Objective
Aim of the project is to develop a system for grinding the ceramic raw materials for the production of porcelain stoneware tile and single-fired whiteware tile with the application of inverters to the mills in order to optimise the grinding cycles and reduce both the grinding times and energy consumption.
The innovation resides in the possibility of varying the velocity of the mills during the grinding cycle in function of the comminution curve of the material being ground. In this way it will be possible to determine the optimum performance of the mills for each grinding cycle, in function of the nature of the material being ground and the grinding conditions, by optimizing both the time of grinding and the consumption of energy. Up until now this has not been possible because of the rigid connection of the motor to the mill.
The application of variable speed to the mills got the results as follows : reduction of grinding time ranging from 13% to 23% and electric energy saving ranging from 18% and 22% at the same time. The differences depended on both the unlike ceramic body and the grinding couple (intented here as nature and density of grinding media and mill lining).
Global energy saving of 0.549 GWh/y and a payback time of 3.44 y were obtained.
The classic production process is :
- raw material preparation.
Raw materials are crushed using dry mills (hammer, toothed, swinging-arm and runner types) or wet mills (ball and pebble types). In the last case, a spray-drying of the slip produced by the mills is needed to obtain powders fit for shaping.
- Shaping and drying.
Shaping is by means of hydraulic presses, the former type being more widely used. A sorting operation is necessary between pressing and drying of the tiles. The drying is generally carried out by fast driers.
- Glazing and decorating.
Glazing machines with screen printing equipment and other accessories are used for this process.
- Firing.
Firing is by means of roller-hearth kilns without muffles. The ceramic article achieves its intended consistency through a series of physical and chemical reactions during this stage.
Discontinuous drum mills operated by alternating current with rigid transmission ratios are used for the production of porcelain stoneware tile and single-fired whiteware tile. The mill velocity to obtain optimum grinding depends on many factors : the nature of the material being ground, the density and size of the grinding media, the ratio between the amount of material being ground and the amount of the grinding media and the size of the mill. Since these factors vary considerably, mill manufacturers select standard sizes and velocities for the mill motors based on the most probable grinding
conditions. In practice, the mill velocity chosen almost never coincides with the optimum velocity for the real grinding conditions.
In addition, it is not possible to vary the velocity of the mill as a function of the communition curve of the material (residue-grinding time). The communition curve shows:
i. an initial phase of mixing of the material to be ground with the grinding media, inwhich there is little variation in the amount of residue;
ii. a second phase of communition in which there is considerable variation in the amount of residue, and
iii. a third phase of "refining" in which the residue decreases and the amount of variation arrives at an asymptotic value. The application of an inverter (device to vary frequency at constant torque) to the mill motor consents the following advantages:
1 - adaptation of the grinding cycles to the real grinding conditions and determination of the optimum mill velocity;
2 - possibility of varying the mill velocity during the grinding cycle, depending on the optimum velocity for each phase of the grinding curve;
3 - elimination of the peak requirements of the motor on the electricity input through gradual start-up with the inverter.
Results obtained with the pilot system have indicated the following further advantages, such that development of the system of an industrial scale is justified:
- reduction of energy consumption by the mills of around 25%;
- reduction of grinding time of around 20%;
- less stress on the mechanical parts, thus longer service lives of the parts
and lower maintenance costs;
- possibility of using electricity during the most economical periods of consumption (according to the Multi Hour Tariff System);
- possibility of increasing the production capacity of the grinding division without increasing the number of mills.
After the planning, construction and start-up of the plant (Phases 1, 2), collection of data during operation regarding the technological, energy and production aspects (Phases 3, 4) and evaluation of data gathered (Phase 5) have been made. Phases 3, 4, 5 also included verification of the system proposed by comparing the data obtained with and without the inverter for the same levels of production.
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Coordinator
40026 Imola Bologna
Italy
The total costs incurred by this organisation to participate in the project, including direct and indirect costs. This amount is a subset of the overall project budget.