Metals
Removal from Acidic Drainage - Chemical Methods
Mine Environment Neutral Drainage at CANMET-MMSL |
MEND Report
3.21.2a
March 1996
Summary
In collaboration
with McGill University's Dept. of Mining and Metallurgical Engineering,
Noranda Technology Centre (NTC) investigated the prospect of recovering
valuable metals from acid mine drainage (AMD) while maintaining
effluent quality and reducing the amount of sludge generated. Several
chemical methods were evaluated to selectively precipitate and recover
metal ions, leading to the development of a conceptual flowsheet
for a three-step precipitation process (Figure 1). This flowsheet
was further evaluated, along with the associated process economics.
The research was funded by Noranda and carried out for the MEND
program.
The original
three-step precipitation process (Figure 1 ) was developed and investigated
in detail at the laboratory scale (Part I) by McGill University.
This process consisted of: a) Fe precipitation in the presence of
surfactant (dodecylamine, DDA, which was expected to alter the surface
properties of Fe(OH)3, thus reducing co-precipitation
of Zn); b) sulphide precipitation to obtain a Zn rich sulphide precipitate;
and c) a final lime treatment to remove residual metals if, necessary
to comply with water quality standards.
Several concerns
were raised pertaining to this process. Solid/liquid (S/L) separation
and materials handling appear difficult. Furthermore, residual DDA
in the effluent may be detrimental to aquatic life. Ultimately,
process control and cost are the key problems for sulphide precipitation.
As a result, alternative processes were sought.
To this end,
three alternatives processes were developed and evaluated by NTC
(Part II). In one process, the first step consisted of Fe(III) precipitation
with CaC03, followed by precipitation of metals with
NaOH/Na2CO3. In another process, Cu was removed
by cementation, using Fe powder. Iron (III) was subsequently precipitated
as a phosphate, using H3PO4; and Zn was removed
as a hydroxide, using Ca(OH)2. These processes are depicted
in Figure 2 and Figure 3, respectively. The use of Na2S
to obtain ZnS/CuS precipitates was further investigated in a reverse
version of the three-step process (Figure 4). Following the removal
of Zn and Cu as sulphides at pH 3.5 in the first step, lime neutralization
in conjunction with aeration at pH 9.5 was applied to simultaneously
precipitate iron and the remaining metal ions, and to produce acceptable
effluent quality together in one step.
The results
obtained from both studies are:
Part I - McGill
Study (Figure 1):
1. With the
use of lime, the iron present in AMD can be completely removed as
ferric hydroxide at pH 3.5, following oxidation with H202.
The solid content of the settled sludge ranges from 6 to 8%.
2. The use
of DDA in the first step to reduce co-precipitation of other metal
ions onto ferric hydroxide sludge slightly improved the subsequent
Zn recovery. However, settling of precipitates in all stages deteriorated
and the content of the leachable metals in the iron sludge increased.
As a result, the use of DDA is not recommended in the process.
3. The iron
sludge required several washing cycles in order to remove leachable
metals.
4. Zn and Cu
can be selectively recovered by using either Na2S H2S
or NaHS in the second step. The Zn/Cu selectivity is closely dependent
on pH (e.g. 3.5) and the alkaline reagent (e.g. NaOH) used for pH
control. However, as the Zn/Cu recovery increases with increasing
pH, the Zn/Cu grade of the sludge decreases.
5. More than
90% Zn recovery and greater than 50% Zn grade can be obtained at
pH 4.5 when lime and Na2S are used to set the pH and
precipitate the Zn.
6. H202,
03 and Trapzene (a CaO2 mixture; patent pending
FMC Corp.) were evaluated as Fe oxidants. 03 was technically
the most effective when it was used in stoichiometric quantities.
7. Lime treatment
of the overflow from the second step to pH 9.5 resulted in an effluent
quality similar to that from the conventional lime neutralization
process.
Part
II - NTC Study (Figure 2, Figure 3, Figure 4):
1.- The McGill
process was further examined in reverse order and as a two-step
process. In this two-step process the removal of Zn/Cu as sulphides
at pH 3.5 was performed first followed by the oxidation of iron
with air and precipitation with lime at pH 9.5. This method required
a large quantity of Na2S (e.g. 3-4x stoichiometric requirement)
and technical difficulties in the separation of the Zn-rich sludge
were encountered.
2. Of the chemical
processes examined, the two-step process yielded the least contaminated
iron precipitate and the highest Zn recovery. In addition, this
process did not produce sludge requiring special disposal. However,
the Zn grade of the precipitate was about 30%.
3. Biological
oxidation could oxidize iron, but the required retention time was
2-6 days. On a cost basis, biological oxidation seems to be at least
one order of magnitude cheaper than chemical methods
4. Although
the use of CaC03 resulted in the least contaminated Fe(OH)3
precipitate and its cost was one order of magnitude less than other
chemicals tested, the iron still needed to be oxidized before precipitation.
5. Process
economics for each process investigated and oxidizing reagent used
were assessed. The cost for each process was compared to the cost
of the lime neutralization treatment plant being operated at Les
Mines Gallen, site of the AMD used in the tests. The most expensive
method was the three-step ZnS precipitation, and the least expensive
was the two-step CaCO3/NaOH process.
6. Use of Po-3
4 to selectively remove ferric iron, following
cementation of copper with iron, and precipitation of zinc with
lime was also explored (based on initial Dahnke's master`s thesis
19856. The process was not technically or chemically feasible.
7. Requirements
for dewatering of each precipitate, generated at each step along
the processes were determined. ZnS precipitates required flocculation
clarifier settling and good filtering (e.g. via filter press).
The processes
investigated suffer from high costs relative to conventional lime
treatment. In particular, the costs of chemical sulphide reagents,
and H202 are not economical. As a result,
further research aimed at optimising the existing flowsheets is
not recommended. unless new concepts which radically reduce costs
or S/L separation steps are involved.
Some alternative
options for developing a process flow sheet should be examined.
Suggestions are listed below in order of priority:
I. Following
the reduction of all iron in the AMD to ferrous iron with SO2,
precipitate ZnC03 in the first step; then oxidize iron
with air and precipitate iron and other residual metal ions with
lime.
II. Investigate
the biological sulphate reduction process and/or a combined biological/chemical
processes.
III. Selective
leaching of Zn from lime sludges should also be explored as a potential
alternative. Particularly, the sludge generated from the two-step
NTC process should be looked at, due to its co-absorption property.
Zn selectively leached out from the sludge can be subsequently precipitated
with Na2CO3.
Any new process
must consider the acceptability of end products from each process
for recycling to a Zn roaster Zn concentrator or lead circuit.
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