FIELD OF THE INVENTION
The present invention relates to a hydrometallurgical process for producing MnO2
or Manganese Metal from furnace dust containing manganese oxides. More
particularly, the invention relates to a hydrometallurgical process for leaching
manganese from furnace dust containing manganese oxides.
BACKGROUND OF THE INVENTION
High purity manganese dioxide is generally produced from high-grade (50% Mn)
manganese dioxide ore. Manganese dioxide being insoluble sulphuric acid, the
ore is reduced to MnO in the region of 800-900°C. The reduced ore is preferably
wet ground in ball mill to finer particle size for better leaching by acid solution.
The slurry from the ball mill is then leached with sulphuric acid at ambient
temperature to 85°C.
By controlling the pH, temperature, aeration, and through addition of MnO2 ore
or small quantities of EMD, the iron impurities along with potassium is
precipitated as a complex salt for example, jarosite. Manganese sulphate solution
is then treated with BaS or H2S to remove heavy metals like copper, nickel,
cobalt and some iron, wherein the pH-value is maintained around 5.5 to 6.5
depending on impurities. The purified solution is used for producing Manganese
or Manganese dioxide by electro-winning (Electro-winning process is described in
Ullmann's (V Ed.), Vol.A16,p.l28).
Electric arc furnace dust of ferromanganese production units is formed from
volatiles and fines collected during wet scrubbing of the off-gas from manganese
alloy smelting furnaces. It contains good amount of manganese. Storage of EAFD

presents a long term environmental concern. It cannot be re-circulated to the
furnace due to its fineness and volatiles present in it.
Furnace dust is a suitable raw material.for MnO2 or Mn metal production. It
contains high amount of manganese and there is no further size reduction is
required before leaching. Replacement of furnace dust with high grade
manganese ore for the production of Mn or MnO2 solves the problems related to
handling and storage of dust. It can also decrease the cost of production of Mn
or MnO2 as furnace dust is a waste material.
PCT Application No. WO 2009/157620 disclose a process for recovering
manganese from electric arc furnace dust which includes acid cleaning followed
by reduction at temperatures more than 500°C and leaching. However, such
process is characterized by high cost due to high temperature reduction and
results in high amount of green house gas emission. Hence, the present
invention is directed to replace the high temperature reduction with reductive
leaching.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a hydrometallurgical process
for leaching manganese from furnace dust containing manganese oxides.
Another object of the invention is to propose a hydrometallurgical process for
leaching manganese from furnace dust containing manganese oxides, which
constitutes a two-stage leaching process for making manganese or MnO2, which
eliminates the pre-reduction stage by directly using a leaching reductant.

SUMMARY OF THE INVENTION
Accordingly, there is provided a two stage process of leaching Mn-oxide
containing furnace dust, for making manganese or MnO2 from comprising the
steps of leaching of Mn-oxide containing furnace dust with sulfuric acid in a first
stage; said acid being present in an amount effective to dissolve soluble
manganese and convert manganese oxide in to manganese sulfate; adding a
base to leach solution to raise pH 1.5 to 2 for potassium removal; leaching the
residue generated in the first stage with a sulfuric acid solution and a reducing
agent, said reducing agent being present in an amount effective to reduce MnO2
to MnO and said sulfuric acid being present in an amount effective to convert
MnO to MnSO4; mixing of leach solution from b) and c); removing iron from the
said solution by adding base to raise pH to 4 to 7; adding a sulfide to said leach
solution to heavy metal precipitates; removing said metal precipitates from the
solution; treating said leach solution in an electrolytic cell having an anode and
cathode to deposit manganese dioxide on anode or manganese metal on
cathode; and recovering said manganese dioxide from anode and manganese
metal from cathode.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1 shows schematically the process according to the present invention.
DETAIL DESCRIPTION OF THE INVENTION
The present invention is directed to a process for making manganese or
manganese oxide from furnace dust comprising the steps of

i) leaching the furnace dust at room temperature with sulfuric acid solution
where the pH of the solution is less than 3 to convert soluble manganese
oxide in to manganese sulfate;
ii) filtering the solution to remove the residue from the leach liquor;
iii) leaching the residue generated in the first step in presence of a reducing
agent with sulfuric acid solution where the pH of the solution is less than
3 to convert soluble manganese oxide in to manganese sulfate;
iv) removal of potassium by jarositing at pH 2 by addition of lime slurry and
filtering;
v) removal of iron by goethiting at pH 4-5 by addition of lime slurry in
presence of oxidizing agent and filtering;
vi) removal of base metals by sulfiding in presence of barium or ammonium
sulfide and filtering;
vii) treating the Purified solution in electrolytic cell containing cathode and
anode to produce Mn or MnO2
Manganese alloy furnace dust contains manganese in the form of MnO, Mn3O4,
Mn2O3 and MnO2. The manganese dissolution of these compounds in the
sulfuric acid is 100, 66.66, 50 and 0% respectively according to the following
equations.


MnO2 is not soluble in sulfuric acid. MnO2 in the dust and MnO2 generated by
equation (2) and (3) is leached by addition of a reductant. This reductant include
Fe-Mn, carbohydrate having formula CmH2nOn where m and n ranging from 1 to
10 and ferrous sulfate. 50-60% of the manganese from the dust is soluble with
out any addition of reductant. Rest of the manganese requires a reductant.
Leaching process developed in the present invention contains two stages. In first
stage, soluble manganese is dissolved in the sulfuric acid solution without any
reductant at room temperature. Solid liquid ratio is 5:1. High temperatures are
not required due to fast reaction of MnO with sulfuric acid. The liquor obtained
after leaching is filtered for the removal of residue. In second stage of leaching,
residue generated from the first stage is leached with sulfuric acid in presence of
carbohydrate or Fe Mn or ferrous sulfate. Leaching with Fe-Mn require no
heating but carbohydrates and ferrous sulfate need temperature 70-90°C. Solid
liquid ratio maintained is 5:1. The said process has the following advantages.
1) Amount of liquid is low in the second stage for a given solid to liquid ratio
as the solid content is decreased due to dissolution in the first stage. It helps in
the energy saving as low amount of energy is required to heat the low amount of
solution.
2) For a given acid concentration, acid consumption will be low.
3) The same is applicable for reductant.

Ferromanganese fines are the preferred reducing agent in our process. Fe-Mn
fines generates during the sizing operation of Ferromanganese. These fines have
low cost compared to lumps and have higher reactivity. MnO2 in the dust and
MnO2 generated during leaching are leached with H2SO4 in presence of
ferromanganese according to the following reactions.

Hydrogen generated during leaching is a strong reducing agent which doesn't
need any temperature for the reaction. So, it is possible to leach maximum
manganese in presence of Fe-Mn at room temperature.
Another reductant used in the present invention is carbohydrate (molasses,
dextrin etc.). Dextrin is produced from starch by acid pyrolysis and molasses is
the by product of sugar manufacturing, generally contains sugars. Insoluble
MnO2 in the dust is dissolved in acid in presence of dextrin or molasses according
to the following equation.

When using a carbohydrate, the temperature of the leach solution should be
raised to 70-98°C. This temperature helps in the reduction of MnO2 to MnO.
The next step in the process of the present invention is to raise the pH of the
leach solution to 1 to 2 by adding lime slurry for the removal of potassium by
jarosite precipitation. The pH is raised to 5 to 7 by the addition of a base and the

addition of an oxidizing agent to convert the iron from the ferrous state to the
ferric state thereby causing the iron to precipitate. Any base and oxidizing agent
can be used, however, the preferred base is calcium hydroxide (Ca(OH)2) and
the preferred oxidizing agents are pyrolusite ore or furnace dust or air. The
aluminum also precipitates in this step. The step of adjusting the pH and adding
an oxidizing agent are accomplished in a conventional manner using
conventional equipment. Removing the precipitated iron, aluminum and silica is
done by filtration in conventional manner. Next, a sulfide is added to precipitate
the heavy metal. Suitable sulfides include ammonium sulfide, alkali metal sulfide
and hydrogen sulfide. Barium sulfide is preferred. The heavy metals which ar e
precipitated in this step typically include nickel, cobalt, arsenic, copper, zinc,
lead, and molybdenum. The addition of the sulfide converts these heavy metals
to sulfides which are precipitated in the solution. Next, the precipitated heavy
metals are then removed from the solution by conventional filtering.
The leach solution is then added to an electrolytic cell which has both cathodes
and anodes. An electrical potential is applied to the anode and cathode which
causes manganese to plate on the cathode or for manganese dioxide to plate on
the anode. In either case, this step of recovering the manganese is accomplished
in a conventional manner using conventional equipment. Finally, the manganese
is removed from the cell in a conventional manner using conventional
equipment. Typically, the manganese dioxide on the anode is has a purity more
than 92% by weight and above.
These and other aspect of the present invention may be more readily
understood by reference to the following examples.

Example 1
This example illustrates using furnace dust and Fe-Mn fines (70% Mn) as the
reducing agent to make MnO2. Chemical analysis of the dust is given in the Table
1. Employing 50 gm of furnace dust along with 200ml water and 25g acid, the
leach step was carried out in a 1000 ml flask with agitation for 30 minutes. After
leaching, solution was filtered. Then the residue from first age leaching was
leached with sulfuric acid in presence of Fe-Mn reductant. Around 22 gm of
residue was treated with 7g Fe-Mn with particle size<150 micron size and 20 gm
acid in 90ml water. Second stage leaching was carried out for 1 hour. It was
determined that >95% manganese in the dust is dissolved in to the solution.

Lime slurry was added to first stage leach solution to raise the Ph to 2 for the
removal of potassium by jarosite precipitation. This solution was mixed with the
second stage leach solution for further processing. Then pyrolusite fines were
added to convert Fe2+ to Fe3+. Lime slurry was added to increase pH to 5 for
removal of iron as goethite.
The slurry was filtered. The required amount of barium sulfide added to form
heavy metal sulfides. The solution was filtered.

The necessary sulfuric acid was added to the purified solution and the solution was
added to the electrolytic cell for MnO2 deposition. The current efficiency was 95%
and battery grade MnO2 recovered from the anode. The depleted solution after
electrolysis can be recirculated to the leaching step in a continuous process as it
contains sulfuric acid.
Example 2
This example illustrate using dextrin as a reductant to make MnO2. First stage
leaching and filtration was performed in the same way as example 1. In second
stage leaching 4g of dextrin was added in place of Fe-Mn and solution temperature
was maintained at 90°C, keeping all other conditions same. Recovery of
manganese into the solution was>95%.
After the leach step, the solution can be further processed in accordance with
example 1 to recover MnO2.
Example 3
This example illustrates the use of molasses as a reductant. Leaching was
performed in the same way illustrated in example 2. Amount of molasses used was
8g in place of dextrin. Mn dissolution was found to be more than 95%.

WE CLAIM :
1. A two stage process of leaching Mn-oxide containing furnace dust, for
making manganese or MnO2 from comprising the steps of
a) leaching of Mn-oxide containing furnace dust with sulfuric acid in a first
stage; said acid being present in an amount effective to dissolve soluble
manganese and convert manganese oxide in to manganese sulfate;
b) adding a base to leach solution to raise pH 1.5 to 2 for potassium
removal;
c) leaching the residue generated in the first stage with a sulfuric acid
solution and a reducing agent, said reducing agent being present in an
amount effective to reduce MnO2 to MnO and said sulfuric acid being
present in an amount effective to convert MnO to MnSO4;
d) mixing of leach solution from b) and c);
e) removing iron from the said solution by adding base to raise pH to 4 to 7;
f) adding a sulfide to said leach solution to heavy metal precipitates;
g) removing said metal precipitates from the solution;
h) treating said leach solution in an electrolytic cell having an anode and
cathode to deposit manganese dioxide on anode or manganese metal on
cathode; and

i) recovering said manganese dioxide from anode and manganese metal
from cathode.
2. The process as claimed in claim 1, wherein the furnace dust comprises
Gas cleaning plant sludge of Blast furnace or submerged arc furnace used
for the production of Ferromanganese or silico-manganese.
3. The process as claimed in claims 1 or 2, wherein the rerducing agent is
selected from Fe-Mn, the group consisting of carbohydrates with general
formula of Ca(H2O)m where m and n are between 1 and 12, molasses and
ferrous sulfate.
4. The process as claimed in claim 1-3, wherein the pH of the leach solution
is about 2 and below.
5. The process as claimed in claim 1-4, wherein slurry density is less than
40% W/W.
6. The process as claimed in claim 1-5, wherein leaching is performed two
stages.
7. The process as claimed in claim 1-6, wherein the manganese recovered
from the purified solution by electrolysis and acid regenerated is re-
circulated to leaching.

ABSTRACT

The invention relates to a two stage process of leaching Mn-oxide containing
furnace dust, for making manganese or MnO2 from comprising the steps of
leaching of Mn-oxide containing furnace dust with sulfuric acid in a first stage;
said acid being present in an amount effective to dissolve soluble manganese
and convert manganese oxide in to manganese sulfate; adding a base to leach
solution to raise pH 1.5 to 2 for potassium removal; leaching the residue
generated in the first stage with a sulfuric acid solution and a reducing agent,
said reducing agent being present in an amount effective to reduce MnO2 to MnO
and said sulfuric acid being present in an amount effective to convert MnO to
MnSO4; mixing of leach solution from b) and c) ; removing iron from the said
solution by adding base to raise pH to 4 to 7; adding a sulfide to said leach
solution to heavy metal precipitates; removing said metal precipitates from the
solution; treating said leach solution in an electrolytic cell having an anode and
cathode to deposit manganese dioxide on anode or manganese metal on
cathode; and recovering said manganese dioxide from anode and manganese
metal from cathode.