Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR RECOVERY OF TECHNICAL GRADE MOLYBDENUM FROM
DILUTED LEACHING ACID SOLUTIONS (PLS), WITH HIGHLY CONCENTRATED
ARSENIC, FROM METALLURGICAL RESIDUES.
SPECIFICATION
The present invention discloses a process for obtaining technical grade
molybdenum trioxide
from diluted leaching acid solutions (PLS), with highly concentrated arsenic,
antimony or bismuth.
Said solution is obtained by leaching of metallurgic residues with a high
concentration of these
impurities, such as casting powders.
More specifically, the present invention discloses a process for recovering
molybdenum through
ionic exchange, through which molybdenum is separated from other metals also
present in said
solution, through the use of ionic exchange resins and controlled
precipitation of As, Sb, and Bi,
with magnesium or iron salts, followed by an ammonium molybdate precipitation,
which is later
calcinated to obtain technical grade molybdenum trioxide.
BACKGROUND OF THE INVENTION.
A mining process or method is the summation of methods through which, starting
from a
deposit, metals and/or metallic compounds of commercial purity and quality are
obtained, in a
profitable manner and with an acceptable environmental impact.
In the known leaching process, one or many mineral values contained in an ore
or a
concentrate are dissolved, generally using an aqueous solution of a leaching
agent. The term
can also be extended to include dissolving secondary materials, such as scrap
metals,
residues, and waste.
Leaching produces an aqueous solution rich in extracted ions of the valued
metal (PLS,
pregnant leaching solution) from which it is possible to separate this metal
and recovering it
with a high level of purity. Also, a solid residue or gravel is produced,
that, ideally, has a
sufficiently low level of valued leached minerals, as to discard it in
tailings or dump.
If the solid leaching residue is impregnated with salts or precipitates that
eventually can
release toxic agents once exposed to the environment, before discard, the
residue must be
properly treated in order to achieve elimination or stabilization of the
potentially contaminant
compounds. In some cases, leaching can have a different objective than the one
previously
described. For example, when a concentrate is leached for selectively
extracting certain
impurities, and thus increase the quality of concentrate (for example during
removal of
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copper from molybdenite concentrates).
In purification and enrichment processes of solutions, leaching processes are
not necessarily
selective and, in consequence, produce leaching solutions containing, besides
the metal of
interest, a range of impurities. This, added to the fact that concentration of
the metal of
interest may not be too high, making impossible a direct recovery of the metal
of interest
from leaching solution, in this way, these solutions must be previously
treated through
purification and enrichment steps.
Purification allows to eliminate impurities, effectively isolating valuable
elements. Enrichment
of solutions is also particularly beneficial for reducing the volumes of
solution to be treated in
subsequent steps of metal recovery. This contributes to lessen investment
costs and
increase efficiency in recovery.
In the present invention, a process for recovery of molybdenum as molybdenum
trioxide,
from diluted acid leaching solutions of metallurgic residues, having a high
concentration of
arsenic, antimony, or bismuth, is disclosed.
The object of the present invention is recovering molybdenum, through ion
exchange,
contained in diluted leaching solutions containing molybdenum, but having a
high
concentration of arsenic, among others, in such a manner that a molybdenum
product is
obtained efficiently and profitably.
Prior art shows documents disclosing molybdenum recovery through ion exchange.
Such is
the case of US Patent 4,891,067, disclosing a process for selective separation
of
molybdenum present in an acid solution at pH 2 and containing molybdenum and
at least
one of the elements in the group conformed by uranium, iron, arsenic,
phosphorous, silicon,
and vanadium. Said process comprises to contact the acid solution with a
stationary phase
consisting of a resin with an active oxime group and eluting said stationary
phase with an
alkaline solution to recover molybdenum. US Patent 4,596,701 discloses a
procedure for
purifying molybdenum trioxide, specifically disclosing a method for preparing
ammonium
molybdate comprising contacting said concentrate with an aqueous solution of
sulfuric acid,
ammonium, ammonium sulfate and persulfate, for solubilizing at least 2% of the
molybdenum values present in said concentrate.
A Chilean patent application CL N 3137-2005 discloses a process for
molybdenum and
copper extraction, which are contained in solidified slags from fusion
processes in copper
concentrates. This document does not interfere with the present patent
application since the
slag does not contain arsenic, therefore treating a technical problem
different to the one
addressed in the present patent application.
None of the previously mentioned processes interfere with the present
application since said
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documents of the prior art show processes with different operative variables,
and therefore, do
not provide the benefits obtained with the present invention, which, as
disclosed in the present
application, consist on efficiently and profitably recovering technical grade
Mo03 from leaching
solutions with high arsenic content. Through the present invention, a
technical grade
molybdenum trioxide is obtained, which means having a molybdenum content over
58%, and a
content of non-regulated metals, such as As, Sb, and Bi, in values lower than
0.1%.
SUMMARY OF THE INVENTION
In accordance with one aspect, there is provided a process for recovery of
technical grade
molybdenum from diluted leaching acid solutions (PLS) containing highly
concentrated
arsenic, from metallurgical residues, wherein the process comprises the steps
of:
(a) contacting a pre-filtered acid leaching solution (PLS), from leaching of
casting
powders, with an anionic ion exchange resin to produce a charged ion exchange
resin;
(b) washing the charged ion exchange resin of step (a) with water;
(c) extracting molybdenum from the washed ion exchange resin using an alkaline
regenerating ammonium solution having a pH value between 8 and 12, to form
ammonium
molybdate in solution, which is recycled for adjusting molybdenum
concentration;
(d) washing the resin of step (c) with water;
(e) adding magnesium and/or iron salts to the ammonium molybdate solution
recovered in step (c) to form a precipitate of Mg3(As04)2 and/or FeAs04 which
is carried to
an arsenic abatement step, and a solution containing ammonium molybdate in
solution;
(f) adding sulfuric acid to the ammonium molybdate solution obtained in step
(e) for
precipitating molybdenum in the form of ammonium molybdate (NH4)4Mo8026) in an
acid
medium having a pH value between 1.5 and 4;
(g) separating the precipitate formed in step (f) by filtrating ammonium
molybdate and
recycling the obtained solution to the PLS solution of step (a);
(h) calcining the precipitate separated in step (g) to obtain molybdenum
trioxide
(Mo03), and ammonia; and
(i) recovering ammonia produced in step (h) for returning to the process as
recycled
regenerating solution.
BRIEF DESCRIPTION OF THE FIGURES
Figure 1. Flowsheet. A "PLS Powder" solution is input to a column containing a
IX (a) resin.
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The molybdenum is extracted from the ion exchange resin with an alkaline
regenerating
ammonium solution. The resulting solution containing molybdenum is cleaned by
immediately adding magnesium sulfate (b), causing precipitation of arsenic
selectively (c).
The cleaned ammonium solution is acidified (d), precipitating a molybdenum
salt. This salt is
recovered by filtration (e) and calcined (f) to generate a high quality
trioxide. The remaining
solution is recycled to step (a).
Figure 2. Detail of the cleaning of ammonium. By adding magnesium sulfate, an
arsenic
precipitate (sampling points indicated) is generated. Molybdenum precipitation
by acid results
in a cake, which will be washed by re-pulping with process water and re-
filtration.
Figure 3. Molybdenum precipitate heating up curve versus time, wherein
temperature
increase in steps, allowing the completion of each of the decomposition sub-
stages, before
proceeding to the next stage (or step). This profile is an example of the
importance of the
knowledge of the chemical decomposition - direct heating to the final
temperature result in a
substandard product.
DESCRIPTION OF THE INVENTION
The process of the present invention discloses recovery of molybdenum through
ion
exchange, followed by an increase in the concentration of molybdenum in the
regenerant,
precipitating impurities, precipitating molybdenum, passing through drying,
and
calcination, to obtain the final product of technical grade molybdenum
trioxide, as shown
in Figure 1.
More specifically, the present invention discloses a process to recover over
70% or more,
specifically around 90% of molybdenum present in PLS as technical grade Mo03.
Specifically, the invention consists in selective recovery of molybdenum, in
the form of Mo03,
from PLS solution generated from leaching of casting powders.
The present invention develops a process for recovering molybdenum through ion
exchange.
The process has been validated in a pilot plant and the fundamental aspects of
the process,
validation steps, and methodology of industrial scaling, as disclosed in the
present invention,
show a novel and inventive process.
The ion exchange process considers two steps which repeat in ion exchange
cycles: charge
and regeneration of the resin; each one followed of water washing steps. The
charge
process of the resin consists on capturing molybdenum ion in molybdate form
from the acid
leaching solution, while the regeneration process of the resin consists on re-
extraction or
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discharge of the molybdate ion using an alkaline regenerating solution. The
equations
corresponding to the process are shown below:
Resin charge step:
H2Mo04 + 2 R-OH ¨> R2-Mo04 + 2 H20
Resin regeneration step:
R2-Mo04 + 2 NH4OH ¨> 2R-OH + (NH4)2Mo04
Wherein R represents the ion exchange resin. The resin used in this process is
of anionic
weak base or anionic weak/string base type, presenting functional groups
selected among
secondary, tertiary, tertiary/quaternary amines, and polyamines.
Molybdenum extraction with ion exchange resins is performed contacting PLS
solution
containing Mo with an anionic resin. This contact can be discontinuous, adding
the resin to
the solution and agitating, or continuously, by using columns.
Ion exchange is highly selective for Mo using a particular resin. However,
traces of
undesirable elements that are contained in the solution can be entrapped in
the resin, in
particular in the present application As, Sb and Bi. Therefore, it is required
to send the
charged solution to the step of impurity removal (figure 2).
Afterwards, charged Mo must be discharged from the resin contacting the
charged resin with
an alkaline solution of ammonium hydroxide, wherein a Mo charged solution is
obtained,
contaminated with As, Sb, or Bi which are partially co-extracted with Mo.
Removal of these impurities can be performed by precipitating with a magnesium
or iron salt,
obtaining a solid product that must be treated for further disposition and
obtaining a
regenerating solution charged with Mo and virtually free of impurities (Fig.
2). Chemical
reactions using magnesium sulfate for arsenic, antimony, and bismuth
precipitations are:
3MgSO4 + H3As04 --> Mg3(As04)2(s) + H2SO4
MgSO4 + 2HSb02 ---> Mg(Sb02)2(s) + H2SO4
Mg504 + 2HBi02 --> Mg(Bi02)2(s) + H2SO4
After removal of As, Sb, and Bi, a charged regenerating solution is obtained,
free of
impurities, which is then sent to the Mo precipitation step (Fig. 2) by adding
sulfuric acid, Mo
precipitates as ammonium molybdate with an efficiency higher than 70%,
according to the
following reaction:
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8(NH4)3M004 + 6H2SO4 -> (NH4)4M08026 + 6(NH4)2SO4 + 6H20,
The last step to obtain molybdenum trioxide is the calcination step, wherein
the ammonium
molybdate precipitate is subjected to controlled heating from 20 to 700 C. The
temperature
profile used (Figure 3) in the calcination step of the present invention
considers:
1.- Heating from 20 to 260 C and holding for approximately 20 minutes to 2
hours, for
removing hydration water.
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2.- Heating from 260 to 370 C and holding for approximately 20 minutes to 2
hours, for
removing water and ammonia.
3.- Heating from 370 to 500 C and holding for approximately 20 minutes to 2
hours, for
removing As as arsenic oxide.
4.- Heating from 500 to 700 C and holding for approximately 20 minutes to 2
hours, for
decomposition and desorption of sulfur and arsenic.
In summary, using the present invention allows to produce molybdenum trioxide
with
impurities, such as As, Sb, and Bi, with values lower than 0.1%. The present
invention in
more detail has the following correlative steps:
1.- Contacting acid aqueous solution (PLS) previously filtered with a pH lower
than 1.6, with
an ion exchange resin of anionic type. Such as for example, the ones described
in Table 1.
2.- Wash with water the resin post-charge to avoid potential precipitation of
solid elements
that are dissolved in PLS due to its acidity (such as iron) which could
precipitate when in
contact with the regenerant due to the basic pH of the ammonium hydroxide
solution.
3.- Extracting molybdenum from the ion exchange resin with an alkaline
solution of
ammonium hydroxide in a concentration range from around 5 g/L to 150 g/L,
reaching a pH
of around 8 to 12, more preferentially between 8.5 and 9.5, in the form of
ammonium
molybdate in solution.
4.- Wash with water the resin post-discharge, similar to step 2.
5.- Adding a magnesium or iron salt, such as for example magnesium sulfate,
magnesium
chloride, or ferric sulfate, to the solution obtained in step 3, to obtain a
pulp with an arsenic
and other impurities precipitate, which is separated in 2 lines: the ammonia
solution obtained,
that contains Mo, passes to the molybdenum precipitation step and the solid
obtained is
carried to an abatement step and external disposition to the process of the
present invention.
6.- Adding H2SO4 to the ammonia solution obtained in step 5 to precipitate the
molybdenum in the form of ammonium molybdate ((NH.4)4Mo8026) in an acid medium
with
a pH between 1.5 to 4, more preferentially at pH 3.3, and in a temperature
range between
50 C to 90 C, more preferentially at 70 C.
7.- Separating the precipitate of step 6, using filtration of molybdate, and
the solution obtained
is recirculated to mix it with the initial solution of PLS.
8.- Calcinating in a ramp or steps of temperature between 20 to 700 C the
filtered product of
step 7 to technical grade molybdenum trioxide (Mo03). This step allows removal
of sulfur,
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arsenic, and ammonium traces, therefore the product complies with the
requirements for
the market of technical grade molybdenum trioxide.
9.- Additionally recovering ammonia generated in calcination step 8 in a
condenser
and/or gas scrubber, for further return to the process as regenerant.
Weak (WBA) and mixed weak/strong (WBA/SBA) anionic ion exchange resins were
tested for the process of the present invention. As an example and without
limiting the
invention, the resins indicated in Table 1 were tested. Resins A170/4675 and
A100 Mo
are manufactured by Purolite. Resins A365
(LEWATIT A 365), MP64
(LEWATIT M+ MP 64) and MP62 (LEWATIT MP 62) are manufactured by Lanxess.
After a large number of batch and in column tests, the results described in
Table 2 were
obtained.
Table 1. Specification of resins used in batch and column tests.
Size Functional Capacity
Resin Type Base Appearance Structure
(pm) Group (eq./L)
Spherical 875 +/_ Macroporous 1.3 (base
A 170/4675 WBA Free Complex Amine
(pearls) 125 PES/DVB Free)
SBA/ Spherical 800 to Macroporous 3.8 (base
A 100 Mo Chloride Tertiary amine
WBA (pearls) 1300 PES/DVB Cl-)
SBA/ Spherical 590 +i_ Macroporous Tertiary/ 1.3
(base
MP64 Free
WBA (pearls) 50 PES/DVB quaternary amine Free)
Spherical 470 +/_ Macroporous 1.7 (base
MP 62 WBA Free Tertiary amine
(pearls) 60 PES/DVB Free)
Spherical Acrylate 3.4 (base
A 365 WBA Free > 400 Polyamine
(pearls) gel/DBV Free)
Table 2. Results obtained for column tests.
Mo Extraction efficiency Mo Re-extraction efficiency
Resin
from PLS from resin
MP64 90% 52.3%
MP62 86% 65%
A365 61% 47%
A170/4675 62% 72.1%
A100Mo 81% 56.1%
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As can be seen in Table 2, all resins present an acceptable percentage of
affinity for Mo,
standing out resins MP64, MP62 and A100Mo. Nevertheless, in Mo re-extraction
capacity
from the resin, A170/4675 and MP62 resins stand out.
The variables that are considered relevant for obtaining a product with less
impurities are
reflected in Table 3.
Table 3. Analyzed operating variables for precipitation of impurities and Mo.
Level
Variable
Low High
Mo concentration in charged regenerating solution 5,000
mg/L 10,000 mg/L
Feed solution pH for precipitation of As 8.5 9.5
Feed solution temperature for precipitation of As room
temperature 60 C
Residence time for precipitation of As 30 min 120 min
As concentration in feed for precipitation of As 2,500
mg/L 8,000 mg/L
Sb concentration in feed for precipitation of Sb 39 mg/L 150
mg/L
Concentration of Bi in feed for precipitation of Bi 29.2 mg/L 64.9
mg/L
Feed solution pH for precipitation of Mo rz 7.3 8.5
Residual solution pH for precipitation of Mo (at 70 C) ==--= 2.8
3.3
Residence time for precipitation of Mo 30 min 120 min
EXAMPLE OF APPLICATION
330 L of PLS solution were contacted with 6 L of Lanxess MP-62 resin (for
exemplification,
and without limiting the invention). The resin was disposed in a static column
which allowed
passing PLS solution at a 4.5 l/h flow rate, after this period, a "refine" or
Mo-free solution was
obtained, as shown in Table 4.
Afterward, 12 L water were passed, at a 4.5 L/h flow rate for washing the
resin. After washing,
the resin was regenerated, extracting the captured Mo, passing an alkaline
ammonium
hydroxide solution (50 g/L NH4OH) through the column. This procedure was
performed for 47
cycles of charge/discharge, using in each cycle 330 L of fresh PLS solution.
The
regenerating solution was not renovated since its concentration increases in
each cycle;
maintaining the pH near to 9.
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Table 4. PLS solution concentration before and after Mo extraction
I [Mo] (mg/L) [Sb] (mg/L) [Bi] (mg/L) [As] (mg/L)
PLS Solution 315 93 78 13,840
Refine solution 26 51 59 13,692
Charged regenerating solution 11,740 2,230 951 8,330
The charged regenerating solution was fed to a 5 L reactor for precipitating
impurities,
contacting 215.8g of magnesium sulfate (5% above stoichiometric value). The
pulp was left
reacting for 120 min. The solid was filtered and washed for disposition. The
results obtained
show a high efficiency in precipitation of impurities, obtaining a solution
with only 41 mg/L of
As and concentrations of Sb and Bi lower than 10 mg/L. No co-precipitation
with Mo was
detected.
Afterwards, the treated or clean solution was loaded to the Mo precipitation
reactor, wherein
it was heated to 60 C. Under this condition, sulfuric acid was added, turning
the solution pH
to 3.3.
In these conditions, a precipitation of Mo of 70% was obtained. The solid was
filtered and
washed with water in a ratio of 3 parts of water for 1 part of solid in
weight. From this
procedure, a solid with the concentrations described in Table 5 was obtained.
Table 5. Ammonium molybdate concentrations.
Mo Sb Bi As
57.4% 0.07% 0.02% 4.1%
This solid was further calcinated in an electrical oven for 3.5 h, reaching a
temperature of
650 C. The results of the concentration for the product are shown in Table 6.
Table 6. Concentrations of elements in the produced molybdenum trioxide.
Mo As Sb Bi K Fe Ca Al Cu Mg
64% 0.085% 0.027% 0.016% 0.026% 0.018% 0.002% 0.002% 0.010% 0.022%
Using the present invention, the final molybdenum product exceeds the
conventional
commercial standards of concentration. The obtained purity is compatible with
the
conventional market of technical grade molybdenum trioxide.
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