilns for iron ore pelletizing in the future. Shotcreting is a fast installation method, and the lining does not have to be replaced as often as a brick lining. The drawback is that removal of the lining for maintenance is more complex.
Many burners used in the grate-kiln plants today are basically a lined steel pipe through which milled coal powder is blown. The combustion equipment used for the heat supply in rotary kilns for cement production is often far more complex than the burners used in the grate-kiln. The use of multi-channel burners for different fuels and different air channels allows adjustment of the flame shape during operation and ensures a stable flame front (Vaccaro, 2006).
Oxidation of magnetite in iron ore pellets occurs fastest between 1100C and 1200C. At higher temperatures the oxidation rate decreases as a result of increasing dissociation pressure and severe sintering in both the oxidized haematite shell (which becomes denser) and the magnetite core (Forsmo, 2008). As fully oxidized pellets already in the grate are desired when using magnetite (Niiniskorpi, 2002), a grate of increased length and increased dwell time in a zone where the temperature is optimal for oxidation, this, in combination with oxygen injection to further improve the oxidation rate, may be an alternative in the future. This could be combined with a shorter kiln in a machine comparable with a straight-grate plant completed with a short kiln, as the rotary kiln is the more sensitive part in the grate-kiln construction. 2ff7e9595c
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