Clive APrestidge, John Ralston and Roger St.C.Smart, Int. J. Miner. Process,. 1993, 33, 205. 15. Finch, J.A. and Dobby, G.S., Column flotation, Pergamon Press, ...
Metals Materials And Processes, 1998, Vol. 10, No.2, pp. 109-]]8. © Meshap Science Publishers, Mumbai, 1ndia.
AMENABILITY OF COLUMN FLOTATION FOR LEAD-ZINC BENEFICIATION - A CASE STUDY S. Prabhakar and G. Bhaskar Raju National Metallurgical Laboratory (Madras Centre), CSIR Madras Complex, Chennai 600 113, India.
Abstract: The amenability of column flotation technology for the beneficiation of lead and zinc was studied by installing a 3.0 inch dia flotation column. Systematic tests were conducted to depress pyrite and graphite and to improve the quality of lead concentrate. The results clearly indicate that the complexity of the existing flotation circuit can be simplified by adopting flotation columns. In the case of zinc, single stage cleaning by flotation column was found to be sufficient instead of four stage cleaning by conventional cells. However two stage colunm cleaning is essential in the place of existing three stage cleaning by conventional cells to obtain high grade concentrates oflead.
A lead-zinc beneficiation plant with a capacity of 3000 tpd at Rampura-Agucha by MIs Hindustan Zinc Limited (HZL) is one of the largest of its kind in India. Separate concentrates of lead and zinc are produced by conventional flotation. In addition to usual roughing and scavenging, a three stage cleaning was employed in th~ case of lead whereas four stage cleaning was adopted to obtain the final concentrate of zinc. The separation of galena has become complicated due to the presence of highly floatable minerals like pyrite, pyrrhotite and graphite. Though sphalerite was liberated at coarser fraction, most of the galena was finely disseminated. It was generally established that the conventional cells are ineffective to process fine particles. Fine gangue minerals will be easily carried into the froth after getting either entrained in the liquid or mechanically entrapped with the particles being floated. Consequently, the quality of the concentrate will be drastically influenced. Particularly, graphite which is present to the extent of 5-10% by weight was found to accentuate the quality problems. Added to this, the presence of non-sulphidic lead and zinc minerals are posing serious problems in obtaining good recoveries. Considering the advantages and 'superiority of flotation columns and the success achieved in recent years, column flotation technology was implemented in many places to improve both recovery and the quality of the concentrate. The chances of physical entrapment of gangue are minimum in flotation column as the turbulent flow conditions are comparatively less. Further, the mineralized froth in the cleaning was subjected to a blanket ofwash water which washes down the entrained gangue minerals. Consequently, the quality of the concentrate would be improved. It was established that the performance of flotation columns over conventional machines was spectacular in various bperations throughout the world1-5. In many applications, it was seen that the concentrate grade that could be obtained in three stages of cleaning by conventional machines was achieved in a single stage by flotation column. Recent globalization has forced the Indian mineral industries to adopt cost effective technologies in order to sustain the global competition. The technology of column flotation appears to be promising to meet such challenges. The present 'study was aimed at exploring the amenability of flotation column for the
beneficiation of lead-zinc ores of Rampura-Agucha. The possibility of minimising cleaning stages was investigated so that the overall production cost could be cut down.
Petromineralogical studies carried out on the primary sulphide ores of Rampura-Agucha, over a period of time indicate the presence of following minerals.
Sphalerite Galena Pyrite Pyrrhotite Graphite Other gangue
12-20
1-2 15-20 10-12 5-10 45-55
Sphalerite, pyrite, pyrrhotite and galena represent the valuable minerals while quartz, feldspar, micaceous minerals, graphite and sillimanite are the gaugue minerals. Sphalerite is the predominant sulphide mineral whereas galena occurs as fine to medium grained veinlets. Mineralogical investigations revealed that the zinc bearing mineral sphalerite is liberated at 150 microns whereas galena occurs in the size less than 25 microns as disseminations in gangue. The striking feature of the mineralogy of Rampura-Agucha ore is the interlocking of galena and sphalerite with each other and also with gangue minerals like quartz, graphite and pyrrhotite.
A plexiglass flotation column 5 m in height and 7.4 cm dia was used. The overall column design permits variation of parameters such as column height, feed injection point, froth height etc. The top section was connected to a launder in a slanting position to ·enable free falling of concentrate. The feed injection point was located at the height of 3.5m. Sintered bronze disc over a bed of glass beads was used as a sparger. The bubble size can be varied by changing the sintered disc of different porosities. Electronic diaphragm metering pumps were used for pumping the feed and discharging the tailings. The pumps can deliver an accurately measured volumes with an error of ± 1%. Facility for digital display of flow rates was also incorporated in the setup. Constant level between the slurry and the froth phase was maintained by using Differential Pressure Transmitter. The output signal generated by the transmitter was looped with the stroke controller of the tailing pump so that the pumping rate could be automatically adjusted to maintain the interface level at a fixed froth depth. Under steady state conditions, the interface level could be maintained at a constant height of ± 1 em. Flow meters with digital display of flow rate and totalizer were used to record wash water and air flow measurements. The schematic diagram of the column is shown in Fig. 1 Samples of target stream i.e., rougher concentrates of lead and zinc were tapped from an appropriate location of the plant directly to a conditioner. Material was screened to
remove the debris, if any. Conditioner tank was designed to prevent the development of fluids circulation patterns due to intense agitation. Initially, the column was filled with water and the level was stabilized with desired flow rates of tailing, feed, wash water and air flow. In the second stage, mineral slurry was introduced by changing the valve position from water line to slurry tank. After ensuring that the column was He FEED uniformly filled with slurry the feed flow of COLLECTION slurry and wash water that is essential to maintain positive bias were readjusted to the desired residence time. The wash water arrangement was designed in such a way GAS_ that it enters as a sprinkle, not as a jet, to preserve plugflow rise of bubbles. The _TAILINGS column was allowed to run in a steady state ~do-J at least for three residence periods before collecting the samples. Samples of concentrate, tailing and feed were collected simultaneously to check the flow rates. Column test procedure suggested by Finch et al6 was followed. The collected samples were proces~ed to determine percent solids and to prepare dried sub samples for chemical analysis. Chemical analysis of lead, zinc and iron was followed by Atomic Absorption Spectrophotometer? whereas Si02 and graphite were estimated by standard analytical methods8. All the reagents used in this study were of commercial grade.
t
.
•
0
tlb!Jj
lJ
The flotation circuit of Rampura-Agucha lead-zinc ore is typical to that of any lead-zinc beneficiation. ZnS04, NaCN and Nigrosine were used as depressants towards zinc, pyrite and graphtie respectively. CUS04 and isopropylxanthate were used as an activator and collector for sphalerite and ethyl xanthate as collector for galena. Zinc concentrates assaying around 50% of Zn were produced with an average recovery of 90%. However, the quality of the final lead concentrate was found to vary between 36-50% of Pb with an average recovery of 50%. Initially, a set of column experiments were performed on the rougher concentrate of galena at various pH conditions of the slurry and the results of the same are shown in Table I. Two distinct changes were noticed while varying pH. The floatability of pyrite decreases whereas graphite increases by increasing the pH towards basic region. However, the recovery of lead remained almost same throughout the pH range studied. It is clear that the lead concentrate was found to be contaminated mainly by pyrite and graphite. Floatability of pyrite and pyrrhotite is more pronounced at neutral pH whereas graphite at basic pH. While graphite is naturally hydrojhobic and tend to float, several arguments were presented for the floatability of pyrite. The low Eh environment that arises from grinding in an iron ball mill was attributed to such floatability of iron sulphides 10. Batch flotation tests conducted on an ore containing PbS, ZnS and iron sulphides confirmed that the iron minerals tend to float rigorously when the pulp Eh
Solids Wash water velocity Air velocity Slurry feed velocity Pot. Ethyl Xanthate MISC NaCN S.No.
Slurry pH
Sample
Wt. (%)
Feed
-
20.34
: 15% : 0.1 cm/s : 0.87 cm/s : 0.23 cm/s :0.1kg/t : 0.07 kg/t : 0.10 kg/t Assay (%)
Pb
Zn
Fe
Carbon
7.00
Conc. Tails
65.2 34.8
29.35 2.92
2.
8.00
Conc. Tails
28.14 3.49
3.62 5.35
14.81 23.48
30.82 1.04
94
3.
9.00
Conc. Tails
67.5 32.5 66.4 33.6
29.56 2.11
3.02 6.52
31.85 0.92
96
4.
10.00
Conc. Tails
51.2 48.8
38.63 1.21
2.95 5.47
12.52 26.59 8.23
Conc. Tails
49.8 50.2
39.84 0.98
1.42 6.91
6.84 28.41
32.81 7.91 35.41 6.02
97
27.66
10.50
Solids Residence time MISC
20.66 30.44 2.18
-
1.
4.18 3.40 5.62
5.
17.67 13.41 25.12
Recovery {%)
"
94
97
: 16% : 420 sec. : 0.05 kg/t
S.No.
Slurry, pH
Nigrosine (kg/t)
Xanthate (kg/t)"
Water (%) Recovery
Recovery (%)
1.
7.0
0.025
Nil
28
9.5
2.
8.0
0.025
Nil
29
10.1
3.
9.0
0.025
Nil
30
11.2
4.
10.0
0.025
Nil
33
14.0
5.
11.0
0.025
Nil
36
18.3
6.
8.2
Nil
Nil
7.
8.2
0.050
Nil
-
8.
8.2
0.075
Nil
-
9.
8.2
0.10
Nil
-
10.
8.2
0.025
0.15
-
11.
8.2
0.050
0.15
12.
8.2
0.10
0.15
-
Solids Wash water velocity Air velocity Slurry feed velocity Pot. Ethyl Xanthate MISC Slurry pH S.No.
NaCN '(kg/t)
Sample
Wt. (%)
Assay (%) Pb
1.
0.10
Conc. Tails
61.2 38.8
2.
0.15
Conc. Tails
47.8 52.2
38.20 1.05 46.42 1.28
Conc. Tails
47.5 52.5
43.96 1.49
3.
0.20
: 15% : 0.1 cm/s : 1.07 cm/s : 0.23 cm/s :0.1kg/t : 0.07 kg/t : 10.5
Zn 4.43 3.26 4.18 4.56 4.21 4.23
Fe 8.84 19.25 6.26 19.82 7.23 18.!H
Recovery (%) 97 97 96
(resulted from grinding) is low. Recently, oxygen reduction on pyrite and galena was studied as a function of pH11. It was noted that the oxidation of xanthate to dixanthogen is facilitated in the presence of pyrite. The dixanthogen thus formed may in turn adsorb on pyrite and enhance its floatability. A few batch flotation experiments using conventional flotation cell were conducted on pure graphite to ascertain its floatability under different conditions and the results of the same are shown in Table II. It is evident that the floatability of graphite is more or less similar in the entire basic pH region. It may be noted that the flotation of graphite is related to the recovery of water. The increase of graphite recovery with water suggests the phenomena of entrainment. It is expected that the contamination due to graphite by entrainment mechanism could be thoroughly restricted in flotation column since the quantity of process water entering into concentrate product is negligible. An azine dye, nigrosine was tried as depressant for graphite and found to be more effective. Also, the floatability of graphite was more in the presence of xanthate. However, it could be restricted by increasing the dosage of nigrosine. Further experiments were conducted in basic environment because NaCN is more stable and effective in alkaline medium and also the rate of oxidation of xanthate to dixanthogen is slow. The depression of pyrite by cyanide was interpreted due to the adsorption of iron cyanide complexes which are hydrophilic 12-14. Hence, all the experiments were conducted at high pH and NaCN to control the flotation of pyrite. It is evident from the results shown in Table III that the pyrite was depressed in the presence of NaCN without affecting the recovery of galena. However, graphite continued to float and affect the grade of the concentrate. Hence, two-stage cleaning was tried to obtain cleaner concentrate of galena. In the second stage cleaning, nigrosine was added to depress graphite. The effect of nigrosine was found to be dramatic on the depression of graphite. It may be noted that a clean galena concentrate assaying 75% of Pb could be achieved by using nigrosine. The column variables like air velocity (Jg) wash water rate (Jb) and percent solids were studied. The effect of Jg on grade and recovery of galena concentrate is shown in Fig. 2.
CONDITIONS: SOLIDS:
17 '/,
"'18C : O·07k9/t
Jb: 0·1
em/~
H"CH:
J.l: 0·23
em/~
Xant lIat. : 0·1 kg/t
CaNOl nONS:
0·2 kg/t
Slurry pH : 10·5 Nigrosin« : O·lkg/t
SOLIDS: 11 'f,
"" BC : 0·07 k'.l/t
J'.l: 1·0 em/~
HaCN : 0·2
ZnS04
JsI; 0·23 em/.
Slurry
: O.l5kg/t
80
100
60
90
40
80
lQ.11thGt.,:
-;'
""9'01'"_,'
0·1 kg/t
''.lIt 10'0 0·1 kg
It
70
100
'0
~O a:
:'
, >-
'..." 0
pH:
ex:
...> a:
0
...a:
"
U
.0 Q.
10
20
...
...>
IX:
0
"
U \oJ
.b Q.
.0
>-
..•
0
a:
50
60
o-s
1·0
1·5
J9(em/~)
Fig. 2. Effect of superficial air velocity on grade and recovery of lead
.0 Q.
Q.
0·04
0·08
0·\2
Jb(cm/s)
Fig. 3. Effect of wasn water bias on grade and recovery of lead
The gas holdup of the collection zone was estimated according to the procedure suggested by Finch & Dobby15 and found to vary between 10-15% by varying Jg fr9m 0.53 cm/s to 1.60 cm/s. It is well known that frother dosage plays a crucial role in controlling feed water penetration into froth. The bubble size could be reduced by increasing frother dosage upto certain limit. Due to reduced bubble size and bubble rise' velocity, the gas holdup automatically increases. Beyond certain frother concentration, the interface will be vanished due to negative bias and the grade will be affected. Thus, proper combination of air rate and frother dosage is vital for effective performance of column. Usually the recovery of feed water increases by increasing Jg. If the entrained feed water is more than the wash water (bias water 0) the cleaning action will be deminished and consequently the quality of the concentrate will be affected. Furthermore, by increasing Jg beyond its critical limit, a homogeneous distribution of bubbles of fairly uniform size rising at an uniform rate (bubbly flow regime) becomes highly unstable and turns into churn turbulent flow regime. Under these conditions, the recovery will be affected due to unfavourable hydrodynamic conditions. Thus there exists a practical upper limit on Jg for smooth operation. In the present investigation, air velocity of 1.00 cm/s was found to be optimum to obtain reasonable grade and recovery. However, in many industrial scale operations, Jg will be around 1.5 cm/s 15. Experiments were conducted to determine the optimum wash water rate. The principal aim of wash water is to increase the grade of recovered concentrate by displacing entrained hydrophilic gangue particles from the froth phase. Effect of wash water on grade and recovery is shown in Fig. 3. It is apparent that the wash water rate of 0.1 cm/s was found to be enough to improve the grade without decrease in recovery: The effectiveness of wash water is typically assessed by measuring the "bias" flow rate of wash water through the stabilized froth zone of the column. In many cases, bias rate of 0.25 cm/s was reported16 to fully suppress the entrainment. Usually, wash water rates are kept as low as possible to minimize unnecessary dilution of reagents. Wash water
,-o,m/_
Jg; Jb
: 0·1
1\ :
J
l~Clt.
emf,
0·13
,mil :
C-1 kg/t
has been reported to give improved cleaning presumably due to the reduced viscosity which aids the phenomena of drainage.
"'ac: O·01kg/t N.CN: 0·2 kg/t Sl""rry
NISlro,ln~
pH:
10-5
:O·'."~/t
Column throughput can be increased by increasing the feed solids upto the point 90 -; where sufficient bubbles are available to a: w > lift the particles. By increasng the feed o u solids beyond the limit, the froth will be overloaded with particles and become too dry to flow properly. Under these circumstances, large quantity of material is circulated or refluxed between the froth and pulp zone. This recycling will enhance the quality of the concentrate. FigA. Effect of feed percent solids on grade and recovery of lead. Bubbles in froth phase will grow in size due to coalescence and lamella will become thinner. This may lead to excessive drainage of concentrate particles back into slurry (froth drop back) and affects the overall recovery. The recoveries could be improved to a certain extent by increasing the Jg within the bubbly flow regime. The effect of percent solids on grade and recovery is shown in Fig. 4. It may be noted from the results that by increasing solids beyond certain point, the recovery was found to decrease without affecting the quality of the concentrate. Another contributing factor may be increased viscosity of pulp. It may be recalled that slurry density and viscocity often have opposing effects on bubble rise velocity and therefore on gas holdup. In column celrs, froth is frequently overloaded due to dilution of frother by the wash water and high solids loadi.':g of the bubbles. Generally, froth overloading has been characterised by the maximum possible solids content in the froth product. Solid to liquid ratios in the range of 0.2-0.4 by volume have been reported as the onset of froth overloading for sulphide mineral flotation systems. Thus, the rate (P) at which a flotation concentrate can be discharged into a froth launder can be given by
.0 ~
p=-~-pJf
l+~ where (p is the solid to liquid ratio by volume, p is the particle density and Jf superficial flow rate of the froth. In the present investigation, high quality concentrates with a recovery of 90% could be achieved by feeding the slurry containing 25% of solids. In order to improve the quality of the lead concentrate, two stage cleaning by column was tried and the results of the same are compared with plant practice (Table IV). It is apparent that the quality of the concentrates obtained by column is much superior compared to conventional cells. The quality of the concentrates obtained after three stage cleaning by conventional cells can be achieved in a single stage operation of flotation column. By adopting one more cleaning by column, high grade concentrates of the Pb assaying around 75% could be achieved. It is also evident that the addition of nigrosine in the second stage of column operation is essential to depress fast floating graphite. Thus, by adopting column technology, high grade concentrates of lead could be obtained with minimum cleaning stages.
Table IV. Stage-wise Beneficiation of Galena and Comparison and Conventional Cells
Between Column
: 15% :0.1cm/s : 1.07 cm/s : 0.23 cm/s : 0.1 kg/t : 0.07 kg/t : 0.20 kg/t : 10.5 :0.1kg/t
Solids Wash water velocity Air velocity Slurry feed velocity Pot. Ethyl Xanthate MIBC NaCN Slurry pH Nigrosine
Assay %
Stage of Beneficiation Plant Feed
Pb
Zn
Fe
Carbon
1.74
11.75
9.92
N.A
13.81 4.1.47
9.01
19.34
N.A N.A
Si02 N.A
4.68
Column Cone. Two Stage (without Nigrosine)
49.49
7.38 7.42
N.A 25.4
4.68
25.4
Column Cone. Two Stage (with 0.1 kg/t Nigrosine)
76.89
4.82
2.89
4.10
1.60
Plant Final Cone.
40.21
5.70
9.74
N.A
N.A
Plant Rougher Cone. Column Cone. Single Stage
N.A
Note: NaCN 0.1 kg/t was added during grinding stage and additional 0.1 kg/t during column test. Slurry pH mentioned above refers to column test only.
Table V. Comparison
Between Column and Conventional
Flotation Cells on the
Cleaning Operation of Sphalerite Rougher Concentrates : 17% : 0.1 cm/s : 1.07 cm/s : 0.23 cm/s : 1.00 kg/t : 0.15 kg/t : 0.09 kg/t : 9.00
Solids Wash water velocity Air velocity Slurry feed velocity Copper sulphate Sod. Isopropyl Xanthate MIBC Slurry pH
Percent assay of sphalerite concentrate
S.No.
Conventional
Column 1.
(Plant)
Zn
Pb
Fe
Zn
Pb
56.51
8.15
52.57
0.68
Fe 9.47
11.13
39.50
0.93
12.85
10.86 9.43
43.25
0.75 0.89
9.26
42.89 43.42
13.14 12.42
10.12
42.82
0.78
2.
46.14
1.17 1.20
3.
47.21
1.12
4.
53.86
5.
54.14
1.28 1.42
6.
51.62
0.99
1.12
11.68 10.42
Column flotation experiments were also performed on the rougher concentrates of zinc to compare the performance of column over conventional cells. Continuous experiments by column were conducted under the similar conditions of reagent dosages maintained in plant operation. Samples were collected simultaneously from the plant to compare the concentrate quality. The result of the same presented in Table V, clearly demonstrate the superior performance of column. Single-stage cleaning by flotation column was found to be effective compared to that of four-stage cleaning by conventional flotation machines. The multi-stage cleaning by conventional machines could be replaced by adopting flotation columns and thus the overall flotation circuit could be simplified.
Amenability of flotation column for the beneficiation of galena and sphalerite was studied by installing laboratory size flotation column within the premises of Rampura-Agucha beneficiation plant. It was established that the quality of the concentrates of lead and zinc obtained by using flotation column (as cleaner) is superior compared to conventional cells. Single stage cleaning by column was found to be sufficient to obtain zinc concentrate suitable for smelting whereas twostage column cleaning was found to be essential to obtain final lead concentrate. By adopting column flotation technology, the overall flotation circuit could be simplified and associated advantages like low maintenance, less power consumption and floor space etc. can be derived to reduce the production cost.
Authors are thankful to Prof.P.Ramachandra Rao, Director, National Metallurgical Laboratory, Jamshedpur for his valuable guidance and permission to publish this work. The authors wish to acknowledge the financial support of M/s.HZL, Udaipur.
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John Ralston and Roger St.C.Smart, Int. J. Miner. Process,
