BELOW IS SUMMARIZED CEMENT MANUFACTURING PROCESS, FOR DETAILS CLICK FOLLOW.
Raw Material Preparation.
Brief Information From Quarrying to Raw Meal Preparation in a Structured Form, For Cement Professionals and New-Comers
Brief Information of Fuel Preparation and Clinkerization For Cement Professionals and New-Comers.
Cement Grinding Process
Brief Information About Cement Grinding Process For Cement Professionals and New-Comers.
The cement manufacturing Process can be broadly classified into following activities:
|Step No||Process description|
|1||Raw Material Acquisition (Quarrying)|
|2||Raw material preparation|
|4||Fuel preparation - Coal grinding|
|6||Cement storage, packing and transport|
Raw Material Acquisition
Most of the raw materials used are extracted from the earth through mining and quarrying and can be divided into the following groups: lime (calcareous), silica (siliceous), alumina (argillaceous), and iron (feurriferous). Since a form of calcium carbonate, usually limestone, is the predominant raw material, most plants are situated near a limestone quarry or receive this material from a source via inexpensive transportation. The plant must minimize the transportation cost since one third of the limestone is converted to CO2 during the pyro-processing and is subsequently lost. Quarry operations consist of drilling, blasting, excavating, handling, loading, hauling, crushing, screening, stockpiling, and storing.
Raw material preparation
The raw material preparation comprises of the following operations:
- Pre-Homogenization (Stockpiling)
- Raw material proportioning, drying and
- Blending and kiln feed
Crushing can be defined as comminution (size reduction) process to reduce the size of Run of mine (ROM) Limestone/sandstone to size (10-30mm) suitable for grinding operation in ball mill, VRM or roller press. Crushing of limestone can be accomplished either by single stage or two stage which depends on the physical, geological properties of limestone and the product characteristics required for grinding (Mill Type). Crushing is usually accomplished by either compression or impact. Primary crushers can be Impact crushers, Jaw crushers or Gyratory crushers. Secondary crushers can be Hammer crushers, Roll crushers, Cone crushers or Hammer mills.
Pre-homogenizing is necessary if the limestone shows large fluctuations in chemical composition and when handling large quantities of raw material. Pre-homogenizing is used primarily for the main components in cement production viz. Limestone and clay/silcastone, and other components of raw materials such as sand and iron-ore are nearly always homogeneous and therefore need not be blended
Different types of homogenizing stores are available.
- Circular homogenizing store with bridge scraper (CHO)
- Longitudinal homogenizing store with scraper chain excavator (SCE)
- Longitudinal homogenizing store with disc reclaimer (LDR)
- Longitudinal homogenizing store with bridge scraper (LHO)
Raw material proportioning
The homogenized limestone and clay/silcastone along with other raw materials like sand and iron ore are reclaimed into separate feed bins. The proportional ratio of the raw materials is fixed using raw mix design and is determined from the chemical analysis of the raw materials. The raw materials stored in individual feed bins are proportionally delivered through weigh feeders at the discharge of the bins to a belt conveyor or a feeder feeding the mill for subsequent drying and grinding.
Raw material drying and grinding
The raw mix that is fed in to the mill has to be dried and ground to the required product
specification (normally 16% retained on 90 microns). This is accomplished by grinding equipments known as mills. Mills belong to a class of equipments that accomplish comminution of materials in the fine range. There are two types of mills which are mostly used in cement plants for grinding raw mix viz. Ball mills and VRM.
Ball mills are essentially fine grinding units capable of grinding material to a fineness say 84% passing 90 microns. The size reduction technique involved here is attrition and impact. In ball mills, Hi-chrome steel balls are used to grind the raw material. The ball mill is located horizontally rotating about the horizontal axis. Material is fed from inlet and ground materials discharged through the other end of the ball mill. For drying the material, hot air is introduced to remove moisture in the raw material. Grinding can be in two ways viz. closed circuit and open circuit grinding. In open circuit grinding, passage of material is only once through the mill. Consequently, a consistent product fineness is difficult to achieve. Installations having a hydraulic roll press prior to mills are common. In closed circuit grinding, a high efficiency separator (O-Sepa or Sepax, Sepol) is in operation which separates the material that is ground from the mill in to coarse and fine. The fine material is the product while the coarse material is recirculated back in to the mill for further grinding to the required fineness.
Vertical roller mills (VRM)
Roller mills are being used frequently now because of low energy consumption when compared to ball mills. Roller mills use pressure and friction for size reduction of raw materials. Material in a rotating grinding table is crushed and ground beneath stationary hydro-pneumatic rollers. Ground material spills over the edge of a lower retention ring where it is carried by a rising gas stream to a classifier on top of the mill. Hot gases cause immediate evaporation of the material’s moisture. The classifier rejects oversize particles and returns them for regrinding. The desired product travels to a dust collector with the gas stream. The material is collected in the dust collector or ESP and ultra-fine dust laden gases are passed to Bag house/ESP for further cleaning before venting out to atmosphere.
The basic principle behind dry blending process is aerating the material in the silo thereby fluidizing it resulting in a complete homogenization of material. Blending is also improved by differential extraction rate from different extraction points in a sequential manner. The efficiency of blending system is calculated by collecting number of samples at feed and discharge points of silo.
Blending effect = v [ (Si2 - Se2 ) / (So2 - Se2 ) ]
where Si = Standard deviation of feed sample either on CaCO3 or Lime saturation factor
S0 = Standard deviation of outlet samples either on CaCO3 or Lime saturation factor
Se = Measurement error
Some of the silo types frequently in use are.
- Inverted cone
- Continuous Flow (CF) silos
- Fuller Random Flow (FRF) silos
- IBAU silos
This is the heart of cement manufacturing process. In dry process technology, the homogenized raw meal from the silos are heated to temperatures around 1000 deg. C in the preheater and calciner to undergo partial calcination (about 93%) of raw material. Different types/configurations of preheater are available.
ILC - Inline Calciner
ILC-E - Inline Calciner with Excess air
SLC - Separate Line Calciner
SLC-I - Separate Line Calciner and Inline calciner
SLC-D - Separate Line Calciner - Downdraft
SLC-D-NOx - Separate Line Calciner - Downdraft Low NOx type
Rotary kiln, in which the cement is burnt at 1300-1500 °C, is a long cylinder rotating on its axis and inclined so that the material fed in at the upper end travel slowly to the lower end. For dry process the rotating speed of the kiln is normally in the range of 2.5-3.5 rpm. The complete calcination of raw meal occurs in the kiln to form clinker. L/D of a typical two support kiln is 10-13 and 16-18 for three support kilns. The inclination of rotary kilns is 3.5%for short rotary kiln and 4% for three support kilns. The diameter of kilns ranges from 3.0m to 6m while the lengths vary from 36m to 95m.
Cooling of clinker occurs here. Material is cooled from temperatures around 1000 deg. C to temperatures as low as 65 degrees above ambient.
Some of the commonly used coolers are:
- CIS cooler
- CFG with RFT cooler
- SF-Crossbar cooler
Coal is normally used as fuel for combustion in preheater, calciner and kiln. The grinding of coal is done in ball mills or roller mills. Due to the high volatile nature of coal, safety measures must be taken to prevent explosion in coal grinding circuits. This is accomplished by the use of explosion vents and explosion doors which vents the pressure build-up during explosion and by inertization systems that immediately floods the system with inert gases like carbon-di-oxide or Nitrogen to replace and vent out O2 from the system.
The clinker from the pyro-processing area is ground in ball mills or roll press + Ball mill combination or vertical roller mills to form cement. Commonly used separators are in closed circuit grinding are: O –sepa, Sepol and Sepax separators to achieve a finish cement fineness of 3000 – 4000 Blaine surface (cm2/gm).
Cement storage, packing and transport
Cement from the cement mills is stored in large silos and then packed in packing machines before being sold out to the consumer market in standard jute/paper bags. Bulk loadout concept is fast catching up to meet the huge requirements at construction sites.