Part 2 Dispatch in bags The cement markets in Latin America are typically consumer-oriented since individuals consume the majority (close to 80% in some cases) of cement in bag form, and brand loyalty often plays a role. Sales are the result of price and quality.

When talking about quality, one refers not only to the quality of the cement itself, but also to the presentation of the product and the integrity of the packing. The optimum packing, the adequate weight, and the integrity of the bags is linked to the equipment used for filling and manipulating the bags.

Another aspect that should be taken into consideration is material flow. To avoid bottlenecks in the production flow, the dimensioning of the packing plant requires taking the following into consideration:

- Available area This determines the main dimensions of the equipment, as well as its location and general arrangement.

- Weight The weight of the bags could be as follows: 50 kg, 42.5 kg, 37.75 kg, 25 kg, and/or 21.4 kg.

- Total production capacity The total capacity of a packing plant can be calculated according to the formula below:

- Dispatch of different products

- Capacity and general arrangement of the storage system The storage area for filled bags and the truck dispatch area will be dimensioned based upon the possible flow of trucks.

- Flow of trucks

Generally, a modern cement plant (Figure 1) has the following equipment and systems:

- Bucket elevator (chain or belt) or screw conveyor;

- Pre-bin with level indicators;

- Rotary feeder;

- Packing machine;

- Truck loading (open or closed trucks);

- Palletizer with load securing; and

- Electrical control and automation.

Packing plants The general arrangement of a packing plant is limited to the available area, especially when the dispatch is carried out under the silos. For this reason, the packing plants need to be compact. When a large amount of products is set to be dispatched, the packing system should be automated, having all the necessary systems for controlling and weighing the filling of the bags.

A modern packing plant should be able to handle different types and sizes of bags. A standard packing plant (Figure 2) consists of the following components :

- Mechanical conveyor for material/spillage transport;

- Vibrating screen for separating foreign bodies and agglomerated cement;

- Packer feed hopper for de-aerating the material and achieving a uniform feed into the packer;

- Shut-off gate and rotary cellular feeders;

- Rotary and in-line packers for filling the cement bags;

- Automatic bag applicator;

- Belt conveyor system for removing and distributing the full bags;

- Bag rejection system for incorrect-weight bags;

- Pneumatic bag cleaning system;

- Spillage collection and transport equipment; and

- Compressed air station.

The cement is fed by either a screw conveyor or bucket elevator. The product is directed via chute with flexible connections to a shaking screen, which screens out lumps and foreign materials. Once discharged from the shaking screen the product is discharged through a flexible connection into a surge hopper with conical discharge. The hopper is equipped with level indicators and with an extra high-fill level indicator, through which a connected interface stops flow when the cement reaches a high level. The hopper is fitted with a manually operated slide valve for maintenance purposes and a rotary valve for feeding the packing machine.

The packing machine is a rotary model with four, six, eight, 10, 12 or more spouts, depending on the required performance. Cement is gravity fed through the packing machine's hopper section to directly linked impellers - one per spout. Empty bags, in roll form, are fed via the applicator to one or several vacuum pick-up station(s) and shooting channel(s), which place a bag on each spout as the packing machine rotates. The weight-control unit checks the weight of the bags on each spout.

Filled bags are discharged from the packing machine onto a belt conveyor that feeds a check-and-control unit consisting of two bar rollers, through which every filled bag passes. The burst bags are automatically emptied in the same way the weight-control unit identifies any bag that is not within tolerance settings. The faulty bags are cut up and emptied as they pass through the bar rollers. The cement is discharged from the check-and-control unit into a horizontal screw conveyor. From there, it is sent to the vertical screw conveyor and the shaking screen for recycling.

Palletizers A safer and a more economic solution for truck loading is the loading of pallets. Fork-lifts move the pallets from the full-pallet storage directly onto the trucks. The time required to load one truck with approximately 400 bags (10 pallets) is about five minutes.

A modern palletizer has the following main characteristics:

- The palletizer is totally open and easy to access through its front.

- Since the palletizer does not have any component fixed to the floor, it is easy to maintain and clean.

- The equipment can be supplied with special software for monitoring and controlling the palletizing process.

- Since the palletizers are designed without rotary table or counterweight in the elevator, they are easy to maintain (Figure 3).

Where required for bag protection, the pallets can be covered with plastic. This operation can be totally automatic, making it possible to handle 100 pallets per hour. A recent trend observed in Latin America is the dispatch of bags covered with plastic without the pallet.

The palletizers are electrically driven, and only require air to block the safety cylinder of the elevator and the safety device for elevating the pallets. The line of bags is prepared step by step, avoiding the deformation of the bags.

Part 3 of this three-part series examines Latin America's bulk dispatch terminal options.

The selection of proper dispatch equipment at a cement terminal could mean substantial savings in the total production cost. In the second of this three-part series, we discuss packing and palletizer plants