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:
- Crushing
- Pre-Homogenization (Stockpiling)
- Raw material proportioning, drying and
grinding
- Blending and kiln feed
Crushing
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-Homogenization

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
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.

Cement grinding

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 load out concept is fast catching up to meet the huge requirements at construction sites.