1. J. Chem. Tech. Biotechnol. 1985,35A,108-1 14
Recommended Flowsheets for the Electrolytic Extraction
of Lead and Zinc from Red Sea Polymetal Ore
Loutfy H. Madkour
Chemistry Department, Faculty of Science, Tanta University, Tanta, Egypt
(Manuscript received 12 April 1984 and accepted 21 November 1984)
The polymetal complex ore Umm-Gheig considered in Egypt as a rather rich
source of lead and zinc is subjected to mineralogical, chemical, spectral, X-ray and
differential thermal analyses. Hydrometallurgical treatments based on leaching,
precipitation and electrodeposition of metal from the ore are established. The
influences of current density, temperature and metal ion concentration on the
Faradic current efficiency are discussed. Advantages and disadvantages of flow-
sheets and various approaches depending on convenient baths for the electro-
deposition of metals are investigated. The results of electron microscopic investiga-
tion confirmed by metal value data given in the A.S.T.M. cards coincide well with
those given by chemical analysis.
Keywords: Umm-Gheig ore; lead and zinc extraction; flowsheet.
1. Introduction
The polymetal deposits of the Red Sea ore belt represent a rather complex morphogenetic type
of mineralisation,' and occupy the littoral zone extending NW-SE for a distance of 130km. The
investigated sample contains 30.7% Zn, 7.99% Pb, 5.05% Fe and 6.58% SOz. The minerals are
hydrozincite, zincblende, smithsonite and cerussite, whereas silica and carbonates constitute the
bulk of the gangue. The minerals present in this complex ore are often found in such close
intergrowth that it is either difficult to obtain suitable high-grade concentrates at high recoveries2by
physical methods, or the recovery of metals from the respective concentrates is poor.
Hydrometallurgical methods based on leaching and precipitation rather than smelting play an
important role in meeting the requirements for the treatment of complex3ores. Nitric acid as a direct
leacher in the decomposition of complex ore has been a~hieved.~The present investigation is
directed to the electrolytic extraction of zinc5-' and leads39from various baths. Also the role of
impurities'&'* on the electrodeposition processes is considered.
2. Experimental
2.1. Sampling
Mineralised horizon ore (500kg) was crushed to 100% minus 1.0mm:
2.2. Chemical and spectral analyses
Analysis13 of the ore sample was carried out at the Egyptian Geological Survey and Mining
Authority.
2.3. Thermal analysis
This was carried out by means of a MOM derivatograph with a-A1203 as reference. The
powdered sample (1g) was heated at the rate of 10°Cmin-' up to 1000°C(constant sensitivities:
DTA, $50; TG, 200 and DTG, %5).
108

2. Flowsheetsfor electrolytic extraction of lead and zinc 109
2.4. Preparation of the ore mother liquor
Ore sample (30g) (<l.OOmrn) was leached with 150cm3 twice-distilled H 2 0 and 300cm3
concentrated (16M)HNO~(S/L=1:15). The leached ore was completely decomposed by
evaporation without taking to dryness. The cooled concentrate was diluted with 300cm3 water
and heated with continuous agitation. The solution of decomposed ore was filtered, washed with
2% HN03and finally collected in a measuring flask.
2.5. Preparation of the standard solution
Lead nitrate and zinc sulphate solutions were prepared by dissolving the appropriate quantity of
the Analar grade product in twice-distilled water plus acid or alkali as required, to give similar
concentrations to those present in the ore mother liquor for comparison.
2.6. Electrolysis system
This comprised a constant current device (25PA-1000 mA); an electrometer for measuring
current through the circuit, the potential of the working electrode vs S.C.E.; a glass electrolytic
cell of 1dm-' capacity, fitted with a thermometer relay, holders for the electrodes and a magnetic
stirrer; electrodes of graphite core, stainless steel, Pt, Ag, Ni and Al.
3. Results and discussion
Table 1 reveals that the ore is rich in Zn and Pb. The arrangement of mineral^'^ according to
their abundance (Figure 1) is as follows: hydrozincite, Zns(C03)2(OH)6;zincblende, ZnS;
smithsonite, ZnC03and cerussite, PbC03.The total loss in weight due to heating up to 1000°Cwas
17.8%.X-ray powder diffraction data shows that galena ismainly converted to cerussite. Nitric acid
is an excellent direct leacher for treatment of the ore as shown in Table 2.
I I I 1 I I I I I I
100 200 300 400 500 600 700 000 900
Temperature ("C)
Figure 1. Thermal analysis of Umm-Gheig ore.

3. 110
Table 1. Chemical and spectral analyses of Umm-Gheig complex polymetal ore
L. H. Madkour
Content Content Content
Component (%) Component (%) Element (parts x
SiOz 6.58 Zn 30.70 Cd 800
Fe 5.05 PbS04 4.86 Ba 400
CaO 4.84 ZnC0, 59.76 co -
so:- 1.28 Moisture 0.97 Ag
AIzO, 0.83 Pb 7.99 Ti 0.06
MgO 2.58 PbCO, 6.02 Ni 40
-
L.O.I." 28.22 Au -
Loss on ignition in weight percent at 11M)"C.
Table 2. Efficiency of leaching the complex ore using (11~)HNO,
acid
Content (%)
Efficiency
Element Ore solid Ore liquor (%)
Zn 30.70 30.42 99.10
Fe 5.05 4.80 95.10
Pb 7.99 7.99 ioo.no
90t
I I I I I l l 1 J
10 20 30 40 50 (0.)
L I I I I I I I I 1
10 30 50 70 (AA)
Current density (mA cm-2)
I I I I I I 1 I I I I I I I /
20 30 40 50 60 70 (0.1
I I I I I I I 1 I I l l
30 40 50 60 70 (AA)
Temperature ("C)
Figure 2. (a) Effect of current density at constant temperature on PbO, deposition from nitrate (A, A)and plumbite (0,
0)solutions at 55 and 60°C respectively. A , 0, pure standard electrolytes; A,0, ore leach solutions. (b) Effect of
temperature at constant current density on Pb02 deposition from nitrate (A, A)and plumbite (0,O)solutionsat 45.5 and
37 mA cm-', respectively. A,0,pure standard electrolytes; A,0,ore leach solutions.

4. Flowsheets for electrolytic extraction of lead and zinc 111
3.1. Electrolytic extraction of metals
Duplicate relations of the current efficiency (a%)with respect to the current density,
temperature and metal ion concentration, either in the case of pure standard electrolytes or ore
leach solutions were drawn according to Faraday’s law in both Figures 2 and 3. In general, lower
current efficiency values are obtained in the case of ore leach solutions due to the effect of
impurities’(’-’2 and the presence of other metal cations during the deposition processes.
The results of chemical analysis reveal that the purities of the metals and the oxides are better
than 99.6%. The surface structure of the anodic oxide films were also investigated under the
electron microscope. The results from the electron microscope, confirmed by the X-ray standard
tables (A.S.T.M. cards) for the oxides, are coincident with those obtained by the chemical
analysis.
-
4
a-I00
a-“ 9 0
85
80
10 20 30 40 50 60
I / I I I I I I 1 J I
20 60 100 140 I80 220 (AA)
Current density (mA Cm-‘)
c0
(0.)
-4
a-
95r
85
I80
I o o ~
90g51
-20 30 40 50 60 70
Temperature (‘C)
Zn‘*lq d K 3 )
Figure 3. (a) Effect of current density at constant temperature on Zn deposition from sulphate (0,0)and zincate (A, A)
solutions at 38 and 50T, respectively. 0.A, pure standard electrolytes; 0,A,ore leach solutions. (b) Effect of temperature
at constant current density on Zn deposition from sulphate (0,0)and zincate (A, A)solutions at 50 and 100 mA cm-’,
respectively. 0, A , pure standard electrolytes; 0, A,ore leach solutions. (c) Effect of zinc ion concentration at constant
current density and temperature on Zn deposition from sulphate (0,0)and zincate (A, A)solutions at 50 mA cm-’, 38°C
and 100 mA cm-’, 50T,respectively. 0, A , pure standard electrolytes; 0,A,ore leach solutions.
3.1.1. Electrolytic extraction of lead
The different factors liable to affect the nature of the deposits including metal ion concentration,
complexing agents, pH, temperature, current density and presence of impurities were studied.
Difficulties associated with the electrodesposition of the lead oxides on the anode, are due to the
ease of their oxidation and non-coherent nature of the deposits. Difficulties arise over controlling
the potential and current density. To obtain a deposited layer of Pb02from Pb(N03)2that does
not fall off the graphite core, it was found necessary to change the anodic potential and
electrolyte temperature in 4 successive stages. The concentrations of Pb(N03)2and HN03in the
electrolyte affects the structure of the PbOz deposit.
The phase composition of Pb02 was determined by X-ray diffraction, the surface structure of

5. L. H.Madkour112
the deposits was studied by electron microscopy. The current density is the principal factor
determining the phase composition of the electrodeposits of Pb02. In case of deposition of Pb02
from Na2Pb02, an increase of applied anodic current density enhances rapid discharge of OH-
ions and leads to the formation of oxide with a higher oxygen content; on the other hand, a
decrease in current density favours the formation of the lower oxygen content oxide. Increasing
the concentration of NaOH to 4M enabled discharge of oxygen to occur and the yield of PbOl
fell. The rapid drop in the apparent coulombic efficiency as the pH is increased is thought to be
the result of the net oxidation state of Pb approximating 4.00. A remarkable decrease in current
efficiency was obtained with a rise of temperature due to enhanced discharge of oxygen at higher
temperatures.
The optimum conditions necessary for the anodic deposition of PbOz were finally reached as
shown in Figure 2. Pb is quantitatively obtained by cathodic deposition from ore leach plumbite
solution (pH39). Also bright, fine crystalline Pb can be deposited ~athodically'~from ore leached
nitrate solution. The distance between the stainless steel electrodes is maintained at 3cm. The
drying of the lead without oxidation is considered to be so difficult" that it is usually to take
advantage of the fact that Pb is deposited as PbOZ anodically; convenient baths are summarised
in Table 3.
Table 3. Suitable baths for the electrolytic extraction of Pb, Zn and PbO, from natural complex ore
Compobition of Current Tcmpera- Current
electrolyte holution density ture efficiency
Bath (in 1 dm3ore liquor) (mAcm-') ("C) Product (Q%) Recovery
Nitratesolution Ore leach nitrate (10% HNO,) pH 5
+3-5 dropsof conc. H2S04tomake
PbO: more adherent to anode
Ore leach nitrate (10%HN03)pH 5Nitrate solution
Plumbite solution Oreleach PbOi- pH. (9-10)
Plumbite solution Ore leach PbOi -,pH (%lo)
Sulphatesolution
Zincate solution
Ammonical solution
Ore leach ZnSO,, Fe2(S0,), +
IPlSgdm-'conc H2S0,
Ore leach ZnOS-, NaOH 3-5 mole
dm3pH(Ic12)
Ore leach in presence of excess
NHJOH Izn(NH3)61(OH)z
Anodic
45.5
Cathodic
8
Anodic
37
Cathodic
20
Cathodic
50
Cathodic
100
Cathodic
80
55 PbO, Anode 97.3%
96.8
55 Pb Cathode 98.7%
60 PbOz Anode 96.3%
25 Pb Cathode 98.9%
38 Zn Cathode 92.6%
50 Zn Cathode 98.1%
25 Zn Cathode 97.6%
98%
98.170
97.6%
67%
92.8%
93.4%
3.1.2. Electrolytic extraction of zinc
The ore mother liquor, freed from Pb ions (after deposition of Pb or PbOz), was evaporated with
the addition of drops of concentrated sulphuric acid to expel nitrogen oxides followed by dilution
and deposition of Zn. The presence of a higher concentration of 4.80% Fe in the ore showed a
deleterious effect on current yield (Table 3). The current efficiency and the ohmic voltage
decreased as the H2S04 content in the electrolyte increased, but the power consumption
remained nearly constant. At pH<2 the current efficiency was low and hydrogen evolution
occurred.
The decrease in the cathodic current efficiency (a%)is related to several factors, including a
decrease in hydrogen overvoltage on certain areas of the zinc electr~deposit,'~the formation of
local galvanic cells in the zinc which lead to increased corrosion, or an increase in the evolution
of hydrogen and the possible alteration of the growth morphology of zinc by impurities.

6. Flowsheets for electrolytic extraction of lead and zinc 113
Zinc is deposited from the ore zincate liquor after the chemical precipitation of Fe impurities
using NaOH, either in the absence of, or in the presence of, Pb ions. PbOz is simultaneously
anodically deposited from the same plumbite medium.
The dissolution of zinc in NaOH to give Zn(OH)',- takes place through ZnO or Zn(OH), as
intermediate. Zincate is considered to decompose slowly in solution and to precipitate ZnO as a
passivating film.
The ore sulphate liquor is treated by an excess of ammonia solution to convert Zn2+ to the
amino complex [Zn(NH3>,J(OH),. The precipitated hydrated Fe203.x-H20is then filtered off so
as to avoid any complication from galvanic cells" which favour the redissolution of zinc by acid
electrolyte. Zinc is subsequently deposited at 25°C without potential control.
Analytical results reveal that the purity of zinc produced is 299.99% and corresponds
completely to the requirements for the production of high grade Zn alloys. The current efficiency
of zinc deposition from SO',- bath and the cell voltage increase as the zinc content of the bath
and the power consumption decreases. In general, it is observed that an increase of metal ion
concentration increases both the rate of deposition and the yield.
After various experiments the optimum conditions necessary for the cathodic deposition of Zn
were reached (Figure 3) the more convenient baths are summarised in Table 3.
Complex
Zn-Pb ore
I HN03 (S/LI:15)
Leaching -I
Filtrotion
t
t
Ore mother liauor - Cake)residue
I 2O/o HNO,
10% 9N03 (byvol)
Electrolvtic cells
solution
Washing
II FiItrotion
I I
Silica Jroduct
To dump
I
1-
1
Lead electrolysis
I Lead product
Brine electrolyte
I5 g dmm3H, SO,
ISulphation t
I No'OH NH'~OH
Caustification Ammoniation
Fe precipitotion/
Na,ZnO, I e a c F Z n
Zinc electrolysis
Zn cathode product
1
Figure 4. Hydrometallurgical process flowsheet for treatment of complex zinc and lead ore as based on initial experimental
8
investigation.

7. 114 L. H. Madkour
3.2. Recommended flowsheets
The recommended flowsheet for the extraction of metals from Umm-Gheig ore sample after
leaching with nitric acid is illustrated in Figure 4. Two other alternate pathways have been
attempted successfully although each has its drawbacks. It was planned to precipitate iron by
adjustment of pH prior to any electrolysis step since iron present in the ore mother liquor
consumes energy in the Fe2+/Fe3+oxidation-reduction reaction and may contaminate zinc during
the electrodeposition process. Either sodium or ammonium hydroxide were used for the purpose.
When sodium hydroxide was used in excess 3-5 mol dm-3 (pH 1@-12), Fe203.x-H20 was
precipitated and discarded after filtration. Zinc and lead were obtained simultaneously from the
same alkaline zincate and plumbite bath solutions by electrolysis. Pb02is deposited at 4mA cm-2
anodic current density, whereas zinc is deposited at 100mA cm-2 cathodic current density. The
disadvantages of this flowsheet, were that metallic lead might deposit cathodically and
contaminate
In an alternative flowsheet, ammonia solution was used in excess to effect the precipitation of
Pb(OH)2and FezO3.x-H20 simultaneously and these were separated by centrifugation. Separation
of Fe,O, * x-H20 which should be accomplished before the electrolysis process is applied, was
achieved by dissolvingthe precipitate in nitric acid, followed by addition of excesssodium hydroxide
(pH 9-10). Metallic lead was deposited cathodically or anodically as dioxide. Zinc was deposited
cathodically from the ore mother liquor [Zn(NH3)6(OH)2]solution. The disadvantages of this
flowsheet are concerned with the numerous treatments such as additions, separations,dissolutions,
electrolysis and the large amounts of reagents consumed.
also excess of alkali is used (180 g dm-3 NaOH).
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