Flotation process

Flotation (of English: to float - float ) is a physico- chemical separation technique for fine-grained solids due to the different surface wettability of the particles. It makes use of the fact that gas bubbles easily attach to hydrophobic, that is difficult to wet by water surfaces and provide the particles buoyancy so that they swim. The prerequisite is that the gas used himself sparingly soluble in water. Under these conditions, also collect the hydrophobic gas bubbles on the hydrophobic particle surfaces. The method is used for example in the mining industry as a treatment method to separate ore and gangue in an aqueous slurry ( suspension). It has replaced the traditional blow in most application areas. Flotation is not one, as is often assumed erroneously to the separation process due to the density.

By definition, according to VDMA 24430 is a separation process, in which dispersed or suspended in water substances transported by adhering gas bubbles at the water surface, where they removed with a reamer.

Implementation

In the floatation air is introduced through a high-speed or lances and finely divided. In floatation surfactants and foam stabilizers contained here stabilize the bubbles. A species of particles can be worse moisten with water and therefore adheres better to the air bubbles. These particles float on the air bubbles and can be skimmed off with the foam. The remaining particles are to remain in the pulp and be pumped out at the end of the flotation process.

Today, 90 percent of all lead, zinc and copper ores are concentrated by flotation. A fine grinding is necessary for this process to adequately digest the ore contained and allow the finely divided air bubbles to transport mineral granules to the surface of the pulp and keep it in the foam blanket. Other substances such as potassium chloride crystals can be concentrated by flotation.

When making paper from recycled paper is a water - waste paper mixture is subjected to flotation to remove the ink. Likewise, the flotation process is used for the treatment of sewage in treatment plants to bind fine particles of dirt and suspended solids. In medicine, flotation has been used in the faeces for the detection of parasite eggs.

In flotation, various excipients are used, which form the so-called flotation medium. In most systems, the flotation medium (e.g., water, oil, etc. ) is recycled to the process, i.e., reused. By adhering to the flotation materials, however, may generate a loss that needs to be replaced. The excipients can be classified as follows:

• Frothers are used to stabilize the air bubbles.
• Collectors make the auszubringenden in the foam mixture fraction water-repellent ( hydrophobic), while the other components are to remain water- attracting ( hydrophilic). In the slurry injected air adhere only to the hydrophobic particles and carries them to the water surface, while the hydrophilic particles remain in the slurry. Collectors are critical to the effectiveness of the method. As a collector, certain sulfur compounds are suitable (such as xanthates, Dithiophosphates, mercaptans ), amines, alkyl sulfonates, as well as some fatty acid salts.
• Pusher ( for example, sodium and potassium silicate in the Kunststoffflotation ) improve the wettability ( hydrophilic ) and to accelerate the lowering in the separation medium.

Flotation in paper recycling

Flotation is an important process step in paper recycling in Europe. With the flotation deinking is the whiteness of the waste paper used are increased by the removal of the printing inks. This exploits that only the hydrophilic fibers are wetted by water, while the hydrophobic ink remains largely unwetted.

The water in the flotation apparatus are added -inking chemicals such as soap, sodium hydroxide and water glass. After the introduction of air, the separated ink particles attach to the air bubbles and then float as foam on the recycled pulp suspension. The foam is skimmed from the surface or aspirated ( skimming ). Then the process repeats several times.

This paper can be processed by this method, it must meet certain requirements. For example, from made ​​in flexo printing newspapers not remove the inks used. They therefore pose considerable problems for recycling. Even low levels of such newspapers in the paper lead to an unacceptably poor whiteness. The ink particles are too small and not hydrophobic, why not work with them flotation. Similar problems are preparing pigmented inkjet inks, digital printing process using liquid toner and UV coatings.

An alternative to flotation offers the washing, which is used more frequently in North America, while in Europe dominated the flotation process. Both processes separate the ink in different ways, more or less selectively extracted from the waste material. In flotation, but where less fibers lost (about 10 per cent, in the wash - depending on the ash content - 20 to 30 percent). Therefore now wins the flotation in North America in importance.

New deinking plants for waste paper processing of graphic papers operate almost exclusively by the Flotationsprinzip. In contrast, the laundry is mainly used in the production of sanitary paper ( paper towels, toilet paper ). Here are all possible fillers are washed out, so that only the soft, long paper fibers remain.