From Roast to Paints
How to Use Pyrohydrolysis for Inorganic Oxide Prime Material for Ceramics
A complete spray roasting plant (as seen in fig. 2) consists of of the reactor, a preconcentration unit in the form of a venturi tube, further absorption columns for acid recycling during an azeotropic process, then followed by a cleaning unit for washing stack gases and the stack itself. The resulting oxide powder is transported via a rotary valve from the truncated steel bottom of the reactor and transferred to a powder bin. The powder may be also be finally pelletized by means of cylinder presses or by a pelltizing disc.
How to Control the Roasting Process
The Process control consists of fully integrated field bus systems such as temperatures, pressures, material flows, concentrations, filling rates, etc., which are observed in a control room by an operator. Quality controls for the produced powders includes measures such as specific weight, chemical composition, impurities like acidic residuals by incompleted reaction, bulk density, crystallographic data, grain size distribution by a granulometer, X-ray data, as well as REM electronic microscope investigations and the specific surface (BET) of the powder.
Since most of processes described here are derived from hydrochloric acidic systems a closer look at the chemical periodical system gives a view of the elements which can undergo a pyrohydrolysis reaction (fig 3).
It also means, that the chloride salts of those components which a suitable for the thermal pyrohydrolysis process must be thermally stable to be able to undergo the pyrohydrolysis reaction inside the thermally heated reactor, instead of escaping through the stack route, as seen in detail in the following equations:
The equations hold for 2-valued elements (n=2) such as Fe, Mg, Co, Ni.
The metals which fulfill these restrictions are also given by their vapor pressures (5) shown in Fig.4.
A survey of the Gibb´s Free Energies Δ G (6) ,which control the different reactions is given in a diagram versus reaction temperatures ( Fig.5) .
There are several important applications for these reactions in the process industry, each of them having its own production routes , industrial use and market. Further applications of iron oxides are colourants based on haematite, which amount of two thirds of world wide produced pigments, followed by chromium oxide. Since those prime materials mostly come from acid recycling plants by spray roast processes, a strict requirement in process parameters such as temperature, oxygen content during process performance, purity of the process water streams, as well as the kinetics during the roasting process have to be defined and strictly controled.