For the industrial production of pure oxides such as iron oxide, alumina or magnesia as well as for mixed oxide products, the thermal chlorides decomposition in hydrochloric solutions has gained world wide importance. Typical, these processes use spray roasting in gas fired reactors to supply raw materials for the ferrite industry, pigments and heat resistant refractories and different types of special ceramics.
The thermal spray roasting technology , also called pyrohydrolysis, due to the chemical actions which happen during the thermal process in a gas fired reactor, has been developed around 60 years ago. In fact, the process is based on an Aman patent,which originally was developed to produce magnesia (MgO) from the Dead Sea brines in Israel. Later on, the Ruthner Company in Austria developed this process to technical perfection for its use in a number of processes – on one hand to recycle pickling acid in steel works, on other side to exclusively produce inorganic pure as well mixed oxides. Its application has been directed towards the steel making technology for recycling hydrochloric acid, as well as mixed acids, like hydrofluoric acid together with nitric acid for its use back into the special steel production process.
A typical spray unit (as shown in fig. 1) consists of a cylindrical brick-lined reactor with a conical truncated bottom, from where the oxide powders are discharged. The solution is sprayed into the reactor by two or more spraying booms, of which each is being provided with an array of spraying nozzles. The nozzles are made of long lasting metals, such as titanium or tantalum, designed to spray the liquid in full tapered mode, usually in an angle of sixty degrees. The whole cylinder is heated by gas burners fixed in special heating chambers.
A Close–Up Look on Pyrohydroysis Reactions
Liquid pressure, spraying angle, temperature inside the heated reactor , as well as salt concentration of the liquid are crucial parameters for results of the pyrohydrolytic process and the quality of the powder being made by it. The aggressive acid environment and high temperatures in between 700 – 900 °C require selected materials, such as refractory material of high quality, acid resistant metal such as titanium and plastic material for piping, such as PVC, PE or PVDF. The chemical procedure of the pyrohydrolysis can be generalized by the following equations:
which holds for n= 2, 4
The closer reaction paths are not yet fully known in detail due to their difficult kinetic and therodynamic performance, however it is assumed that they move along a chain of thermally controlled hydrolysis reactions followed by an oxidation, hence the term „pyrohydrolysis reaction“.
The powders formed in this type of process consist of hollow spheres, of very light specific weight. Sometimes, a further treatment, such as milling, is necessary – eventually followed by thermal treatment in rotary kilns – to achieve a certain crystalline rearrangement. This can be of high importance for example during production of ferrites, pigments, catalysts or special ceramics.
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