High Iron Calcines Treatment in Electrolytic Zinc Plant of
Metallurgical Met-Mex Peñoles , S.A de C.V
Claudio Rocha Rosales
Metalurgica ,Met-Mex Peñoles S.A. C.V.
Electrolytic Zinc Plant
Boulevard Laguna 3200 Poniente, Colonia Metalurgica
Torreon, Coahuila, Mexico
claudio_rocha@penoles.com.mx
ABSTRACT
Peñoles is a mining group with integrated operations in the smelting and
refining of non-ferrous metals, and is also a chemicals producer. Peñoles is the
world’s foremost producer of refined silver, metallic bismuth and sodium
sulphate and is the leading Latin American producer of refined gold, lead and
zinc. The Met-Mex Peñoles Zinc Plant commenced operations in 1973 with an
annual production capacity of 105,000 t/y. Year after year the production has
been increased, and at present, the production capacity is 240,000 t/y. In this
paper, the leaching process,particularly the operation of the jarosite circuit, is
reviewed ,specifying process improvements to treat high iron calcines .
INTRODUCTION
The purpose of the leaching process is to recover the metals, mainly zinc,
through a series of extraction stages with sulphuric acid solutions, which
progressively increase the zinc recovery. After the series of acidification
stages,the solution, which is rich in iron, is sent to the jarosite circuit to
separate the zinc from the iron [1].
The first step of the leaching process is the neutral leach circuit which produces
a zinc sulphate solution that is sent to the purification section for the removal
of impurities such as cadmium, cobalt and copper, after this solution is sent to
the tankhouse to feed the electrolytic cells.
REVIEW OF THE LEACHING PROCESS
Leaching Process
The Leaching Process consists of the following stages, which are presented in
Figure 1.
• Neutral Leaching
• Fourth Purification
• Acid Leaching
• Hot Acid Leaching
• Jarosite Formation
• Jarosite Acid Washing
• Sulphates Purge
• Lead–Silver Residue Filtration
• Jarosite Filtration
• Jarosite Neutralization
LOW SILVER CALCINE LEACHING
HIGH SILVER CALCINE LEACHING
spent calcine
MnO2
spent
Neutral
Leaching
FourthPurifica
tion
ZnSO4
Neutral
Leaching
Neutral solution
Neutral Thickener
calcine
Fourth
Purification
Neutral solution
NH3
to Purification
calcine
to Purification
Neutral Thickener
Jarosite
Acid Leaching
Acid Leaching
Jarosite Thickener
Acid Thickener
Acid Thickener
spent
H2SO4
Acid Wash of Jarosite
spent Hot Leaching
spent
Hot Leaching
H2SO4
Hot Thickener
filtrate
Acid Wash Thickener
Water Wash Thickener
Purge
Sulphate
solids
PRESS
FILTERS
TRAY
FILTERS
filtrate
filtrate
solids
Lead-Silver
Cake to
Lead Plant
Hot Thickener
Ca(OH)2 Spent
solids
PRESS
FILTERS
Jarosite Cake
Hard Water
to Water
Treatment
CaCO3 +
Ca(OH)2
Neutralization
Jarosite
Neutralized
Jarosite to
Container
Figure 1 – Leaching circuit Flow Diagram
filtrate
filtrate
Lead-Silver
Cake to
Lead Plant
solids
solids
Neutral Leaching
The purpose of this stage is to leach most of the zinc present as ZnO in the
calcine, and the zinc extraction is in the order of 85%. In this circuit, there is a
reaction tank in which the acidity is controlled to a pH 1 to 1.5 . The attack
acid is composed of spent electrolyte, manganese acid solution from the
cellhouse and the zinc solution recovered from the purification section.
Fourth Purification
The purpose of this circuit is to integrate the zinc that was recovered in the
residue treatment stages, because it is a solution with an average acidity of 15
g/L and an average iron concentration ranging from 4 to 8 g/L. This solution
is used to leach calcine and simultaneously hydrolyze the iron and precipitate
impurities like arsenic, antimony,indium, selenium and tellurium. In this
stage,there is a reactor tank controlled to a pH of 4.0 to 4.5 .
Acid Leaching
The purpose of this operation is to leach the remaining zinc, present as ZnO,
that was not leached in the previous stages to achieve a combined extraction
in the order of 90%. There are three reactors where electrolyte is added to
achieve a high zinc extraction. The free acidity in the first tank ranges from 15
to 25 g/L .
Hot Acid Leaching
The purpose of this circuit is to leach the zinc ferrites (ZnO.Fe2O3), and in this
way, to increase the accumulated zinc extraction to 98%. In addition, a leadsilver residue is generated that is filtered and sent to the adjacent lead smelter
to recover the lead and silver values. There are 10 reactor tanks, where
sulphuric acid and spent electrolyte are added to maintain a free acidity in the
first tank from 150 to 170 g/L; steam is added to the tanks to maintain a
temperature of 85°C.
Jarosite Formation
This operation eliminates iron from the solution by forming jarosite which is
filtered and taken out of the circuit. During jarosite precipitation, other
impurities such as arsenic, antimony, indium, selenium and tellurium are coprecipitated and are also eliminated from the circuit.
Jarosite Acid Washing
This stage consists of six reactors which help extract the metals in the calcine
that is added as a neutralizing agent in the jarosite formation circuit, in order
to increase the overall recovery of zinc. Spent electrolyte is added to obtain a
free acidity of 60 g/L and steam is used to maintain a temperature of 92°C.
Sulphates Purge
The purpose of this circuit is to bleed sulphates from the circuit; however, it
also bleeds other salts such as manganese and magnesium sulphates, as well
as chlorides and fluorides, to acceptable operating levels. The sulphates bleed
is important because it allows an increased addition of sulphuric acid to the
circuit and this achieves an increase in zinc recovery.
Lead – Silver Residue Filtration
This stage is used to recover the lead and silver contained in the zinc
concentrates and also contributes to the solids balance in the leaching circuit.
The filtration of this residue, with adequate washing, also helps to recover the
soluble zincin the filter cake.
Jarosite Filtration
The purpose of this stage is to remove the jarosite to balance the solids and
reject impurities from the circuit. It also increases the overall zinc recovery by
recovering the soluble zinc as wash liquor from the filter cakes.
Jarosite Neutralization
The jarosite coming from the filters is neutralized with mixture of Ca(OH)2
and CaCO3 to produce a physically stable material that can be stockpiled and
to stabilize any minor metals content that could possibly be leached from the
jarosite by rain and subsequently reach the water table.
Year after year quality concentrates decrease ,and the Met-Mex Peñoles Zinc
Plant treats concentrates with high iron ,cobalt,silica,iron ferrous contents to
produce a calcine with 11.5 -12.5 % Fe and 0.018 to 0.030 % Co. Table 1 shows
the typical analysis of the calcine treated.
Table 1 - Typical analiysis of the Peñoles calcine
Element
Average Content
Range
(wt %)
(wt %)
Zn
57
56 - 58
Fe
12
11.5 - 12.5
Pb
1.0
0.8 - 1.20
Cu
1.20
1.00 - 1.40
Fe ++
1
0.8 - 1.2
Cd
0.42
0.4 - 0.44
Insol.
2.0 - 4.0
3
Co
0.24
0.018 - 0.30
Ni
0.004
0.003 - 0.007
To get treat high iron calcines , Next Improvements were done in Metalurgica
Met-Mex Peñoles Zinc Plant .
PROCESS IMPROVEMENTS
1.- Increase Acid free in Weak Acid Leaching from 10 to 20 gr/l .
2.- Increase Acid Free in Hot Acid Leaching from 130 to 160 gr/l .
3.- Increase Retention Time in Jarosite Precipitation from 6 to 7.5 hours .
4.- Flows Balance .
5.- PH Automatic Control in Fourth Purification Reactor .
6.- PH Automatic Control in Jarosite First Reactor .
7.- Increase Temperature in Jarosite Reactors from 88° C to 92° C .
8.- Increase Jarosite Filtration.
9.- Increase Thickening Surface in Acid Wash Jarosite.
10.- Increase Oxidation.
1.- Increase Acid free in Weak Acid Leaching from 10 to 20 gr/l :
We Install a Decanter ( D-1) in underflow of Neutral Thickeners 32 and 33 to
separate neutral solution/ solids .
2.- Increase Acid free in Hot Acid Leaching from 130 to 160 gr/l :
Reduce water in reagents preparation to increase spent flow.
3.- Increase Retention Time in Jarosite Precipitation from 6 to 7.5 hours .
We install Reactor # 6 (320 m3) in head-board of jarosite stage.
4.- Flows Balance .
Implemented Automatic Control Flows * and get best performance in
chemical reactions.
5.- PH Automatic Control in Fourth Purification Reactor .
We achieve increase efficiency leaching calcine .
6.- PH Automatic Control in Jarosite First Reactor .
We got increase iron precipitation in jarosite stage .
7.- Increase Temperature in Jarosite Reactors from 88° C to 92° C .
Install Reactor # 6 in Jarosite stage whit automatic heating system.
8.- Increase Jarosite Filtration.
We change one silver-lead press filter to jarosite filtration .
9.- Increase Thickening Surface in Acid Wash Jarosite stage .
We enable big one 400 m2 thickener to jarosite acid wash stage .
10.- Increase Oxidation.
We install Neutral leaching Reactor # 35 and Jarosite Reactor # 6 both whit
Oxidation System ( oxigen) (by Outotec inc.) to oxidize iron ferrous to iron
ferric .
In the Figure 2 are indicated each one improvements that were done in the
diferentes stages of Leaching Process to can treat high iron calcines
Figure 2 – Low Silver Leaching circuit Flow Diagram
Because of the high impurity contents, several reactors are used for jarosite
precipitation. The jarosite precipitation circuit is arranged with nine reactors
tanks (see Figure 2). The process is initiated when the solution rich in Fe (15 to
30 g/L) coming from the Weak Acid Leaching each thickener overflow is fed to
the first reaction tank # 6. Here calcine is added to neutralize the acid.
In reactor # 6, ammonia (4.0 to 5.0 g/L) is added to form the jarosite, and the
solution in the reactors is heated with steam to a temperature of 92°C.
Additional calcine is added in reactors # 7 and 8 to continue lowering the acid
content (see Table 2).
Table 2 - Typical process variables values in jarosite precipitation circuit
Variable
Average Content
Range
(gr/l)
(gr/l)
Fe (Reactor 6) inlet
20
15 - 25
Fe (Reactors 28 and 32 ) outlet
6
4-8
Acidity (Reactor 6 )
22
15 - 30
NH3 (Reactor 6)
4.5
4-5
The reactions for jarosite formation are:
2 NH3+ 2 H2O
2 NH4OH
3 Fe2(SO4)3+ 10 H2O + 2 NH4OH
(1)
(NH4)2Fe6(SO4)4(OH)12+ 5 H2SO4 (2)
The formed jarosite is sent to three thickeners for sedimentation. The overflow
is a zinc solution containing 4-8 g/L Fe, and this is sent to the Fourth
Purification stage to recover the zinc and to make use of the iron in solution to
precipitate any remaining impurities.
Jarosite from the underflow of the thickeners is sent to the acid wash stage, to
leach the calcine that was added for acid control in the jarosite circuit. After
acid washing, the washed jarosite is sent to thickener No. 35. The overflow of
thickener No. 35 is returned to the jarosite precipitation circuit because of its
high iron content, and the underflow is sent to two washing thickeners (No. 34
and No. 134) to lower the soluble metals content. The overflow of these
thickeners is sent to the Jarosite Thickeners to help to clarify them, and the
slurry of the underflow is pumped to six press filters . The jarosite is
neutralized with mixture of Ca(OH)2 and CaCO3 to stabilize the heavy metals
and to give the residue the proper physical characteristics to be transported
by truck to the outside plant storage areas. A typical jarosite analysisis shown
in Table 3.
Table 3 - Typical analisis of the Peñoles jarosite
Element
Average Content
Range
(wt %)
(wt %)
Zn
3.5
03-abr
Fe
27
26 - 28
Pb
1.7
1.5 - 2.3
Cu
0.4
0.3 - 0.5
S
13
12 - 14
Cd
0.18
0.16 - 0.20
Insol.
5.5
5.2 - 5.8
Ar
150
140 - 160
Co
0.007
0.004 - 0.01
Ni
0.004
0.002 - 0.006
REMOVAL OF IMPURITIES DURING JAROSITE PRECIPITATION
The jarosite precipitation stage is very important in the leaching process
because most of the iron from the solution, as well as a considerable amount
of the impurity content, is eliminated in this circuit, as shown in Table 4.
Table 4 - Impurities Removal in jarosite circuit
Element (Reactor 6) inlet
(Reactors 28 - 32) outlet
(mg/l)
(mg/l)
Ar
550
150
Sb
41
26
Se
17
12
Te
32
25
Si
879
414
Sn
4.86
3.22
In
48.4
20.36
Cl
355
284
F
46
42
Mn
14530
14180
The iron precipitated as jarosite also serves as a purge and helps reduce the
recirculation of impurities that have to be removed in the Fourth Purification
circuit and in the hot and cold purification stages in the purification area.
Tests have shown that in the Peñoles jarosite circuit, the amount of iron
removed in the process correlates with the amountof impurities removed
(arsenic, antimony, indium, etc.) There is also a direct relation with the
variables that are important for the jarosite formation and the amount of iron
precipitation. The correlation between iron removal and impurities
elimination is shown in previous studies (see REFERENCES).
The most important variables for jarosite precipitation are temperature, acid
concentration of the solution andthe ammonia concentration.These variables
are plotted against the amount of iron precipitated in Figures 8 to 10,
respectively. For an efficient precipitationofjarosite these variables must be
closely monitored and controlled within specific ranges.
CONCLUSIONS
Year after year around of world quality concentrates are decrease ,is dificult
to find good concentrates,Iron,Silica,Lead,Cooper ,Iron Ferrous,Germanium
are increase,causing problems at normal operationes ,is necesary to continue
improvements the process to achieve daily production to meet customer
requirements .
REFERENCES
1. G. Garcia and C. Valdez, “Jarosite Disposal Practices at the Peñoles Zinc
Plant”, Iron Control and Disposal, J.E. Dutrizac and G.B. Harris, Eds.,
Canadian Institute of Mining, Metallurgy and Petroleum, Montreal,
Canada, 1996, 643-650.
2. C. Rocha and J. Ayala , “Impurity Removal in the Jarosite Precipitation
Circuit of the Electrolytic Zinc Plant of Met-Mex ,Peñoles S.A de C.V” .
Iron Control Technologies, J.E Dutrizac and Patricio Riveros Eds.,
CANMET
Mining and Mineral Sciences Laboratories Ottawa
Canada,2006 , 501-512 .