By Heinz Krivanec*
The spray roaster process (pictured) was originally developed
by J. Aman in 1958 for the decomposition (pyrohydrolysis) of
MgCl2 brines. Consequently, it is also well known as
the Aman Process. However, its most common application is the
regeneration of spent hydrochloric pickle liquors in the steel
industry.
Pyrohydrolysis is a combination of
the evaporation of water in chloride solutions and the chemical
reaction between metal chlorides and water at elevated
temperatures to form the corresponding metal oxides and
hydrogen chloride. The CMI spray roaster is a cylindrical,
refractory lined steel vessel with conical top and bottom
sections. Several burners are fitted tangentially to the
circumference of the cylindrical section at its base. These
burners (fired with liquid fuel or natural gas) inject hot
gases into the roaster to create a vortex specifically designed
for the size of the plant and the application.
Since the evaporation and chemical
reactions at high temperatures are very fast processes the
resulting oxide is formed as a very fine solid. The fineness of
the oxide product is significantly increased when processing
dilute metal chloride solutions, resulting in a very fine,
chemically reactive oxide powder. Depending on the type and
process conditions, the oxide powder has a bulk density between
0.2 - 0.8 kg/l and a specific surface area between 2 - 20
m²/g (B.E.T.).
The oxide primary particles are
generally irregular in shape with a mean equivalent spherical
diameter of approx. 0.1µm, while the diameter of the
agglomerates ranges up to 500 µm.
Depending on its application, the
spray roaster oxide may be subjected to further treatment. MgO
for refractories for instance is hydrated to Mg(OH)2
in order to wash out any chlorides of metals which do not
hydrolyse and which are contained in the oxide from the CMI
Spray Roaster.
Subsequently the Mg(OH)2
is filtered to separate the chlorides. Further steps include
calcination, usually in
‘Nichols-Herreshoff’Multiple Hearth Furnaces
(MHF), producing CCM with >98% w/w MgO and sintering in
Vertical Shaft Kilns (VSK) or rotary kilns producing sintered
magnesia with >99% w/w MgO.
The CMI NESA¨ MHF consists of a
series of circular hearths, placed one above the other and
enclosed in a refractory-lined steel shell. A vertical
rotating shaft through the centre of the furnace carries arms
with rabble blades, which stir the charge and move it in a
spiral path across each hearth.ÊMaterial is fed to the
top hearth, and rabbled across it to pass through drop holes to
the hearth below. It passes in this way over and across each
hearth to the bottom where the product is discharged through
one or more ports.
Heated gases flow counter-currently to heat the charge to
reaction temperature and to carry on the desired reaction. The
CMI NESA¨ VSK consists of a cylindrical shell made of
special steel, which is lined with high temperature
refractory.
A very specific crystal structure
is needed if the Mg(OH)2 is to be used as an
additive, for example as a halogen-free flame retardant filler
for plastics. This is achieved by hydration of MgO under
carefully controlled conditions.
Magnesia from natural MgCI2 brines
Spray roasting can be used to
produce magnesia from natural MgCl2 brines. Such
brines are found in arid countries, most often they are the
products of solar evaporation ponds. Such ponds are used to
produce raw materials for the chemical industry, for example
sodium chloride (NaCl), potassium chloride (KCl) or bromine
(Br2). These brines typically contain 400 - 420 g/l
MgCl2, 30 - 40 g/l CaCl2, NaCl, KCl and
can be used as a feedstock for the production of magnesia by
spray roasting.
In the early 1970s such an
application with a capacity of 50,000 tpa of MgO sinter was
built in connection with the potash production from the Dead
Sea waters in Israel, by ICL. The plant is still in operation
and has been extended to three spray roaster units, with a
combined capacity of 70,000 tpa, which feed a common washing,
calcination (supplied by CMI NESA¨), briquetting, and
sintering line. About 700,000 tpa of HCl (18-20% w/w) is
produced as a by-product which is used in the production of
phosphoric acid and other phosphates from phosphate ores. A
similar installation was built in Germany which processed
MgCl2 brines produced by leaching an underground
bischofite deposit.
Magnesia from synthetic MgCl2
solutions
Subsequently, processes have been
developed for the preparation and purification of synthetic
magnesium chloride solutions made from HCl and magnesite ores
or ore tailings. These hydrometallurgical processes allow the
use of low grade raw materials which usually cannot be treated
by conventional methods or which would yield products of
insufficient quality. As an example a 25,000 tpa MgO sinter
installation was operated in Serbia until 1990 using magnesite
and flue-dust as raw materials.
These were leached in a semi
continuous leaching process using 18% w/w regenerated HCl and
giving a solution of not more than 300 g/l MgCl2,
therefore pre evaporation was necessary. Further developments
included an acidulation instead of a pre- evaporation in order
to reduce energy consumption. The first application using
acidulation was located in Hačava (Slovakia), a 25,000 tpa
installation for the production of +99% w/w MgO sinter using
locally mined magnesite and flue dust. The use of acidulation
achieved the most economic MgCl2 concentration, of
about 420g/l.
The isotropic concentration limit
of the HCl solution may be avoided by feeding not simply water,
but also magnesite ore fines suspended in water to the
adiabatic absorption column (acidulation). Thus, HCl reacts in
the absorber with the magnesite producing magnesium chloride,
enabling the solution to absorb additional hydrogen chloride.
The concentrations of HCl and MgCl2 in the solution
are therefore only dependent on their solubilities in water at
actual operating conditions. The acidulation process is
controlled at a concentration of about 390 g/l of
MgCl2 in the acidic liquor. In this way, not only
the production of pure magnesium chloride brines but also the
thermal decomposition to magnesia is achieved in a single
continuous process. HCl is needed only for the initial start-up
of the plant and to make up chloride losses arising from the
presence of metals which do not hydrolyse in the Spray
Roaster.
Current applications and
future trends
Spray roaster magnesia applications
are focused on the relatively small chemical grade market of
about 150,000 tpa which represents 7% of the current world
magnesia production. Due to its purity it is used for the
production of high purity chemicals as well as for specialities
such as food additives and pharmaceuticals. It should also be
mentioned that high purity MgO is a truly recyclable
neutralising agent when used in chloride systems. New
applications in the field of organic chemistry, such as the
production of biodegradable plastics, are likely to emerge and
boost future demand.