From Roast to Paints How to Use Pyrohydrolysis for Inorganic Oxide Prime Material for Ceramics

Author / Editor: Dr. Wolfgang Kladnig / Dominik Stephan

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.

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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.

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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

or also

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.

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.

Iron Oxides — More than Just a Waste Product of Recycling

The iron oxides that are formed in this process are in the sole form of haematite, that is Fe2O3, with small amounts of FeO (wuestite) or also sometimes Fe3O4 (magnetite). First an unwanted by –product of the recycling process of spent pickling acids in steel industry of normal steel production, it became later on an important product as prime material for the ferrite industry.

In combination with further divalent metals like zinc, manganese, nickel, barium or strontium, and after a thermal treatment to produce certain crystal structures which provide soft or hard magnetic properties. These terms hold for permanent (hard) or demagnetizable (soft) properties which find numerous applications in permanent magnets (hard) or ferrites (soft) for electro devices and for radar applications and many electronic communication systems. Soft ferrites have a lower eddy current loss and higher resistances which enables them to switch magnetic properties very fast, up to 15 000 to 20 000 times a second. Hard ferrites find wide application in permanent magnets, mostly in form of Ba, Ni, Sr-ferrites.

Manganes containing hard ferrite prime material can also be processed by pyrohydrolysis in a so called co-spray-roasting by combing solutions of FeCl2 with MnCl2, which are made dissolving iron and manganese metal scrap by hydrochloric leach in proper towers. This process is used in some plants in Europe, USA and Asia. Further applications of iron oxides are in food stuff, cosmetics, as catalysts for chemical processing, in toners, plastics and rubbers as well as in inks and paints. Last but not least, not to forget their vast use as fire extinquishers in big scale burnings like fire fighting during forest fires.

Alumina from Pyrohydrolysis — Uncommon, Yet not Unwanted

Alumina (Al2O3) from pyrohydrolytic processes are uncommon and only made when aluminium stripes are being pickled with hydrochloric acid. Starting with fresh hydrochloric acid of 20 wt%, the aluminium is being pickled to produce ( AlCl3)n solutions of a certain polymeric structure which allow high concentrations of Al in solution.

Alumina (Al2O3) from pyrohydrolytic processes are uncommon and only made when aluminium stripes are being pickled with hydrochloric acid. Starting with fresh hydrochloric acid of 20 wt %, the aluminium is being pickled to produce (AlCl3)n solutions of a certain polymeric structure which allow for a high concentrations of aluminium-Ions in the solution.

The pyrohydrolysis process is run at temperatures of around 750 °C, resulting in a pure white powder of Al2O3 with fluffy consistence. Further processing to produce ceramics or refractories require milling in ball or attritor mills. The final product can easily be pressed into the required forms by isostatic presses.

A co-spraying process can be performed to produce refractory material to make for example Al2O3.MgO and will be detailed in the next chapter. Important during the pressing process is the avoidance of hollow spheres or voids in the final ceramic. Unmilled spray roasted powder has an enormous shrinkage, thus proper milling is a must.

Producing Magnesia for Pharma and Liefstyle

The production of spray roasted magnesia (MgO) usually starts with natural carnallitic brines or Dead Sea water, as well as from hydrochloric solutions deprived from leachings of serpentines, or natural magnesites. From these raw materials, pure solutions are produced, by special cleaning processes, like repeated solvation , precipitation and washing processes, to extract impurities like Fe, Na, K, Ca, B which are unwanted for example for refractory use or the production of ultra-pure magnesia required in pharmaceutical or cosmetic industry.

Solutions for spray roasting in thermally heated reactors contain a higher concetration of MgCl2, nearly twice as such during the production of iron oxide. The temperature requirement is around 100°C higher than with other processes. Specific energy consumption to make MgO amounts 8000 – 8500 kcal/kmol. A pure white powder of hollow spheres is produced by this reaction which requires further milling to be processed in refractories.

An other refractory material such as Al2O3.MgO can be made by a co-spray-roasting process, starting from solutions of MgCl2 and AlCl3 . Also the production of Cr2O3.MgO, an equal important material starts from MgCl2 and CrCl3 solutions. Production temperatures in both cases amount around 850 – 900°C.

Titanium Dioxide — Pigments from the Reactor

Titanium dioxide ores, which are processed to make TiO2 pigments, start mostly from ilmenites, Fe2O3.TiO2, or natural rutile or slags from the titanium production. Besides the conventional routes like the sulfate or chlorine processes, also the leachings of ilmenites with hydrochloric acid is used. The solution then containing iron chloride FeCl2, will be further pyrohydrolyzed to recycle the acid (HCl) for further processing during ore leachings, whilst the residual impure titanium dioxide , TiO2, will be further cleaned by washing, followed by drying, calzining and milling, to finally produce pure rutile (TiO2).

In pigment production, the crystal parameters such as purity, crystal form and size, grain size and the contents of trace impurities are of great importance and are prerequisites for the proper chosen production route.

Further applications of titanium dioxide include the use for catalysts, which mostly are used in de-NOxing catalysts or also for use in solar cells.

A Niche Product of Pyrohydrolysis: Chromium Oxide

Chromium(III) oxide (Cr2O3) is an important prime material for colorants, as well as in its use in refractories. Further applications are in leather making and in catalysis. Producing Cr2O3 by spray roasting uses hydrochloric solutions of ClCl3, which can be made by HCl solvation of chromium scrap. The process requires temperatures and concentrations in hydrochloric solutions, similar to the iron oxide process, i.e. around 200 g/l. The result is a dark green powder, with particle forms and of sizes of 0,1 – 3 mm. There is no industrial production on big scale known to make chromium oxide by the spray roasting route, in contrast to the above mentioned routes.

Can Nickel and Cobalt Oxides be Produced By Spray Roasting?

Ore leaching by hydrochloric acid, even at higher temperatures, is used to process Lateritic nickel ores from mines in New Caledonia, Canada. In many cases, nickel and cobalt ores are found with combinations of both elements.

There are currently experiments to determine the possibility to use spray roasting for oxides production with these materials. Currently, however, there exists only one spray roasting unit in Europe (Belgium) for the cobalt oxide production.

Special Oxides from Rare Earths — In High Demand for Electronics

The metals known as Rare Earths are mostly found in the form of sands in Australia, Brazil and South Africa. Special process designs allow a production of the corresponding chloride forms, all above the chlorine rout by a gas phase reaction, but also by leachings with hydrochloric acid solutions.

Zicnconium oxide (ZrO2) can be made starting from a solution of ZrOCl2, while yttrium oxide ,(Y2O3) relies on solutions of YCl3. Both elements can easily be used to produce the corresponding oxides at temperatures of around 800° C in a spray roaster. The resulting powder has to be be milled afterwards for further processing. This process is used in both Japan and the USA to make Y2O3 and ZrO3, which are important materials for electronics, ceramics, electro ceramics (supra conductors), catalysts, for opto electronics and for special steel processing technologies.

Dare to be Different: Alternative Spray Roasting Process Routes

All processes mentioned above are those derived from the so called hydrochloride route, which means by starting from acidic chloride salt solutions. But there are also several different routes that are currently being developed, some of them of possible future industrial importance.

Hydrochloric acid pickling is often applied during mild steel (normal steel) production. In contrast to this, high alloyed steel (special steel) needs a more rigid procedure of pickling, using mixed acids like hydrofluoric/nitric acid mixtures (HF/HNO3). Those solutions of mixed acids can also be recycled by thermal methods of a spray roast process, using a different construction and different materials like the above mentioned ones.

The process was originally developed by the Andritz company during the 1980´ies and is now successfully applied in steel industries. The resulting materials, usually mixtures of iron oxide together with nickel and cobalt oxides, are usually being reduced to the proper metals to get then recycled for the processing of high alloyed steels.

Besides steel, also titanium metal is being pickled with hydrofluoric acid (alone). The resulting liquid mix of titanium fluorides can be spray roasted to give hydrogen fluoride (HF) as well as TiO2. This material may be used to make pigments, however in most cases will be returned back into the titanium production route.

On academic scale there were some developments to make mixed ferrites from HNO3 solutions containing together with manganese and iron nitrates to produce mixed oxides for ferrite applications and further to make aluminium titanate AlTiO5, a prime material for the corresponding ceramics, which are of industrial importance for use in engineering ceramics.

Pyrohydrolysis Has Come a Long Way From Acid Recycling

Pyrohydrolysis which is a process originally developed as a revolutionary method to recycle pickling acids based on hydrochloric acid to modernize steel manufacturing and to result in much higher pickling velocities, better surface properties of pickled steel and beneficial for environmental protection combined with added profit, has now turned into new processing routes for inorganic oxide production. The big and ever rising demand in prime material for ceramics, electro ceramics, refractories and pigments has been completed by process routes which were described here. Some 800 plants using this technology have been established world wide producing millions of tons of oxide materials or acids for steel pickling.

And yet, the development has not stopped at this point: Meanwhile, a big number of engineering companies in the world enlarge this list of accomplishments of better and more economic process routes. In that aspect the original developments by this technology, starting from its homeplace in Austria, have met international reputation. Reason enough for optimism: Given the current speed of development, further growth can be expected for pyrohydrolysis

* The author is a technological and scientific specialist.