Chrome Ore Processing

Chromite deposits are mined by both underground and surface techniques. Much of
the ore is rich enough to be used directly: for production of ferrochromium, a
rich, lumpy ore containing more than 46 percent Cr2O3 and having a chromium-iron
ratio greater than 2:1 is preferred, but ores with a lower ratio and as little
as 40 percent Cr2O3 are also used. (Ores high in alumina are preferred for
processing into refractory brick.) As finely divided ores, which do not smelt
efficiently, come under greater exploitation, a number of processes are employed
to agglomerate them for more satisfactory use in furnaces. Fines can be blended
with fluxes and coke (the principal source of carbon) and then preheated or
“prereduced” before being charged into an electric smelting furnace.

If carbon and Cr2O3 are combined in a molar ratio of 3:1 and subjected to
increasing temperature, a number of oxidation-reduction reactions will ensue
that will produce first a series of chromium carbides and finally, at 2,080∞ C
(3,775∞ F), pure chromium and carbon monoxide. (This will take place at 1
atmosphere, or about 100 kilopascals, of pressure, but reducing the pressure
will lower all of the reaction temperatures.) This theoretical reaction does not
account for the presence, in commercial practice, of impurities in the metal and
slag that may alter reaction temperatures and cause undesirable reactions of
their own. For this reason, while a ferrochromium of very low carbon content
(less than 0.1 percent) can in principle be produced in a single stage of
smelting, in practice not all carbon is eliminated owing to the presence of
magnesia, alumina, and silica in the ore and the use of silica as a flux to
lower the melting point of the slag. In practice, therefore, the primary product
is usually a high-carbon ferrochromium that can subsequently be refined to a
low-carbon product. If pure chromium is desired, iron must be removed from the
ore or from an intermediate ferrochromium product by hydrometallurgical
techniques .

Extraction and refining High-carbon ferrochromium  :

Most ores smelted with coke in an electric furnace produce metals that are
saturated with carbon. For ferrochromium, the saturation point is approximately
9 percent, but actual carbon content varies with the condition of the ore and
the composition of the slag. For example, with a lumpy, refractory ore and a
slag containing approximately equal amounts of magnesia, alumina, and silica, a
ferrochromium is produced that contains 4 to 6 percent carbon and less than 1.5
percent silicon. This is a result of high temperatures generated by a viscous
slag, of a slowly reacting bulky ore, and, possibly, of refining of the molten
metal by undissolved ore in the slag. When the rate of the reducing reaction is
increased by using fine ore, or when the slag is made less viscous by adding
lime to the melt, the carbon level of the ferrochromium approaches saturation.
Adding more silica to the charge produces what is called charge ferrochromium, a
carbon-saturated alloy with an increased silicon content. Some South African
ores produce charge ferrochromium containing 52–54 percent chromium, 6–7 percent
carbon, and 2–4 percent silicon; ores from Zimbabwe with a higher chromium-iron
ratio yield a product that is 63–67 percent chromium, 5–7 percent carbon, and
3–6 percent silicon. During the smelting of high-carbon or charge ferrochromium,
slag and metal are tapped from the furnace into a ladle and separated by
decanting or skimming. The recovery of chromium from the ore varies: in a good
operation 90 percent is recovered in the molten metal; of the 10 percent
remaining in the slag, some is in metallic form and can be recovered by mineral
processing techniques. The smelting of charge ferrochromium consumes 4,000
kilowatt-hours of electric power per ton of alloy produced, compared with 4,600
kilowatt-hours per ton for high-carbon ferrochromium.

Ferrochrome silicon – Extraction and refining:

If silica is added to the charge until its weight equals that of the ore, the
smelting processes will yield what is known as ferrochrome silicon. Containing
38–42 percent silicon and less than 0.1 percent carbon, this alloy is used as a
deoxidizer in the production of stainless steel and as an intermediate material
in the production of low-carbon ferrochromium. Because of the greater energy
required to reduce silica to silicon, the smelting of ferrochrome silicon
consumes 7,600 kilowatt-hours per ton of product.

Low-carbon ferrochromium – Extraction and refining 

When chromite and lime are melted in an open electric-arc furnace and then
contacted with ferrochrome silicon, a low-carbon (0.05 percent) ferrochromium
can be obtained. In an alternate process, high-carbon ferrochromium is oxidized
and then blended with additional high-carbon ferrochromium. The briquetted
mixture is placed in a large vacuum furnace, which is heated by graphite
resistors to 1,400∞ C (2,550∞ F) at a reduced pressure of 30 pascals. The carbon
is removed from the alloy (going off as carbon monoxide) to a level as low as
0.01 percent.

Chromium metal – Extraction and refining

Pure chromium is produced either by the thermal reduction of Cr2O3 with aluminum
or by the electrolysis of trivalent chromium solutions. The aluminothermic
process begins with the roasting of fine ore, soda, and lime in air at 1,100∞ C
(2,000∞ F). This creates a calcine containing sodium chromate, which is leached
from the insoluble gangue and then reduced and precipitated as Cr2O3. The Cr2O3
is blended with finely divided aluminum powder, charged to a refractory-lined
container, and ignited. The combustion quickly generates temperatures in excess
of 2,000∞ C (3,600∞ F), giving a clean separation of chromium from the slag. The
purity of the metal, varying from 97 to 99 percent chromium, depends on the
starting materials. In the electrolytic process, milled high-carbon
ferrochromium is leached by a mixture of reduced anolyte (electrolytic solution
recycled from the anode side of the smelting cell), a chrome alum mother liquor
based on a solution of ammonium sulfate recycled from a later stage in the
process, and sulfuric acid. The resultant slurry is cooled, and silica and other
undissolved solids are filtered from the solution. The iron forms crude ferrous
ammonium sulfate crystals, which also are filtered out. The mother liquor is
then clarified in a filter press, and about 80 percent of the chromium is
stripped by precipitation as ammonium chrome alum. The mother liquor is sent
back to the leach circuit while the chrome alum crystals are dissolved in hot
water and fed into the cathode portion of an electrolytic cell. The cell is
divided by a diaphragm in order to prevent the chromic and sulfuric acids formed
at the anode from mixing with the catholyte (cathode electrolyte). With the
passage of electric current from a lead anode to a thin stainless-steel cathode
sheet, chromium is plated onto the cathode and, every 72 hours, is stripped from
the sheet, sealed in stainless steel cans, and heated to 420∞ C (790∞ F) to
remove water and hydrogen. This electrolytic chromium contains 0.5 percent
oxygen, which is too high for some applications; combining it with carbon and
heating the briquettes to 1,400∞ C (2,550∞ F) at 13 pascals lowers the oxygen
content to 0.02 percent, resulting in a metal more than 99.9 percent pure.