From the limestone quarry through to the final product,we have the right measuring technology for the cement industry
Portland cement is the basic ingredient of concrete. Concrete is formed when Portland cement creates a paste with water that binds with sand and rock to harden. Cement is manufactured through a closely controlled chemical combination of calcium, silicon, aluminium, iron and other ingredients. Common materials used to manufacture cement include limestone, shells and chalk or marl combined with shale, clay, slate, blast furnace slag, silica sand and iron ore. These ingredients, when heated at high temperatures, form a rock-like substance that is crushed into the fine powder that we commonly know as cement.
The most common way to manufacture Portland cement is through a dry method. The first step is to quarry the principal raw materials, mainly limestone, clay and other materials. Holding the right amounts of all the key ingredients for the production process is important and storage vessels come in many shapes and sizes including concrete bunkers and steel blending silos. Each vessel is fitted with mechanical or pneumatic discharge and filling points, and an effective level measurement must be easy to install and simple to maintain. A Solicap M capacitance point level rope probe is used to prevent overflow of bunkers and a robust rod probe to indicate low levels in blending silos. Ropes and probes are protected with an active build up compensation, for bulk solids with a caking tendency and a two-stage overvoltage protection against static discharges from metal silos.
After quarrying the rock is crushed. This involves several stages. The first crushing reduces the rock to a maximum size of about 15cm. The rock then goes to secondary crushers or hammer mills for reduction to about 8cm or smaller. A crusher is an expensive piece of equipment and to avoid damage to the liner and optimise energy consumption, a certain level must always be maintained. You can rely on the Prosonic S ultrasonic level measurement solution to monitor your crushers. Ten sensors can be linked into the Prosonic S transmitter saving you cost and set-up time in the field. Vertical mill process optimisation and grind effectiveness is a balance of air flow, optimum pressure and temperature control.
Mill product load is measured with a Deltabar S differential pressure transmitter. Deltabar S offers extensive diagnostic messaging so that troubleshooting any problem is easy. Air quantity measurement with a Deltabar S differential pressure transmitter optimises the drying process. Water metering is used to cool the mill components with Promag W electromagnetic flow meters. Oil is used on mills for lubrication to prevent damage to bearings and shafts. Various measurements are needed to ensure the correct and trouble-free operation of the mill. A Liquiphant vibration level switch is used to measure the oil level in the reservoir. This switch is not affected by any contaminants, such as metal filings, in the oil. A Ceraphant T pressure switch is used to ensure that the system pressure is maintained, and should the pressure drop, the crusher can be stopped before any damage is caused.
The crushed rock is combined with other ingredients such as iron ore or fly ash and ground, mixed and fed to a cement kiln. The cement kiln heats all the ingredients to about 1500°C in huge cylindrical steel rotary kilns lined with special firebrick. Kilns are frequently as much as six metres in diameter – large enough to accommodate an automobile and longer in many instances than the height of a 40-storey building. The large kilns are mounted with the axis inclined slightly from the horizontal. The finely ground raw material or the slurry is fed into the higher end. At the lower end is a roaring blast of flame, produced by precisely controlled burning of powdered coal, oil, alternative fuels, or gas under forced draft. As the material moves through the kiln, certain elements are driven off in the form of gases. The remaining elements unite to form a new substance called clinker. Clinker comes out of the kiln as grey balls, about the size of 3mm to 25mm- sized marbles.
The basic cement kiln system comprises a preheater in which feed material is prepared by heat exchange with hot exhaust gases from the rotary kiln. The challenge in installing a robust temperature measurement system includes the selection of cost-effective sensors and transmitters requiring minimum maintenance over the plant lifetime. For temperature measurement in the upper cyclones of the preheater, where sensor erosion is a major maintenance challenge, an Omnigrad TAF11 with an advanced ceramic thermowell made of silicon nitride is recommended. This material has been successfully tested with up to five times longer lifespan compared to AlSl310. For the bottom cyclones, we recommend Omnigrad TAF16 thermometers with a bar stock thermowell made from a nickel cobalt which offers unparalleled resistance against aggressive gases, stress cracking and high tensile strength against material impact.
Pyroprocessing is generally regarded as the heart of the cement-making process. It is the stage in which most of the operating costs of cement manufacture appear and is also therefore the area where most of the opportunities for process improvement exist. Significant maintenance time can be saved by automating air cleaning cycles in the preheater cyclones. Clogging in the cyclones happens due to chemistry of the raw meal and the hot exhaust gases. Monitoring is important to avoid downtime and hazardous situations. Gammapilot FMG60 offers non-invasive point level or continuous measurement to monitor build up before complete blockage occurs. Trigger points can be easily configured to proactively schedule maintenance. Pressure can be monitored and blockages in the cyclone are detected using a combination of Ermeto piping and Cerabar S pressure sensors with high-purity ceramic (99.9% Al2O3) measurement cells, which are overload-resistant. Accurately control the blending of raw meal as it passes through each cyclone and ensure instrument accuracy is not affected by plant vibration by selecting a remote sensor and housing combination. Carefully monitor the adverse effects of alternative fuels on the clinker process chemical reactions and process conditions in the cyclones with a combination of Gammapilot FMG60 scintillators and high-accuracy Cerabar S gauges. In addition, Cerabar S sensors offer integrated overvoltage protection of electronics against lightning strike and power surges.
The crushed rock is combined with other ingredients such as iron ore or fly ash and ground, mixed and fed to a cement kiln.
Clinker is discharged red-hot from the lower end of the kiln and generally is brought down to handling temperature in various types of coolers. The heated air from the coolers is returned to the kilns, a process that saves fuel and increases burning efficiency. After the clinker is cooled, cement plants grind it and mix it with small amounts of gypsum and limestone. Cement is so fine that 500 grams of cement contains 150 billion grains. The cement is now ready to be transported to ready-mix concrete companies to be used in a variety of construction projects. The rate of cooling can be critical to the clinker quality and performance of cement.
A clinker cooler package includes:
- Differential pressure flow sensors for metering and monitoring cooling air consumption.
- Soliwave non-intrusive microwave blockage barriers to detect material back up in silos under the clinker cooler grate.
- Non-invasive Gammapilot radiometric scale for mass flow on clinker screw conveyors or belt feeders (indicates clinker density and infers free lime levels).
- Omnigrad TAF11 and TAF16 temperature sensors and transmitters, which last on average 10 times longer than regular stainless steel thermocouples.
- The Solicap S high temperature point level switch for fine-grained and coarse solids (designed with a sword probe which prevents build up and associated alarm conditions). The robust sensor Solicap S design is unique in the market as it permits a maximum lateral loading of up to 800Nm.
Clinker is stored in large storage silos, domes and buildings and the total capacity should amount to at least 14 days’ kiln production to ensure a continuous cement supply. Endress+Hauser level solutions can assist you maintain supply to the mills and prevent overspills of clinker dust which may be hazardous to air quality.
Continuous level measurement in clinker silos can be done with a non-contact Micropilot M radar measuring device (max. 400°C/752°F at antenna); for greater distances, use the version with parabolic antenna. Point level detection can be done with capacitance high-temperature rope probe (max. 400°C/752°F). Level measurement in cement silos is challenging and a continuous sensor health check is key in improving reliability and availability of measurements. Micropilot FMR57 is the radar level sensor best suited for measurements in high silos. The parabolic antenna facilitates very small emitting angles and is used for narrow multi-chamber silos. Dust, filling noises, temperature layers and gas layers do not affect the measurement. Use the Micropilot FMR57 health check diagnostics to monitor for deteriorations in measurement integrity caused by cement build up, steep angles of repose and rat holing. The dual output module with HART digital communications can automate radar cleaning cycles, activate silo air fluidisation and make reliable stock level measurements. The HistoROM data management chip records measurement and set-up data in each radar. Plug in touch screen displays copy and transfer set-up data to new or replacement radars. You can reach the highest reliability even in the presence of obstructions in the silos due to Multi-Echo Tracking evaluation algorithms.