Cement is often referred to as Portland cement, a generic term used for nearly all cement. It is the mixture from which many types of concrete are made and is called such due to its resemblance to a type of building stone quarried in the early part of the 19th century. This stone came from the Isle of Portland, located off the southern coast of England. While the scale of cement production has certainly increased over the past two centuries, the basic methods remain largely unchanged for cement manufacturing. However, equipment used in the process has advanced considerably to make cement manufacturing safer and more efficient.
Cement Manufacturing: Equipment & Processing
Cement manufacturing equipment needs to withstand the heat and the abrasive material from which cement is made. Making cement involves combining raw materials including limestone, clay, and shale, which are milled into a very fine powder after being mined. These materials are then mixed with other ingredients according to their end-use. This mixture is then fired in a kiln at temperatures between 2732-2912˚F (1400-1500˚C).
Modern Cement Manufacturing Process
Mainly made from limestone and clay, mining these raw materials entails blasting or drilling using heavy mining equipment to extract them from quarries. Once these raw materials are extracted, they are processed further with cement manufacturing equipment onsite or taken to a plant that turns them into usable cement. Manufacturing equipment such as crushers and mills are used to break these materials down. Once processed, they go on to a finishing mill.
The four stages for manufacturing cement involve:
- Crushing and grinding raw material,
- Blending materials in correct proportion,
- Kiln drying to drive certain elements to form clinker, and
- Grinding the clinker into a finished product.
Crushing & Grinding
During this stage of cement manufacturing, equipment first crushes the harder materials into smaller particles. This usually happens in two stages, using a vertical roller (or other similar types of) mill to grind the material down. Depending on the exact process used for cement production, the material will be either wet or dry. When using dry grinding, raw materials are first dried in cylindrical rotary dryers. Meanwhile, softer materials go through wash mills, where they are vigorously stirred in water to produce a fine slurry that then passes through screens to remove oversized particles.
These raw mixes seek to approximate the needed chemical composition for particular types of cement as they are crushed and ground. Different batches of cement are stored in silos when using dry processing techniques or in slurry tanks when using wet processing. To reach the right mixture, compressed air circulates through the dry material in silos to agitate it. The wet process uses either compressed air or mechanical means to stir the slurry tanks. From here, the material is then fed into the kiln.
Modern rotary kilns are used most widely today to make cement. These can reach up to 660 feet (200 meters) long and 20 feet (6 meters) wide. They consist of a cylindrical shell lined with heat-resistant materials – either cast or brick – to protect the outer shell of the kiln. Rotating slowly on an axis, the raw material is fed from the upper end, sliding slowly down to the lower end where it is fired with fuel, most commonly natural gas, petroleum, or crushed coal. Temperatures within the kiln range from 2460-2820°F (1350-1550°C), producing a substance called clinker, which has a lumpy consistency. It then passes through coolers that use incoming air to cool the product. This material can then be ground immediately into cement or stored for later processing.
Clinker and gypsum are ground together into a fine powder before being milled further. This mixture passes through the mill, though the process may include separating coarser material that is then returned for further grinding. Sometimes the grinding process uses small amounts of specific mineral compounds to aid grinding, such as silica dioxide, calcium oxide or tricalcium silicate. This finished cement then gets pumped into bulk containers or paper bags.
Cement Manufacturing Equipment
The abrasiveness of material and high temperatures involved in processing requires that cement manufacturing equipment be particularly robust. For North American cement manufacturing plants, vertical roller mills commonly handle the raw materials that will become cement, often with a throughput of 150-750 tons per hour. Bulk cement manufacturing equipment like rotary feeders help transfer raw materials at different points during production. When feeding the raw cement mixture into the mill, a system of weight belts helps control the rate of feed.
Rotary Airlocks Within Cement Manufacturing Systems
One key component within modern cement manufacturing equipment that helps control the flow of dry cement powder is the rotary airlock valve. These are also known as rotary airlocks, rotary valves, or rotary feeders. They increase safety within a cement plant, acting as a flame barrier and reducing the chance of dust explosions.
Rotary airlocks accomplish this by preventing air loss during the transfer of dried bulk cement powder, controlling the rate at which the cement passes through the system while limiting steam or other vapor, or isolating areas with temperature differentials. Rotary feeders allow metering with minimal air loss, enabling control of the feed rate while keeping air leakage to a minimum within pneumatics systems. Additionally, these mechanisms can be used to help feed the fuel that powers the kiln.
Vertical roller mills are fed either through a flap-gate system or a rotary feeder. Both function with cement manufacturing equipment as airlocks to prevent unwanted air – also known as false air – from entering the mill and separator.
When false air enters the system:
- The ID fans that control the flow rate into the kiln need to be larger and faster, thus more costly.
- Larger, quicker fans use more electricity, so they are more expensive to operate.
- Turbulence occurs inside the mill, disrupting milling operations and often requiring that the material be reground.
As is the case in any industrial setting, downtime of cement manufacturing equipment costs both time and money. Modern rotary feeders offer greater reliability than older rotary feeders or flap-gates which require more maintenance. Flap-gates need constant tweaking to ensure the mechanism that opens and closes them works correctly while also requiring regular replacement of flap seals to preserve its airlock capabilities.
When it comes to mills used in the final phase of cement manufacturing, equipment like rotary airlocks function similarly to when used with raw materials, minimizing the amount of false air coming into the mill. Though rotary airlocks do the same job, they must deal with more demanding conditions when acting as finish mill feeders. These include:
- Air volume that is typically higher.
- Additional materials that can be even more abrasive, such as gypsum, limestone and other additives.
- Highly abrasive and fine clinker dust, along with high-speed airflow, essentially sandblast the internal components of cement manufacturing equipment like rotary feeders.
To curtail and control airflow going into the mill, rotary feeders need to withstand this sandblasting effect. For this reason, modern cement manufacturing equipment uses high-performance, abrasion-resistant steel.
Feeding the Kiln
Coal remains a common choice for fueling cement kilns throughout North America. Because raw coal is still used in cement manufacturing, equipment that can grind coal onsite is necessary for most cement plants. As with the actual milling of cement, manufacturing equipment used to break down the coal requires mechanisms with airlock capabilities.
These must be able to handle capacities from 10-100 tons hourly. Often, the raw coal comes in sizes of four inches or less and contains moisture. If shipped on open rail cars or open-top trucks, surface moisture also accumulates. The situation worsens if the coal is stored outside. Moisture creates an inherent stickiness that makes milling difficult. Flap-gates and conventional rotary feeders struggle to handle this raw coal, often underproducing cement as a result.
Using Alternate Fuels
Cement manufacturing plants also run kilns on alternate solid fuels, such as wood or waste, including used tires, plastics, nutshells, or even garbage. For these alternative fuels, conveyancing relies on either pneumatics to directly inject the kiln’s main burner or gravity-feeding by dumping the fuel into the calciner.
For both types of fuel systems, a rotary feeder functions like an airlock. Pneumatic conveying systems operating at 10 pounds per square inch or less need rotary feeders for introducing alternate fuels. In gravity-fed systems, they segregate negative pressure from atmospheric pressure to keep the calciner’s pressure balanced.
The most cutting-edge rotary airlock valves help in conveying and reducing these alternate fuels. These rotary airlocks feature blades that reduce oversized chunks that would otherwise interrupt the flow of fuel and jam the feeder. These types of airlocks decrease the amount of downtime and allow cement plants to utilize fuel that requires very little pre-processing. Additionally, when using pneumatic conveying systems for these alternative fuels, rotary feeders also act as an airlock, though they may also require metering capabilities as well.
Dust Collection Systems
As is the case in many other industries, dust collection is necessary to ensure safety within cement manufacturing. Equipment must deal with heated and highly abrasive dust, especially from the kilns. This dust not only has the potential to damage equipment, but it can also cause health problems in workers and even explosions.
Airlocks are important mechanisms within dust collection systems, helping prevent explosions and fires due to the dust from cement manufacturing. Equipment used in cement production dust collection systems most commonly features rotary airlock valves, although other types of airlocks can be used as well. Rotary airlock valves are one of the most versatile and vital components in many types of cement manufacturing equipment.
While the primary purpose of rotary airlocks involves preventing air under high pressure from infiltrating lower pressure environments and causing contamination, this element of its operations can also be used for safely conveying abrasive dust waste. Along with protecting equipment, protecting the health of workers and preventing explosions, rotary airlock valves help in the transitioning waste from dust collectors, which are pressurized, to dust collectors, which are unpressurized.