“The good must be put in the dish The bad you may eat if you wish” - already as a child you learn with Cinderella the advantages of a good sorting technique: to be better than a competitor means to be successful. Visual or optical sorting is probably the first separation method men. Who was able to distinguish a good from bad flint – the first 'industrially' used mineral – or, later on, who could distinguish copper minerals from waste rock, and this one was able to start a power-saving production of tools and/or pure metals. Later the composition of the melted material became more sophisticated – adding tin to the copper results in the harder bronze. With a better preparation of the melt by using pure minerals better products could be achieved. When crossing to the Iron Age new minerals were used and the know -how of sorting became even more important, because iron minerals are looking often very similar to the surrounding waste rock. A high quality of the 'optical sorter' who picked the minerals was asked. What are the properties to distinguish minerals from waste rock easily? Color, shine, reflection and others. All these properties can also be used to make decisions by a sorting machine. Even more, optical properties visible only in the UV or IR light can be examined by a sorting machine. Here follows a small chapter with definitions of the process technology. To separate mineral materials without dissolving it in a solvent, mostly two processes are used: the classification and the sorting. Classification is defined as a separation according to geometrical properties as length, surface area or volume. Sorting is carried out according to physical properties like the density, magnetic or electric behavior, to physical-chemical characteristics like the Zeta potential and according to other properties which can be called mineralogical properties, like color, shine, UV or IR-reflection and others. But there are some difficulties. Some of the above mentioned properties are overlapping in their reaction to the separation forces. These are e.g. the geometry or particle size and the density. Let's describe it with a very old process: the separating of the wheat from the chaff. One throws the threshed grain into the air. The wind carries off the light chaff with its big surface, while the smaller, compact wheat drops to the ground. Was it a separation of heavy and light or of small grains and large chaff? Was the separation influenced by the density or just by the particle size and form? The farmer or the farmer's wife who have invented this process did not matter this problem. Only the result was important, they wanted to receive grain without the chaff. And, they didn't want to lose grain with the chaff. Since there is a big difference in the behavior of the two materials wheat and chaff it seems to be obviously a sorting process and sorting was the goal. But if you just look at it with the eyes of a physician you can doubt it. The problem described can appear often separating two materials with different densities. We will discuss it later in a separate chapter. When sorting is applying optical methods, differences in density are fairly unimportant. Using optical detectors for sorting any signal of a detector can be a criterion. According to the definitions made above geometrical, mineralogical as well as physical properties can be applied. Even today we do not have detectors with sufficient high resolution, to utilize all properties of the material in order to get optimal separation results. However, the technical development is proceeding and we will participate! You are invited to discuss with us these problems and their solutions. Click on our homepage once again – the considerations applying sorting will be continued.