All blending plants should include the following equipment:
1 Weighing or other proportioning device(s)
2 Dry mixer
3 Loading or bagging equipment
In addition, some control equipment is desirable but not essential if the other equipment is reliable.
Selection
This information does not set out to recommend equipment over any other but there are various types and their characteristics and advantages are different. The capacity of the equipment should be chosen to match output requirements in terms of tonnes per hour and tonnage per week.
Weighing
In batch operations, the whole equipment may be mounted on a load cell or belt weighers may be used. For continuous blending, individual belt weighers may be fitted to the belts feeding the raw materials to the mixer. The size and type of such equipment should be chosen to match the plant size and outputs required.
Unit Batch Weighing
These are the most popular systems. The outputs range from 20 to 100 tonnes per hour. Smaller systems have a floor mounted hopper with load-cell digital read out, fed from a front end loader. The raw materials are weighed in the proportions determined by the formulation. Batches are then conveyed to the mixing unit. Larger units are normally fed from overhead holding hoppers.
Continuous Weighing
These systems provide a continuous feed to the mixer by belt conveyors and may be continuous belt weighers or constant rate feeders. In the former the raw materials are fed onto fixed speed extractor weighbelts fitted with load-cells. Variations in weight are transmitted to the control unit which adjusts the hopper gate opening. Constant rate feeders have a fixed hopper gate opening and the belt speed is continuously adjusted to the predetermined rate of output. A load cell continuously monitors the amount of material on the extractor conveyor and sends signals to the motor speed control unit.
In both systems, information from the individual weight or speed controllers is fed to a central control unit which can be pre-set for the required output.
Volumetric Feeding
For some blending operations it is sufficient to use volumetric measuring of the raw materials to obtain the correct proportions in the blend. A separate hopper, usually fed by front end loader, must be provided for each raw material. All the components are handled simultaneously and the units are controlled to give the required blend ratio. This system however, relies on consistent raw material densities and many uncontrollable variables may cause changes in the bulk density. One of these factors will be the particle sizes of the granules and these may vary throughout a storage heap.
Mixers
The blending equipment is a major determining factor in the production of good quality blends. Different designs of mixers are available such as rotating drums, cone ended tilting mixers, stationary mixers with internal paddles, vertical mixers and volumetric mixers with variable speed screws. In addition, blending may be carried out without the use of a specific mixer. In this system all raw materials are fed onto a collector belt conveyor and, as each material is constantly fed at the correct weight, all materials are constantly layered onto the belt in the correct formulation ratio. Thus a cross section of that belt will at any time have the correct nutrient content. Subsequent mixing at belt changeovers as the blend moves through the remainder of the plant ensures a correct mix at all times.
The choice of design will depend on individual circumstances and is outside the scope of this resource. However, one aspect of mixing which is of importance is the mixing time. Mixing times are normally around two minutes and times above 5 minutes are not recommended because of the risk of particle breakdown and deterioration of the mixing quality.
After installation, the mixer must be tested to determine that satisfactory mixing is achieved. It is recommended that a coefficient of variation of less than 10% is achieved. Descriptions of mixing tests can be found in references [1] and [2].
Loading Equipment
At all stages after the mixing, great care must be taken to avoid segregation of the blend components. This means that coning must be avoided at all stages.
The final section of the blending plant may include receiving hoppers for bulk loading or bagging but in the simplest case the blended material may be loaded directly onto trucks from a conveyor system.
Further information on equipment may be obtained from manufacturers or by reference to the Proceedings of the International Fertiliser Society listed below.
Testing/Calibration
As a minimum control, records must be kept of all formulations, batch records, weight or volume settings and raw material analyses and sources. In addition the following are to be recommended:
- Regular checks of weighing or volumetric measuring equipment (minimum frequency, once per month).
- Frequent check weighing for bagged material.
- Random checks of the chemical analysis of finished products (minimum frequency, once a week).
Further information is given in the sections on Quality Control and Sampling.
Operations
Full written operating instructions must be available for all personnel. Operating procedures should be well documented and reviewed at regular intervals. Accreditation to ISO 9000 is not essential but is recommended as it offers an independent assessment of all procedures.
References
Links to related IFS Proceedings
87, (1965), Bulk Blending of Fertilisers: Practices and Problems, T P Hignett.
255, (1987), Blending in Ireland, T M Young.
256, (1987), Latest Technology and Methods Used for Bulk Blending and Fertiliser Storage in the United States, L Taylor.
387, (1996), Theory of Fertiliser Blending, G E N Lance.
388, (1996), Technology of Fertiliser Blending, J E Leonard.
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