System for composing a process feed The present invention concerns a system for composing a process feed of different material fractions. The invention is applied to composing the feed of material fractions, where the firm material is in a grain-like form or as corresponding loose material. In this connection the grain-like material stands for firm material in the form of small-sized particles, even powdery, said material having gravitational flowing properties. One or some of the frac- tions, however, may be comprised of material in very small-sized particles which require assisting activities in order to achieve a gravitational flowing condition, such as fluidiza- tion.

One application of the present invention is composing the feed for a melting furnace of a glass production process. The feed of the glass furnace consists mainly of quartz sand and different chemicals, such as calcium carbonate, sodium carbonate, aluminium oxide, so- dium sulphate, colouring agents etc, depending on the use of the glass to be manufactured.

The different material fractions of the feed must be in carefully predetermined proportions.

The feeds have been composed as batch feeds comprising fractions that are carefully weighed out and mixed each feed separately at a time to be conveyed to the furnace. It is well known to control the implementation of composing of a batch with a weighing device, connected to a mixing device.

Glass production as an example in this presentation is a continuous process that has been designed to continue uninterrupted even more than ten years, and it does not allow any sig- nificant disturbances in the material feed during this period of time. This requires a very high level of reliability of the feeding device, and therefore the device has been secured by parallel devices to be taken into use in different phases of composing the feed, when neces- sary. One essential device in this connection is the mixing device to guarantee the uniform- ity of the final feed, and a conventional feeding device comprises at least two of them par- allel in working condition.

Despite the careful weighing of the fractions to be conveyed to the mixing device in use respectively, cases have been noticed by control weighing in connection with the mixing, where the weight of the batch to be mixed does not correspond to the default value deter- mined by the batch weighing. Part of the material fractions may have been left as congeries in the elements of the device between weighing and mixing, or congeries of material hav-

ing gradually piled up from preceding batches have come off from these elements. Such a batch has to be removed as rejected material in order to avoid problems in the further proc- ess. As origin of the problems have turned out to be the elements of the device, which had to be added because of the alternate use of said mixing device. Such parts of the device are for example intermediate cyclones, flow dividers as well as flow spouts and pipes connect- ing them together and to the weighing devices.

The problems mentioned above have been remarkably reduced by means of the system of the present invention, characterized by the attached claim 1. The characteristics of the spe- cial embodiments of the invention have been described in the attached independent claims.

The invention will be described in more detail in the following, with reference to the at- tached drawings, wherein Figure 1 is a general view of the system in accordance with the present invention for compiling a feed batch for a glass furnace; Figure 2 illustrates one feeding/weighing device useful in the system in accordance with the present inventions, and; Figure 3 is an alternative feeding/weighing device to be used in the system of the invention.

In the device of Figure 1, the raw material batch to be fed to the glass furnace is composed of material fractions, the most part of them being stored in silos, marked with reference number 1, which are a part of the device. The material fractions are taken from these silos fraction by fraction by means of an appropriate feeding device 2, each silo in this embodi- ment having its own device. The feeding devices 2 unload the material fraction into the collecting weighing devices 3,4 and 5. In this embodiment the operation of the weighing devices 4 and 5 is designed to control the feeding device 2 of respective material fraction for collecting the desired quantity of said fraction into the weighing device. In this em- bodiment, the weighing device 3 is meant to receive quartz sand, which is the main frac- tion of the feed batch, and it has advantageously an operation, which will be described later. It may of course have the same operations as the weighing devices 4 and 5, but cer- tain benefits to be described later can be gained with the special operations.

In the embodiment described herein, the weighing devices 3,4 and 5 are provided accord- ing to figures 2 and 3 as scales equipped with collection reservoirs 6,7 and 8, and they can be emptied by means of conveyor belts located in connection with the collection reservoirs.

The weighing devices 4 and 5 are formed in the presented embodiment of separate collec- tion reservoirs connected to a common weighing unit. The weighing devices 4 and 5 can naturally be constructed as complete devices, each including its own weighing unit.

In order to realize the basic idea of the present invention, the conveyor belt has been made to unload to one or the other direction. By means of this unloading operation, the material fractions collected to the weighing unit for a feed batch can be directed to a mixer 9, re- spectively 10, being in use at that moment. No dividing equipment or intermediate cy- clones are needed, that would form elements causing congeries on the way of the weighed material flow, and thereby inaccuracies in quantities of the material fractions to be fed to the mixer.

In the implementation of the invention, one weighing unit is preferably used, for instance the weighing unit 5 for dry material fractions and respectively the weighing unit 4 for damp material fractions, which further reduces the accumulation of the material and forma- tion of layers in the device.

In some embodiments it is advisable to use separate weighing devices for material frac- tions which tend to react with each other.

In case any of the material fractions forms a major part of the feed batch to be composed, such as the quartz sand in the feed of the glass furnace of the given example, it is advisable to reserve a separate weighing device for this material fraction, such as the weighing de- vice 3 in the device in accordance with Figure 1. This weighing device can advantageously be made to operate with the loss-of-weight principle. Thereby the material fraction needed for each weighing batch will be portioned out to the reservoir 6 of the weighing device as an excess, and the exact portioning of the fraction will be performed as said operation after the lightening when the weighing device in question is emptied. The emptying is then advantageously performed timely controlled so, that the emptying time will be matched with the emptying duration of the other weighing devices (the devices 4 and 5). The operation significantly facilitates the operation of the mixer 9 or 10 in use.

In the above description of the system, the weighing devices 3,4 and 5 are equipped with conveyor belts as an example only. Naturally, in stead of conveyor belts other devices hav- ing bidirectional conveying capabilities can be used, such as screw conveyor, shaking con- veyor etc.