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Super-scavenging of zinc tailings by FASTFLOT 2500 at Pasminco Mining, Broken Hill
MineraL~ Engineering, Vol. 10, No. 4, pp. 357-366, 1997
Pergamon
PII:S0892--6875(97)00013-7
© 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0892-6875/97 $17.00+0.00
S U P E R - S C A V E N G I N G O F Z I N C T A I L I N G S B Y F A S T F L O T 2500 A T P A S M I N C O M I N I N G , B R O K E N HILL M.W. CHUDACEK§, S.H. MARSHALL§, M.A. FICHERA§, J. B U R G E S S ? a n d F . L . B U R G E S S ? § M.D. Research Co. Pty Ltd, PO Box 22, North Ryde 2113, Australia t Pasminco Mining, PO Box 460, Broken Hill 2880, Australia (Received 25 September 1996; accepted 15 December 1996) ABSTRACT A relocatable single stage prototype of the full scale FASTFLOT 2500 cell was tested on super-scavenging of zinc tailings at Pasminco Mining, Broken Hill. After commissioning of the plant and rectification of minor problems the plant operated very stably with liquid level control better than +_2 mm most of the time, giving the operators excellent control over the concentrate grade~recovery curve. Modest reconditioning of the tailings stream by copper sulphate and Cytec 7016 led to substantial improvement of zinc recovery at about the same grade. With conditioning, the single stage plant recovered 13.1 to 57.4% of available zinc at grade ranging from 4.0 to 25.5% Zn, from a feed grade of 0.27 1.01% Zn. The recovered marmatite was predominantly in the -9 #m range where mechanical flotation cells exhibit very slow kinetics, and sometimes in the + 120 ftm range, where the main plant historically shows poor recovery. Considering that the stream has 25 minutes residence time in the main plant circuit, super-scavenging results by the FASTFLOT process with 1 minute residence time confirm its expected superior performance in the fine particle range when compared to conventional mechanical cells. ©1997 Elsevier Science Ltd Keywords Froth flotation; flotation kinetics; fine particle processing; sulphide ores; tailings INTRODUCTION Deployment of the FASTFLOT 2500 at Pasminco Mining, Broken Hill South Concentrator had three key objectives. Firstly, this study was to conclusively determine economic data for the feasibility of superscavenging zinc from the final residue. Impetus for the study was that upwards of 2 t/h zinc are lost in tailings, some of it in a fine liberated form which a conventional circuit has difficulty in recovering. The initial FASTFLOT 250 pilot plant study as reported by Chudacek et al. [1,2], yielded encouraging data which needed to be confirmed on a full scale FASTFLOT 2500. The second objective was to assess the performance of the FASTFLOT 2500 prototype on long term duty under arduous field conditions. The Pasminco Broken Hill South operations use one of the coarsest grinds in Australia. Hence, it was perceived that if FASTFLOT 2500 could operate stably with such a coarse feed, it would provide a substantial safety margin for operation with fine slurry, a typical area of FASTFLOT deployment. The third objective was to test FASTFLOT support plants such as high pressure pump and jet water filtration systems on a long term basis. Presented at Minerals Engineering '96, Brisbane, Australia, August 26-28, 1996
357
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M.W. Chudaceket
al.
T H E FASTFLOT PROCESS The FASTFLOT process depicted in Figure 1, differs from known flotation devices in that high velocity thin clean liquid jets are used as energy and air carriers. In operation the high velocity thin jets (7) accelerate the surrounding air envelope and plunge between two layers of mineral slurry (5) fed into the diverging mixing nozzle (3) at much lower velocities. The shear rate, due to the velocity differential between these three streams, results in shredding of the gas envelope into very small bubbles and provides very intense agitation accompanied by a high turbulence. There are also high deceleration levels present in the mixing nozzle, as the thin jet with velocity typically about 60 m/s is decelerated to less than 3 m/s over a path of about 0.3 m. Further, the very high turbulence in the mixing zone creates high acceleration levels in small eddies. The FASTFLOT high velocity clean liquid jet technique has the advantage over other high intensity mixing techniques because there are no moving parts, such as the impeller of a mechanical flotation machine, in contact with the abrasive slurry. The wear rate of the mixing nozzle, which has a simple geometry, is very small because the high velocity jet core is isolated from the nozzle walls by the much slower moving layers of slurry. The functioning of the rest of the cell can be seen from Figure 1. A multiphase slurry flow is discharged from the mixing nozzle (3) into the cell bulk compartment where it flows past longitudinal stabilising baffles (8) and the flow velocity is attenuated by entrainment of bulk slurry. The resulting lower shear rate promotes air bubble coalescence leading to reduction of interfacial area of bubbles and hence to mineral concentration effects on the bubbles surface due to gangue rejection. These enlarged gas bubbles loaded with hydrophobic particles disengage from the stream, rise towards the liquid surface and are diverted by bubble guides (9) towards the froth discharge end of the cell. This arrangement promotes a uniform froth generation over the entire surface of the bulk compartment. The rising air bubbles induce an upward slurry flow which is diverted by the bubble guides towards the discharge end of the cell resulting in a defined surface flow capable of carrying even a very frail froth towards the froth weir. As the layer of froth travels towards the froth weir it drains liquid and entrained gangue mineral particles and thickens due to crowding against the weir. It is subsequently discharged by the rotary paddle (10) into the froth launder (11). The slurry flow at the discharge end of the cell is partly discharged via flexible ducts into the discharge launder (12) which is adjustable vertically to control cell liquid level. The rest of the slurry flow is diverted upwards by a curved bottom where in turn the slurry flow is diverted horizontally by a flow guide (13) towards the recycle gate (4) which controls the recycle flow (5).
Fig. 1 Schematic diagram of FASTFLOT design A cell cross section view. (1) Feed distributor; (2) Feedbox; (3) Mixing nozzle; (4) Recycle gate; (5) Recycled mineral slurry; (6) High pressure manifold containing injection nozzles; (7) High velocity thin jet; (8) Stabilising baffles; (9) Bubble diverting guides; (10) Rotary paddle; (11) Froth launder; (12) Discharge launder; (13) Flow guide.
Super-scavenging of zinc tailings
359
The operation of the cell is controlled as follows. Mixing intensity, shear rate and acceleration levels are controlled by the velocity of the thin jets (7) which is in turn controlled by the operating pressure in the manifold (6). Air entrainment rate increases with increased jet velocity, but may be also controlled by changing the magnitude of the recycle stream through vertical adjustment of the recycle gate (4). Froth height and froth flow are controlled by the liquid level in the cell. The liquid level in the cell is controlled by moving the flexibly connected discharge launder (12) up or down. The feeder box (2) in connection with the surface baffle assists in formation of the evenly distributed primary feed layer.
EXPERIMENTAL PLANT The host plant final tailings stream was removed via a 150 mm dia branch-off from a 300 mm dia pipe. The 150 mm branch-off was fed into a 100 nun dia pipe leading to a variable speed 80/50 slurry pump (No 1) which in turn fed a ca 3000 litre conditioning tank. The conditioned slurry was removed via a variable speed 80/50 slurry pump (No 2) which fed the FASTFLOT 2500 cell mounted on an open container. The open container housed also all necessary control equipment and switch gear as well as concentrate and tailings sumps and pumps. The whole plant was designed for unattended continuous operation with an automated shutdown procedure in the case of any unit failure. After commissioning and some revisions the whole plant operated to specification.
EXPERIMENTAL PROCEDURE Most of the experiments were conducted with ca 10 minutes conditioning. Copper sulphate was added to the feed pipe of slurry pump No 1 and Cytec 7016 was added into the conditioning tank. The FASTFLOT plant was sampled when it reached a steady state for the given experimental condition. The slurry feedrate was not controlled by feedback loop control, but set by variable frequency speed control of the slurry feed pump. The feedrate was reasonably steady most of the time. The decision not to control was deliberate as it would expose the plant to occasional fluctuations of the feedrate as in normal operating conditions. The liquid level was controlled via a feedback loop. This arrangement managed to control the liquid level within + 2 m m for + 16% variation of the flowrate. Hence variation of flowrate of this magnitude had virtually no effect on the quality of the concentrate. After the plant was operating at steady state for 15 - 20 minutes feed and concentrate samples were taken. Tailings were seldom sampled because of potential segregation in the tallings sump and also to minimise the number of samples to be analysed. Three samples of feed were taken; sample A before concentrate A collection, sample B after concentrate A collection and sample C after concentrate B collection. Hence the quality of feed over the entire period of concentrate sampling was monitored. Concentrate was collected in six dishes simultaneously sampling all six concentrate launders. Each dish sampled a 200 mm width of the particular launder giving a total sampling width of 1200 mm for a 2500 mm wide cell. Hence 48% of the cell output was sampled for a period of 15 - 60 seconds depending on the cell output. The duplicate samples A and B were found to be in reasonable agreement considering the industrial scale of the tests. The average of these samples is reported here. Concentrate sample A was related to the mean of feed samples A and B and concentrate sample B was related to the mean of feed samples B and C. The samples were dewatered, dried, split and analysed for Pb, Zn, Fe, Cu and Ag. The raw data were processed by a spreadsheet program. All key operating parameters were recorded continuously on a chart recorder. Peripheral parameters were recorded manually. R E S U L T S AND DISCUSSION Altogether 35 experiments were conducted with various feed rates ranging from 206 to 424 L/min, solids feed rates ranging from 78 to 149 kg/min and feed grades ranging from 0.27 to 1.01 % w/w Zn. Final tailings assaying less than 0.4 % w/w Zn were achieved by the main plant when it was operated at its very best. Tailings with residual Zn in 0.4 - 0.6% w/w range represents a typical operation and tailings above 0.6% w/w Zn suggest problems in the concentrator circuit accompanied by substantial losses of zinc to the tailings. The most interesting experiments are presented in Table 1.
21 22 23 24 27 28 29 30 31 32 33 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62
Exp No
s
L/min
400 4OO 424 419 315 303 3o4 304 4oo 403 403 206 311 307 3O6 299 302 299 3Ol 305 3o2 3o3 302 303 302 304 302 305 304 3O6 303 304 304 302 305
119.4 104.7 110.6 145.5 112.2 107.8 110.8 77.9 116.8 126.8 135.0 101.6 148.6 148.4 133.5 103.9 102.7 95,6 106.0 107.0 105.9 107.4 107.7 107.2 110.0 109.1 99,4 100.9 93.0 83.9 116.6 119.8 124.4 121.8 107.7
kg/min
Solids Rate
19120 24/26 24•26 18/20 18•20 16/18 15/17 15117 20/22 15/18 15/17 15/17 15/17 15/17 15/17 15/17 22/24 25/27 25/27 22/24 20/22 22124 17/20 17/19 18120 15/17 15/17 14/17 15/17 16/18 15/17 15/17 15/17 15/17
Liquid Level mm
9.03/8.75 9.03/8.99 8.92/9.03 9.00/8.99 8.80/8.90 9.00/8.96 8.87•8.97 8.65/8.86 8.69•8.90 8.76/8.61 8.46/8,60 8.45/8.44 8.46/8.49 8.36/8.40 8.22 8.49/8.46 8.45/8,27 8.36/8.33 8.54/8.50 8.54/8.52 8.54 8.53/8.47 8.49/8.48 8.54/8.37 8.45/8.55 8.56/8.50 8.51/8.55 8.54/8.50 8.62/8.58 8.80/8.80 8.58/8.59
pH
75 70 68 97 129 120 112 76 68 68 75 64 66 69 67 67 67 66 66 66 64 65 62 39 45 64 63 62 60 60 65
g/t
C u S O 4. 5H=O
11 13 11 11 12 11 11 11 11 11 11 11 11 10 7 7 10 11 10 10 10 11
12 12 12 16 18 17 16 11
g/t
Cytec 7016 0.37 0.42 0.40 0.32 0.34 0.40 0.49 0.44 0.61 0.51 0.38 0.68 0.94 1.01 0.91 0.48 0.46 0.49 0.32 0.36 0.38 0.43 0.82 1";.82 0.84 0.86 0.42 0.40 0.45 0.40 0.27 0.28 0.35 0.42 0.37
%Zn
Feed Grade 5.70 3.20 5.15 8.55 5.70 6.60 7,05 4.26 7.21 8.31 3.89 23.35 5.99 24.45 25.55 3.98 4.34 7.32 3.98 4.97 4.22 3,68 22.10 17.15 17.45 20.50 5.30 5.00 4.40 3.95 3.00 3.00 4.50 6.10 3.50
%Zn
Conc Grade 15.6 7.7 13.0 27.1 16.8 16.4 14.3 9.6 11,7 16.2 10.5 34,3 6.3 24.8 28.1 8.4 9.5 15.0 12.3 8.9 8.9 8.6 26.9 21.0 20.8 24.1 12.8 12.5 9,7 9.9 11.3 10.7 12.9 14.5 9.6
Enrich. Ratio
TABLE 1 FASTFLOT 2500 superscavenging of final Zn tailings at Pasminco Mining, Broken Hill
47 47 45 45 59 61 60 60 47 47 47 85 59 60 60 61 61 61 61 60 61 61 61 61 61 60 61 60 60 60 61 60 60 61 60
Res Time
Rate
Feed
6.7 12.2 10.2 5.1 18.7 17.4 20.1 16.4 15.7 17.5 16.1 54.1 2.3 57.3 57.4 19.3 18.6 14.8 7.8 13.6 11.0 11.1 44.9 45.0 47.5 46.7 30.2 31.2 25.5 32.6 15.3 14.9 14,4 14.0 31.7
Recov. %Zn
3.18 2.70 1.39 4.27 4.53 6.57 3.39 6.79 6.82 4.86 22.49 1.99 49.50 42.00 5.71 5.26 4.18 1.59 2.10 2.17 3.06 23.90 23.65 26.24 26.06 7.46 7.45 6.42 6.56 2.83 3.01 3,75 4.28 7.47
1.75
Recov. Zn kg/h
y~
e~
Super-scavengingof zinc tailings
361
Effects of Reconditioning The tailings stream had been in the zinc circuit for 25 minutes so it was likely that reconditioning may be required for effective recovery of fine particles which are known to be extremely sensitive to the chemical environment. FASTFLOT studies reported by Chudacek et al. [1] indicated improvement of recovery and grade with modest reconditioning on this stream. In the present work, substantial improvement of grade and recovery was achieved after reconditioning the final tail by ca 90 g/t CuSO4.5H20 and 15 g/t Cytec 7016 during preliminary batch cell studies. While the data in Table 1 do not permit direct comparison between many unconditioned and conditioned experiments as they were conducted mostly at different flowrates, Exp Nos 23 and 33 may be compared as they were both conducted at 400 L/min feed rate. Conditioned Exp No 33 with feed grade 0.38% w/w Zn compared to unconditioned Exp No 23 with feed grade 0.40% w/w Zn suggests a 71.5% improvement in Zn recovery at a 32% relative loss of grade. Comparing conditioned Exp No 62 at 300 L/min feed rate, to the higher feedrate (400 L/min) but unconditioned Exp No 21, with an identical feed grade of 0.37% w/w Zn we see 4.7 times improvement in recovery at 39% (relative) loss of grade. While part of the improvement is attributable to longer residence time in the cell during Exp No 62 compared with Exp No 21, not all improvements can be attributed to this factor alone. This would suggest that conditioning improves recovery. There is one interesting comparison to be made in the higher Zn feed grade series of experiments. Comparing Exp No 41 where both copper sulphate and Cytec 7016 promoter were added, with Exp No 40 where only copper sulphate was added, it appears that omission of the promoter resulted in a substantial loss of grade and recovery. This demonstrates how sensitive is the reagentisation of fine particles. Here it is interesting to note that during reagentisation of another Zn stream in the same plant, deliberate omission of promoter Cytec 7016 led to improved Zn recovery and grade. This confirms that both reagents must be in precise and specific balance for each task in hand. While higher dosages of copper sulphate, 97 - 129 g/t and higher Cytec 7016 dosages were used in Exp Nos 30 - 33, these did not seem to be effective. If we compare Exp No 29 with Exp No 32 at about the same feed Zn assays and pH regime, we see that Exp No 29 with less reagents dosage had a better recovery than Exp No 32. Lowering the reagents dosage to 39 - 45 g/t copper sulphate and 7 g/t of Cytec 7016 in Exp Nos 55 and 56 did not have much impact on recovery and grade in Exp No 55 but seemed to lower recovery, enrichment ratio and concentrate grade in Exp No 56. While overdosing of reagents is costly and counter productive, underdosing may depress recovery so 60 - 70 g/t of copper sulphate and 10 12 g/t Cytec 7016 was considered a reasonably safe dosage and used in all subsequent experiments.
Effect of pH It was the intention to keep pH at a value of about 9.00. However, the feedback loop controlling the pH was not operational at that time, therefore a manually adjusted dosing pump was used to control the pH, hence some drift of values in Exp Nos 30 - 33. Exp Nos 39 - 42 were deliberately conducted without NaOH addition at a pH of about 8.6. It is interesting that all but the already mentioned Exp No 40 showed dramatic improvement in grade and recovery. While these improvements can also be partly attributed to higher feed grades, as well as to the appropriate ratio of copper sulphate to promoter Cytec 7016, pH is likely to have a dominant effect. Exp Nos 43 - 49 at lower pH range 8.22 - 8.45, had lower recovery than experiments conducted at pH 8.6 and similar reagents dosage. Hence in subsequent experiments the pH was kept at 8.6 + 0.1, and all of these yielded good recovery and enrichment ratios. The optimal pH is very close to 8.6.
Setting of the froth/pulp interface The vertical distance of the froth/pulp interface from the discharge lip, referred to as liquid level, determines the quality of concentrate as this distance determines the froth depth and froth character in the cell. During experiments, this liquid level was controlled to better than 4- 2 mm. The objective of superscavenging was to maximise recovery so the liquid level was kept at 15/17 mm below the discharge lip for most of the experiments. A higher setting than this would have resulted in the partial discharge of
362
M.W. Chudacek et al.
pulp into the concentrate which is unacceptable practice for FASTFLOT operation. Lower settings than 15/17 were tried but these usually led to loss of recovery with no, or marginal, grade improvement. Effect of feed grade The effect of feed grade on percentage recovery, enrichment ratio and zinc yield can be seen from Figures 2 - 4. These Figures only contain experiments close to the optimal pH of 8.6. The recovery/feed grade plot in Figure 2 suggests that recovery improves from ca 15% at 0.3% w/w Zn feed grade to ca 55% at 1.0% w/w Zn feed grade. This behaviour was expected as a lower grade feed would have a higher proportion of locked in minerals which cannot be recovered and also a higher proportion of liberated ultrafine minerals which are difficult to recover. Feed with high grades was encountered when there were problems with regrind mills resulting in a coarser grind of Zn feed. The main plant historically has had difficulties in recovering marmatite particles over 120/~m size. Chudacek et al. [2], explained how these large marmatite particles are recovered by the FASTFLOT process. Hence it is anticipated that a high grade feed would produce a FASTFLOT concentrate with value metal being split between ultrafine and coarse fraction with relatively little value metal being present in intermediate fractions where the main plant is very effective. It is hoped this 'anomalous' distribution will be confirmed once the key experiments are analysed in detail.
60
•
J
50 A
~'
40
> O u g:
30
c N
20
10 0
' 0.2
' ' ' 0.4 0.6 0.8 Feed Grade Zn (%) w/w
' 1
1.2
Fig.2 Zinc recovery from super-scavenging of final plant tailings by the FASTFLOT 2500 vs tailings feed grade. Pasminco Mining, Broken Hill. The relationship between the enrichment ratio and the feed grade is given in Figure 3. The plot suggests an increase of enrichment ratio from ca 10 times at feed grade 0.25 % w/w Zn to around 27 times at feed grade range 0.8 - 1.0% w/w Zn. Experiment No 39 is responsible for the unusually high enrichment ratio of 34 times at a feed grade of 0.68% w/w Zn. It seems the plant was particularly well set for this experiment and that the chemical environment was just right. The reasons for this very high enrichment ratio will hopefully become clear during detailed analysis of the experiment. The increase of enrichment ratio with feed grade increase was anticipated. However if the feed grade is increased further, say to 2.0 %, a levelling of enrichment ratio is to be anticipated as the maximum concentrate grade from this type of feed is in the 42 - 46% w/w range at the very best. The relationship between plant zinc yield per hour per cell, and feed grade is given in Figure 4. The plot suggests a 20 times increase of the yield for a feedgrade increase from 0.25 to 1.0% w/w Zn. This trend was also anticipated, as with higher feed grade at the same grind, more value metal is available for extraction. Conversely, at lower feed grades, say below 0.25% w/w Zn, yield becomes insignificant and
Super-scavenging of zinc tailings
363
this projects into the economics of process.
40
o 30 IX
,o
"Flu 10
O
I
0.2
I
0.4
I
0.6 0.8 Feed Grade Zn ( % ) w/w
i
1.2
1
Fig.3 Zinc enrichment ratios for super-scavenging of final plant tailings by FASTFLOT 2500 vs tailings feed grade. Pasminco Mining, Broken Hill.
60 50m
~40 ,=~ ---
ID
-
30
u
>- 2 0 c N
10 0
I
0.2
0.4
I
I
0.6 0.8 Feed Grade Zn ( % ) w/w
I
1
1.2
Fig.4 Zinc yield rate from super-scavenging of final plant tailings by FASTFLOT 2500 vs tailings feed grade. Pasminco Mining, Broken Hill. Looking globally at all Figures 2 - 4, we notice two clusters of points. The significance of these two clusters are as follows. The lower cluster in the feed grade range 0.25 - 0.4% w/w signifies the main plant operating stably at its best. When the main plant is in this region, the Zn yield is a relatively small 2.5 8 kg/h/cell and the super-scavenging process is certainly uneconomical. The higher cluster of points signifies the main plant having operating difficulties such as a regrind problem, ore change, incorrect reagentisation etc. When the main plant exhibited operating difficulties, recoveries were significant, in the range of 22 - 49.5 kg/h/cell. Under such conditions, super-scavenging by FASTFLOT would highly likely be economical. Interestingly, the FASTFLOT plant operated very stably during the above main plant upsets
364
M.W. Chudacek et al.
and with standard conditioning simply 'mopped up' most residual zinc which the main plant left behind due to its upsets. This is quite an achievement if we consider that the stream has spent ca 25 minutes in the main plant zinc circuit and only 1 minute in a single stage FASTFLOT plant. Hence we can say that FASTFLOT acted as a safety screen containing zinc losses due to main plant upsets.
Zinc yield projection for full size plant The data described above shows substantial and nonlinear Zn yield variation with change of feed grade. To obtain data for future economic analysis of the project it was necessary to estimate the Zn yield of a full size plant for a typical and representative period of main plant operation. Two periods of one calendar month duration, were selected on the advice of our host plant management for such analysis. Hourly averages of the feedrate, Zn and Pb assays for all three milling sections of the plant were taken, as well as Zn assays of the tailings. From respective hourly averages of Pb and Zn recovery and their grades, a solids rate of the tailings was calculated. Using the recovery vs feed grade plot in Figure 1, the Zn yield was calculated hour by hour and summarised to yield a monthly figure. This approach was necessary to contain the impact of frequent fluctuation of plant performance reflecting itself in fluctuation of the tailings Zn assay. The detailed analysis for the month of March 1996 yielded full plant availability 86.1% and a yield of 516.2 tonnes per month (t/m) of pure Zn equivalent. Similar analysis for the month of May 1996 yielded full plant availability of 83.5% and a yield of 511.4 t/m of pure Zn equivalent. The agreement of the figures suggest that the above methodology was reliable enough to obtain approximate plant availability and Zn yield of a full scale plant for economic analysis of the project.
Scale-up relationship The usual scale-up relationship from our 250 mm wide pilot plant FASTFLOT 250 data is based on the width ratio, as the cross section of FASTFLOT cells is the same at all scales. This means the output factor is 10 times for FASTFLOT 2500 (2500 mm wide) and 12 times for FASTFLOT 3000 (3000 mm wide). However, the wall effects which retard the movement of the froth bed in the cell are much more pronounced on the 250 mm scale than on larger scales. This provides a safe scale-up margin. Projected Zn yield rate figures for FASTFLOT 2500 based on the FASTFLOT 250 x 4 pilot plant study reported by Chudacek et al. [1] for 0.43 % w/w Zn feed were in the range of 3.6 - 3.9 kg Zn/h/cell, while the actual FASTFLOT 2500 for the same feed grade at optimum pH suggests yield rates of 6.5 - 7.5 kg Zn/h/cell. For a broader pH range of 8.35 - 9.00 and a feed grade in the range of 0.40 - 0.45 % w/w Zn, the Zn yield rate was in the 3.1 - 7.5 kg/h/cell range. The lowest value for the experiment at pH 8.35 is clearly suboptimal. Also, the experiment at pH 9.0 yield rate was only 3.9 kg Zn/h/cell confirming the necessity of operating at optimal pH. While there have been numerous improvements to both the FASTFLOT 250 x 4 pilot plant and the FASTFLOT 2500 plant and the chemistry has been optimised since the original study, it seems that full plant prediction based on pilot plant data was reliable. The improvement on the original prediction from pilot plant data can be attributed to the following factors in descending order of significance.
Reagents regime optimisation.
1.
2.
.
Automatic liquid level control on the FASTFLOT 2500 compared to manual control on the FASTFLOT 250. Use of more advanced jet nozzles when compared to the original study.
4.
Reduced effect of the side wall for the larger scale.
5.
Better understanding of the process and better operating practice.
Super-scavengingof zinc tailings
365
FASTFLOT plant performance While some problems were encountered with plant feed removal from the host plant, the FASTFLOT 2500 performed without major hiccups from the beginning of testing. Minor shortcomings of the prototype such as the feed distributor were rectified. Also, a screening system using a polyurethane screen with 4 mm opening had to be introduced to remove large particles from the tailings stream, as well as wood splinters or plastic from blasting cartridges which would otherwise block the mouth of the mixing nozzle and hence disrupt the process. After a few modifications of the feeding trough with screen, feed distributor and washing nozzle manifold, reliable continuous operation could be sustained. Operators were only required to clean the screen of the entrained debris by occasional brushing. This operation did not take longer than 2 - 3 minutes. Accumulated debris on the screen was automatically dumped every 10 -35 minutes depending on the debris load. Simultaneously, with the opening of the dumping gate, a series of process water jets were actuated to assist in quick debris discharge to the dump trough. Thin water jets acting as energy and air carriers became coherent and stable after the initial problems with the filtration of jet water were overcome. In later stages of the study when a prototype of an automatic sand filter was commissioned, the FASTFLOT cell ran without visible deterioration of jet quality and blockage of the final chance filters protecting jet nozzles for many days until tailing hose failure precipitated automatic shutdown of the plant. Most remarkable was the stability of the liquid level control which held the liquid level to + 2 mm for a 16% change in the feed rate. This control is certainly more than adequate. When operated with the normal plant grind of d80 160 #m, cell discharge tubes seldom blocked due to sanding up. The blockage with this feed was never as severe as to actuate the automatic backflushing procedure. However, when the main plant re-grind was off and feed ds0 240/~m, discharge tubes sanded more frequently and the backflushing system was occasionally actuated. After some design modification of backflushing injection ports, the cell routinely recovered from sanded tubes automatically, without any operator intervention. It is important to note here that the FASTFLOT application range is for fine particle flotation and hence for grinds typically d80 < 75 #m. During our pilot plant studies on FASTFLOT 250 we never experienced any sanding with feed of d80 < 75 #m so sanding problems are not anticipated for the FASTFLOT 2500. However, even these finer streams encounter occasional coarse particles, hence a new design of FASTFLOT B-2 was developed featuring bottom discharge of the pulp via a series of flap valves as outlined in Figure 5. This new design has been successfully tested and
~ " "
? " - :." "." ," "÷'C ".'7 ~ )
~
~
Fig.5 Schematic diagram of FASTFLOT B-2 cell. (1) Feed manifold (2) Feed trough (3) Screen (4) Debris dump gate (5) Debris trough (6) Primary feed (7) High pressure manifold with injection nozzles (8) Thin high velocity jets (9) Recycle flow gate (10) Mixing nozzle (11) Stabilising baffles (12) Tailings flap valve 13) Flow deflector (14) Froth bed (15) Rotary paddle (16) Concentrate
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M.W. Chudaceket al.
retains all the metallurgical advantages of former Design A, including excellent liquid level control but is more robust and also cheaper to manufacture. Air sparging and backflushing of discharge tubes were disposed of, hence cell operating costs are lower.
CONCLUSIONS 1.
2. 3. 4. 5. 6. 7. 8.
.
10.
The prototype FASTFLOT 2500 cell was successfully tested on a very difficult stream with particle size distribution well outside its original specification. All shortcomings of the original design have been overcome and experience gained during long term tests have led to a more robust and cheaper to manufacture FASTFLOT B-2 cell design. A major feature of plant performance was its stability and particularly its liquid level control which was better than + 2 nun. The scale-up factor for pilot plant data was found to be conservative, giving a reasonable safety margin for economic prediction of a full scale plant. Modest reconditioning of the main plant tail with copper sulphate and reagent Cytec 7016 led to substantial improvement of zinc recovery at about the same or better grade. Operating at pH 8.6 _ 0.1 led to better recoveries than operating outside this range, pointing to extreme sensitivity of fine particles to the chemical environment. Recovery, enrichment ratio and zinc yield per cell increased with increasing Zn grade of main plant tailings, as expected. When the main plant exhibited operational difficulties FASTFLOT zinc yield was 20 times the yield when the main plant operated at its very best. This suggests that FASTFLOT, besides its super-scavenging duty, can be used as a 'safety net' to contain value metal losses during main plant instability. Considering that the stream had ca 25 minutes residence time in the main plant Zn circuit, the recoveries and grades achieved during a mere one minute residence time in a single FASTFLOT cell are certainly remarkable by any standard. The technique used for determining average monthly Zn yield and full plant availability was found to be adequate for economic analysis of the project.
ACKNOWLEDGEMENT Authors indebted to Pasminco Mining and MD Research for their courageous support of the difficult and extensive study.
REFERENCES 1.
2.
Chudacek, M.W., Marshall, S.H., Estrina, H. & Fichera, M.A., FASTFLOT process - a unique tool for effective recycling. Filtration and Separation, 32(4), 293-303 (1995). Chudacek, M.W., Estrina, H, Marshall, S.H., Fichera, M.A, Burgess, J. & Burgess, F.L., Super-scavenging of zinc residue by the FASTFLOT process at Pasminco Mining, Broken Hill. A u s l M M Annual Conference, 207-211 (1995).
Pergamon
PII:S0892--6875(97)00013-7
© 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0892-6875/97 $17.00+0.00
S U P E R - S C A V E N G I N G O F Z I N C T A I L I N G S B Y F A S T F L O T 2500 A T P A S M I N C O M I N I N G , B R O K E N HILL M.W. CHUDACEK§, S.H. MARSHALL§, M.A. FICHERA§, J. B U R G E S S ? a n d F . L . B U R G E S S ? § M.D. Research Co. Pty Ltd, PO Box 22, North Ryde 2113, Australia t Pasminco Mining, PO Box 460, Broken Hill 2880, Australia (Received 25 September 1996; accepted 15 December 1996) ABSTRACT A relocatable single stage prototype of the full scale FASTFLOT 2500 cell was tested on super-scavenging of zinc tailings at Pasminco Mining, Broken Hill. After commissioning of the plant and rectification of minor problems the plant operated very stably with liquid level control better than +_2 mm most of the time, giving the operators excellent control over the concentrate grade~recovery curve. Modest reconditioning of the tailings stream by copper sulphate and Cytec 7016 led to substantial improvement of zinc recovery at about the same grade. With conditioning, the single stage plant recovered 13.1 to 57.4% of available zinc at grade ranging from 4.0 to 25.5% Zn, from a feed grade of 0.27 1.01% Zn. The recovered marmatite was predominantly in the -9 #m range where mechanical flotation cells exhibit very slow kinetics, and sometimes in the + 120 ftm range, where the main plant historically shows poor recovery. Considering that the stream has 25 minutes residence time in the main plant circuit, super-scavenging results by the FASTFLOT process with 1 minute residence time confirm its expected superior performance in the fine particle range when compared to conventional mechanical cells. ©1997 Elsevier Science Ltd Keywords Froth flotation; flotation kinetics; fine particle processing; sulphide ores; tailings INTRODUCTION Deployment of the FASTFLOT 2500 at Pasminco Mining, Broken Hill South Concentrator had three key objectives. Firstly, this study was to conclusively determine economic data for the feasibility of superscavenging zinc from the final residue. Impetus for the study was that upwards of 2 t/h zinc are lost in tailings, some of it in a fine liberated form which a conventional circuit has difficulty in recovering. The initial FASTFLOT 250 pilot plant study as reported by Chudacek et al. [1,2], yielded encouraging data which needed to be confirmed on a full scale FASTFLOT 2500. The second objective was to assess the performance of the FASTFLOT 2500 prototype on long term duty under arduous field conditions. The Pasminco Broken Hill South operations use one of the coarsest grinds in Australia. Hence, it was perceived that if FASTFLOT 2500 could operate stably with such a coarse feed, it would provide a substantial safety margin for operation with fine slurry, a typical area of FASTFLOT deployment. The third objective was to test FASTFLOT support plants such as high pressure pump and jet water filtration systems on a long term basis. Presented at Minerals Engineering '96, Brisbane, Australia, August 26-28, 1996
357
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M.W. Chudaceket
al.
T H E FASTFLOT PROCESS The FASTFLOT process depicted in Figure 1, differs from known flotation devices in that high velocity thin clean liquid jets are used as energy and air carriers. In operation the high velocity thin jets (7) accelerate the surrounding air envelope and plunge between two layers of mineral slurry (5) fed into the diverging mixing nozzle (3) at much lower velocities. The shear rate, due to the velocity differential between these three streams, results in shredding of the gas envelope into very small bubbles and provides very intense agitation accompanied by a high turbulence. There are also high deceleration levels present in the mixing nozzle, as the thin jet with velocity typically about 60 m/s is decelerated to less than 3 m/s over a path of about 0.3 m. Further, the very high turbulence in the mixing zone creates high acceleration levels in small eddies. The FASTFLOT high velocity clean liquid jet technique has the advantage over other high intensity mixing techniques because there are no moving parts, such as the impeller of a mechanical flotation machine, in contact with the abrasive slurry. The wear rate of the mixing nozzle, which has a simple geometry, is very small because the high velocity jet core is isolated from the nozzle walls by the much slower moving layers of slurry. The functioning of the rest of the cell can be seen from Figure 1. A multiphase slurry flow is discharged from the mixing nozzle (3) into the cell bulk compartment where it flows past longitudinal stabilising baffles (8) and the flow velocity is attenuated by entrainment of bulk slurry. The resulting lower shear rate promotes air bubble coalescence leading to reduction of interfacial area of bubbles and hence to mineral concentration effects on the bubbles surface due to gangue rejection. These enlarged gas bubbles loaded with hydrophobic particles disengage from the stream, rise towards the liquid surface and are diverted by bubble guides (9) towards the froth discharge end of the cell. This arrangement promotes a uniform froth generation over the entire surface of the bulk compartment. The rising air bubbles induce an upward slurry flow which is diverted by the bubble guides towards the discharge end of the cell resulting in a defined surface flow capable of carrying even a very frail froth towards the froth weir. As the layer of froth travels towards the froth weir it drains liquid and entrained gangue mineral particles and thickens due to crowding against the weir. It is subsequently discharged by the rotary paddle (10) into the froth launder (11). The slurry flow at the discharge end of the cell is partly discharged via flexible ducts into the discharge launder (12) which is adjustable vertically to control cell liquid level. The rest of the slurry flow is diverted upwards by a curved bottom where in turn the slurry flow is diverted horizontally by a flow guide (13) towards the recycle gate (4) which controls the recycle flow (5).
Fig. 1 Schematic diagram of FASTFLOT design A cell cross section view. (1) Feed distributor; (2) Feedbox; (3) Mixing nozzle; (4) Recycle gate; (5) Recycled mineral slurry; (6) High pressure manifold containing injection nozzles; (7) High velocity thin jet; (8) Stabilising baffles; (9) Bubble diverting guides; (10) Rotary paddle; (11) Froth launder; (12) Discharge launder; (13) Flow guide.
Super-scavenging of zinc tailings
359
The operation of the cell is controlled as follows. Mixing intensity, shear rate and acceleration levels are controlled by the velocity of the thin jets (7) which is in turn controlled by the operating pressure in the manifold (6). Air entrainment rate increases with increased jet velocity, but may be also controlled by changing the magnitude of the recycle stream through vertical adjustment of the recycle gate (4). Froth height and froth flow are controlled by the liquid level in the cell. The liquid level in the cell is controlled by moving the flexibly connected discharge launder (12) up or down. The feeder box (2) in connection with the surface baffle assists in formation of the evenly distributed primary feed layer.
EXPERIMENTAL PLANT The host plant final tailings stream was removed via a 150 mm dia branch-off from a 300 mm dia pipe. The 150 mm branch-off was fed into a 100 nun dia pipe leading to a variable speed 80/50 slurry pump (No 1) which in turn fed a ca 3000 litre conditioning tank. The conditioned slurry was removed via a variable speed 80/50 slurry pump (No 2) which fed the FASTFLOT 2500 cell mounted on an open container. The open container housed also all necessary control equipment and switch gear as well as concentrate and tailings sumps and pumps. The whole plant was designed for unattended continuous operation with an automated shutdown procedure in the case of any unit failure. After commissioning and some revisions the whole plant operated to specification.
EXPERIMENTAL PROCEDURE Most of the experiments were conducted with ca 10 minutes conditioning. Copper sulphate was added to the feed pipe of slurry pump No 1 and Cytec 7016 was added into the conditioning tank. The FASTFLOT plant was sampled when it reached a steady state for the given experimental condition. The slurry feedrate was not controlled by feedback loop control, but set by variable frequency speed control of the slurry feed pump. The feedrate was reasonably steady most of the time. The decision not to control was deliberate as it would expose the plant to occasional fluctuations of the feedrate as in normal operating conditions. The liquid level was controlled via a feedback loop. This arrangement managed to control the liquid level within + 2 m m for + 16% variation of the flowrate. Hence variation of flowrate of this magnitude had virtually no effect on the quality of the concentrate. After the plant was operating at steady state for 15 - 20 minutes feed and concentrate samples were taken. Tailings were seldom sampled because of potential segregation in the tallings sump and also to minimise the number of samples to be analysed. Three samples of feed were taken; sample A before concentrate A collection, sample B after concentrate A collection and sample C after concentrate B collection. Hence the quality of feed over the entire period of concentrate sampling was monitored. Concentrate was collected in six dishes simultaneously sampling all six concentrate launders. Each dish sampled a 200 mm width of the particular launder giving a total sampling width of 1200 mm for a 2500 mm wide cell. Hence 48% of the cell output was sampled for a period of 15 - 60 seconds depending on the cell output. The duplicate samples A and B were found to be in reasonable agreement considering the industrial scale of the tests. The average of these samples is reported here. Concentrate sample A was related to the mean of feed samples A and B and concentrate sample B was related to the mean of feed samples B and C. The samples were dewatered, dried, split and analysed for Pb, Zn, Fe, Cu and Ag. The raw data were processed by a spreadsheet program. All key operating parameters were recorded continuously on a chart recorder. Peripheral parameters were recorded manually. R E S U L T S AND DISCUSSION Altogether 35 experiments were conducted with various feed rates ranging from 206 to 424 L/min, solids feed rates ranging from 78 to 149 kg/min and feed grades ranging from 0.27 to 1.01 % w/w Zn. Final tailings assaying less than 0.4 % w/w Zn were achieved by the main plant when it was operated at its very best. Tailings with residual Zn in 0.4 - 0.6% w/w range represents a typical operation and tailings above 0.6% w/w Zn suggest problems in the concentrator circuit accompanied by substantial losses of zinc to the tailings. The most interesting experiments are presented in Table 1.
21 22 23 24 27 28 29 30 31 32 33 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62
Exp No
s
L/min
400 4OO 424 419 315 303 3o4 304 4oo 403 403 206 311 307 3O6 299 302 299 3Ol 305 3o2 3o3 302 303 302 304 302 305 304 3O6 303 304 304 302 305
119.4 104.7 110.6 145.5 112.2 107.8 110.8 77.9 116.8 126.8 135.0 101.6 148.6 148.4 133.5 103.9 102.7 95,6 106.0 107.0 105.9 107.4 107.7 107.2 110.0 109.1 99,4 100.9 93.0 83.9 116.6 119.8 124.4 121.8 107.7
kg/min
Solids Rate
19120 24/26 24•26 18/20 18•20 16/18 15/17 15117 20/22 15/18 15/17 15/17 15/17 15/17 15/17 15/17 22/24 25/27 25/27 22/24 20/22 22124 17/20 17/19 18120 15/17 15/17 14/17 15/17 16/18 15/17 15/17 15/17 15/17
Liquid Level mm
9.03/8.75 9.03/8.99 8.92/9.03 9.00/8.99 8.80/8.90 9.00/8.96 8.87•8.97 8.65/8.86 8.69•8.90 8.76/8.61 8.46/8,60 8.45/8.44 8.46/8.49 8.36/8.40 8.22 8.49/8.46 8.45/8,27 8.36/8.33 8.54/8.50 8.54/8.52 8.54 8.53/8.47 8.49/8.48 8.54/8.37 8.45/8.55 8.56/8.50 8.51/8.55 8.54/8.50 8.62/8.58 8.80/8.80 8.58/8.59
pH
75 70 68 97 129 120 112 76 68 68 75 64 66 69 67 67 67 66 66 66 64 65 62 39 45 64 63 62 60 60 65
g/t
C u S O 4. 5H=O
11 13 11 11 12 11 11 11 11 11 11 11 11 10 7 7 10 11 10 10 10 11
12 12 12 16 18 17 16 11
g/t
Cytec 7016 0.37 0.42 0.40 0.32 0.34 0.40 0.49 0.44 0.61 0.51 0.38 0.68 0.94 1.01 0.91 0.48 0.46 0.49 0.32 0.36 0.38 0.43 0.82 1";.82 0.84 0.86 0.42 0.40 0.45 0.40 0.27 0.28 0.35 0.42 0.37
%Zn
Feed Grade 5.70 3.20 5.15 8.55 5.70 6.60 7,05 4.26 7.21 8.31 3.89 23.35 5.99 24.45 25.55 3.98 4.34 7.32 3.98 4.97 4.22 3,68 22.10 17.15 17.45 20.50 5.30 5.00 4.40 3.95 3.00 3.00 4.50 6.10 3.50
%Zn
Conc Grade 15.6 7.7 13.0 27.1 16.8 16.4 14.3 9.6 11,7 16.2 10.5 34,3 6.3 24.8 28.1 8.4 9.5 15.0 12.3 8.9 8.9 8.6 26.9 21.0 20.8 24.1 12.8 12.5 9,7 9.9 11.3 10.7 12.9 14.5 9.6
Enrich. Ratio
TABLE 1 FASTFLOT 2500 superscavenging of final Zn tailings at Pasminco Mining, Broken Hill
47 47 45 45 59 61 60 60 47 47 47 85 59 60 60 61 61 61 61 60 61 61 61 61 61 60 61 60 60 60 61 60 60 61 60
Res Time
Rate
Feed
6.7 12.2 10.2 5.1 18.7 17.4 20.1 16.4 15.7 17.5 16.1 54.1 2.3 57.3 57.4 19.3 18.6 14.8 7.8 13.6 11.0 11.1 44.9 45.0 47.5 46.7 30.2 31.2 25.5 32.6 15.3 14.9 14,4 14.0 31.7
Recov. %Zn
3.18 2.70 1.39 4.27 4.53 6.57 3.39 6.79 6.82 4.86 22.49 1.99 49.50 42.00 5.71 5.26 4.18 1.59 2.10 2.17 3.06 23.90 23.65 26.24 26.06 7.46 7.45 6.42 6.56 2.83 3.01 3,75 4.28 7.47
1.75
Recov. Zn kg/h
y~
e~
Super-scavengingof zinc tailings
361
Effects of Reconditioning The tailings stream had been in the zinc circuit for 25 minutes so it was likely that reconditioning may be required for effective recovery of fine particles which are known to be extremely sensitive to the chemical environment. FASTFLOT studies reported by Chudacek et al. [1] indicated improvement of recovery and grade with modest reconditioning on this stream. In the present work, substantial improvement of grade and recovery was achieved after reconditioning the final tail by ca 90 g/t CuSO4.5H20 and 15 g/t Cytec 7016 during preliminary batch cell studies. While the data in Table 1 do not permit direct comparison between many unconditioned and conditioned experiments as they were conducted mostly at different flowrates, Exp Nos 23 and 33 may be compared as they were both conducted at 400 L/min feed rate. Conditioned Exp No 33 with feed grade 0.38% w/w Zn compared to unconditioned Exp No 23 with feed grade 0.40% w/w Zn suggests a 71.5% improvement in Zn recovery at a 32% relative loss of grade. Comparing conditioned Exp No 62 at 300 L/min feed rate, to the higher feedrate (400 L/min) but unconditioned Exp No 21, with an identical feed grade of 0.37% w/w Zn we see 4.7 times improvement in recovery at 39% (relative) loss of grade. While part of the improvement is attributable to longer residence time in the cell during Exp No 62 compared with Exp No 21, not all improvements can be attributed to this factor alone. This would suggest that conditioning improves recovery. There is one interesting comparison to be made in the higher Zn feed grade series of experiments. Comparing Exp No 41 where both copper sulphate and Cytec 7016 promoter were added, with Exp No 40 where only copper sulphate was added, it appears that omission of the promoter resulted in a substantial loss of grade and recovery. This demonstrates how sensitive is the reagentisation of fine particles. Here it is interesting to note that during reagentisation of another Zn stream in the same plant, deliberate omission of promoter Cytec 7016 led to improved Zn recovery and grade. This confirms that both reagents must be in precise and specific balance for each task in hand. While higher dosages of copper sulphate, 97 - 129 g/t and higher Cytec 7016 dosages were used in Exp Nos 30 - 33, these did not seem to be effective. If we compare Exp No 29 with Exp No 32 at about the same feed Zn assays and pH regime, we see that Exp No 29 with less reagents dosage had a better recovery than Exp No 32. Lowering the reagents dosage to 39 - 45 g/t copper sulphate and 7 g/t of Cytec 7016 in Exp Nos 55 and 56 did not have much impact on recovery and grade in Exp No 55 but seemed to lower recovery, enrichment ratio and concentrate grade in Exp No 56. While overdosing of reagents is costly and counter productive, underdosing may depress recovery so 60 - 70 g/t of copper sulphate and 10 12 g/t Cytec 7016 was considered a reasonably safe dosage and used in all subsequent experiments.
Effect of pH It was the intention to keep pH at a value of about 9.00. However, the feedback loop controlling the pH was not operational at that time, therefore a manually adjusted dosing pump was used to control the pH, hence some drift of values in Exp Nos 30 - 33. Exp Nos 39 - 42 were deliberately conducted without NaOH addition at a pH of about 8.6. It is interesting that all but the already mentioned Exp No 40 showed dramatic improvement in grade and recovery. While these improvements can also be partly attributed to higher feed grades, as well as to the appropriate ratio of copper sulphate to promoter Cytec 7016, pH is likely to have a dominant effect. Exp Nos 43 - 49 at lower pH range 8.22 - 8.45, had lower recovery than experiments conducted at pH 8.6 and similar reagents dosage. Hence in subsequent experiments the pH was kept at 8.6 + 0.1, and all of these yielded good recovery and enrichment ratios. The optimal pH is very close to 8.6.
Setting of the froth/pulp interface The vertical distance of the froth/pulp interface from the discharge lip, referred to as liquid level, determines the quality of concentrate as this distance determines the froth depth and froth character in the cell. During experiments, this liquid level was controlled to better than 4- 2 mm. The objective of superscavenging was to maximise recovery so the liquid level was kept at 15/17 mm below the discharge lip for most of the experiments. A higher setting than this would have resulted in the partial discharge of
362
M.W. Chudacek et al.
pulp into the concentrate which is unacceptable practice for FASTFLOT operation. Lower settings than 15/17 were tried but these usually led to loss of recovery with no, or marginal, grade improvement. Effect of feed grade The effect of feed grade on percentage recovery, enrichment ratio and zinc yield can be seen from Figures 2 - 4. These Figures only contain experiments close to the optimal pH of 8.6. The recovery/feed grade plot in Figure 2 suggests that recovery improves from ca 15% at 0.3% w/w Zn feed grade to ca 55% at 1.0% w/w Zn feed grade. This behaviour was expected as a lower grade feed would have a higher proportion of locked in minerals which cannot be recovered and also a higher proportion of liberated ultrafine minerals which are difficult to recover. Feed with high grades was encountered when there were problems with regrind mills resulting in a coarser grind of Zn feed. The main plant historically has had difficulties in recovering marmatite particles over 120/~m size. Chudacek et al. [2], explained how these large marmatite particles are recovered by the FASTFLOT process. Hence it is anticipated that a high grade feed would produce a FASTFLOT concentrate with value metal being split between ultrafine and coarse fraction with relatively little value metal being present in intermediate fractions where the main plant is very effective. It is hoped this 'anomalous' distribution will be confirmed once the key experiments are analysed in detail.
60
•
J
50 A
~'
40
> O u g:
30
c N
20
10 0
' 0.2
' ' ' 0.4 0.6 0.8 Feed Grade Zn (%) w/w
' 1
1.2
Fig.2 Zinc recovery from super-scavenging of final plant tailings by the FASTFLOT 2500 vs tailings feed grade. Pasminco Mining, Broken Hill. The relationship between the enrichment ratio and the feed grade is given in Figure 3. The plot suggests an increase of enrichment ratio from ca 10 times at feed grade 0.25 % w/w Zn to around 27 times at feed grade range 0.8 - 1.0% w/w Zn. Experiment No 39 is responsible for the unusually high enrichment ratio of 34 times at a feed grade of 0.68% w/w Zn. It seems the plant was particularly well set for this experiment and that the chemical environment was just right. The reasons for this very high enrichment ratio will hopefully become clear during detailed analysis of the experiment. The increase of enrichment ratio with feed grade increase was anticipated. However if the feed grade is increased further, say to 2.0 %, a levelling of enrichment ratio is to be anticipated as the maximum concentrate grade from this type of feed is in the 42 - 46% w/w range at the very best. The relationship between plant zinc yield per hour per cell, and feed grade is given in Figure 4. The plot suggests a 20 times increase of the yield for a feedgrade increase from 0.25 to 1.0% w/w Zn. This trend was also anticipated, as with higher feed grade at the same grind, more value metal is available for extraction. Conversely, at lower feed grades, say below 0.25% w/w Zn, yield becomes insignificant and
Super-scavenging of zinc tailings
363
this projects into the economics of process.
40
o 30 IX
,o
"Flu 10
O
I
0.2
I
0.4
I
0.6 0.8 Feed Grade Zn ( % ) w/w
i
1.2
1
Fig.3 Zinc enrichment ratios for super-scavenging of final plant tailings by FASTFLOT 2500 vs tailings feed grade. Pasminco Mining, Broken Hill.
60 50m
~40 ,=~ ---
ID
-
30
u
>- 2 0 c N
10 0
I
0.2
0.4
I
I
0.6 0.8 Feed Grade Zn ( % ) w/w
I
1
1.2
Fig.4 Zinc yield rate from super-scavenging of final plant tailings by FASTFLOT 2500 vs tailings feed grade. Pasminco Mining, Broken Hill. Looking globally at all Figures 2 - 4, we notice two clusters of points. The significance of these two clusters are as follows. The lower cluster in the feed grade range 0.25 - 0.4% w/w signifies the main plant operating stably at its best. When the main plant is in this region, the Zn yield is a relatively small 2.5 8 kg/h/cell and the super-scavenging process is certainly uneconomical. The higher cluster of points signifies the main plant having operating difficulties such as a regrind problem, ore change, incorrect reagentisation etc. When the main plant exhibited operating difficulties, recoveries were significant, in the range of 22 - 49.5 kg/h/cell. Under such conditions, super-scavenging by FASTFLOT would highly likely be economical. Interestingly, the FASTFLOT plant operated very stably during the above main plant upsets
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M.W. Chudacek et al.
and with standard conditioning simply 'mopped up' most residual zinc which the main plant left behind due to its upsets. This is quite an achievement if we consider that the stream has spent ca 25 minutes in the main plant zinc circuit and only 1 minute in a single stage FASTFLOT plant. Hence we can say that FASTFLOT acted as a safety screen containing zinc losses due to main plant upsets.
Zinc yield projection for full size plant The data described above shows substantial and nonlinear Zn yield variation with change of feed grade. To obtain data for future economic analysis of the project it was necessary to estimate the Zn yield of a full size plant for a typical and representative period of main plant operation. Two periods of one calendar month duration, were selected on the advice of our host plant management for such analysis. Hourly averages of the feedrate, Zn and Pb assays for all three milling sections of the plant were taken, as well as Zn assays of the tailings. From respective hourly averages of Pb and Zn recovery and their grades, a solids rate of the tailings was calculated. Using the recovery vs feed grade plot in Figure 1, the Zn yield was calculated hour by hour and summarised to yield a monthly figure. This approach was necessary to contain the impact of frequent fluctuation of plant performance reflecting itself in fluctuation of the tailings Zn assay. The detailed analysis for the month of March 1996 yielded full plant availability 86.1% and a yield of 516.2 tonnes per month (t/m) of pure Zn equivalent. Similar analysis for the month of May 1996 yielded full plant availability of 83.5% and a yield of 511.4 t/m of pure Zn equivalent. The agreement of the figures suggest that the above methodology was reliable enough to obtain approximate plant availability and Zn yield of a full scale plant for economic analysis of the project.
Scale-up relationship The usual scale-up relationship from our 250 mm wide pilot plant FASTFLOT 250 data is based on the width ratio, as the cross section of FASTFLOT cells is the same at all scales. This means the output factor is 10 times for FASTFLOT 2500 (2500 mm wide) and 12 times for FASTFLOT 3000 (3000 mm wide). However, the wall effects which retard the movement of the froth bed in the cell are much more pronounced on the 250 mm scale than on larger scales. This provides a safe scale-up margin. Projected Zn yield rate figures for FASTFLOT 2500 based on the FASTFLOT 250 x 4 pilot plant study reported by Chudacek et al. [1] for 0.43 % w/w Zn feed were in the range of 3.6 - 3.9 kg Zn/h/cell, while the actual FASTFLOT 2500 for the same feed grade at optimum pH suggests yield rates of 6.5 - 7.5 kg Zn/h/cell. For a broader pH range of 8.35 - 9.00 and a feed grade in the range of 0.40 - 0.45 % w/w Zn, the Zn yield rate was in the 3.1 - 7.5 kg/h/cell range. The lowest value for the experiment at pH 8.35 is clearly suboptimal. Also, the experiment at pH 9.0 yield rate was only 3.9 kg Zn/h/cell confirming the necessity of operating at optimal pH. While there have been numerous improvements to both the FASTFLOT 250 x 4 pilot plant and the FASTFLOT 2500 plant and the chemistry has been optimised since the original study, it seems that full plant prediction based on pilot plant data was reliable. The improvement on the original prediction from pilot plant data can be attributed to the following factors in descending order of significance.
Reagents regime optimisation.
1.
2.
.
Automatic liquid level control on the FASTFLOT 2500 compared to manual control on the FASTFLOT 250. Use of more advanced jet nozzles when compared to the original study.
4.
Reduced effect of the side wall for the larger scale.
5.
Better understanding of the process and better operating practice.
Super-scavengingof zinc tailings
365
FASTFLOT plant performance While some problems were encountered with plant feed removal from the host plant, the FASTFLOT 2500 performed without major hiccups from the beginning of testing. Minor shortcomings of the prototype such as the feed distributor were rectified. Also, a screening system using a polyurethane screen with 4 mm opening had to be introduced to remove large particles from the tailings stream, as well as wood splinters or plastic from blasting cartridges which would otherwise block the mouth of the mixing nozzle and hence disrupt the process. After a few modifications of the feeding trough with screen, feed distributor and washing nozzle manifold, reliable continuous operation could be sustained. Operators were only required to clean the screen of the entrained debris by occasional brushing. This operation did not take longer than 2 - 3 minutes. Accumulated debris on the screen was automatically dumped every 10 -35 minutes depending on the debris load. Simultaneously, with the opening of the dumping gate, a series of process water jets were actuated to assist in quick debris discharge to the dump trough. Thin water jets acting as energy and air carriers became coherent and stable after the initial problems with the filtration of jet water were overcome. In later stages of the study when a prototype of an automatic sand filter was commissioned, the FASTFLOT cell ran without visible deterioration of jet quality and blockage of the final chance filters protecting jet nozzles for many days until tailing hose failure precipitated automatic shutdown of the plant. Most remarkable was the stability of the liquid level control which held the liquid level to + 2 mm for a 16% change in the feed rate. This control is certainly more than adequate. When operated with the normal plant grind of d80 160 #m, cell discharge tubes seldom blocked due to sanding up. The blockage with this feed was never as severe as to actuate the automatic backflushing procedure. However, when the main plant re-grind was off and feed ds0 240/~m, discharge tubes sanded more frequently and the backflushing system was occasionally actuated. After some design modification of backflushing injection ports, the cell routinely recovered from sanded tubes automatically, without any operator intervention. It is important to note here that the FASTFLOT application range is for fine particle flotation and hence for grinds typically d80 < 75 #m. During our pilot plant studies on FASTFLOT 250 we never experienced any sanding with feed of d80 < 75 #m so sanding problems are not anticipated for the FASTFLOT 2500. However, even these finer streams encounter occasional coarse particles, hence a new design of FASTFLOT B-2 was developed featuring bottom discharge of the pulp via a series of flap valves as outlined in Figure 5. This new design has been successfully tested and
~ " "
? " - :." "." ," "÷'C ".'7 ~ )
~
~
Fig.5 Schematic diagram of FASTFLOT B-2 cell. (1) Feed manifold (2) Feed trough (3) Screen (4) Debris dump gate (5) Debris trough (6) Primary feed (7) High pressure manifold with injection nozzles (8) Thin high velocity jets (9) Recycle flow gate (10) Mixing nozzle (11) Stabilising baffles (12) Tailings flap valve 13) Flow deflector (14) Froth bed (15) Rotary paddle (16) Concentrate
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M.W. Chudaceket al.
retains all the metallurgical advantages of former Design A, including excellent liquid level control but is more robust and also cheaper to manufacture. Air sparging and backflushing of discharge tubes were disposed of, hence cell operating costs are lower.
CONCLUSIONS 1.
2. 3. 4. 5. 6. 7. 8.
.
10.
The prototype FASTFLOT 2500 cell was successfully tested on a very difficult stream with particle size distribution well outside its original specification. All shortcomings of the original design have been overcome and experience gained during long term tests have led to a more robust and cheaper to manufacture FASTFLOT B-2 cell design. A major feature of plant performance was its stability and particularly its liquid level control which was better than + 2 nun. The scale-up factor for pilot plant data was found to be conservative, giving a reasonable safety margin for economic prediction of a full scale plant. Modest reconditioning of the main plant tail with copper sulphate and reagent Cytec 7016 led to substantial improvement of zinc recovery at about the same or better grade. Operating at pH 8.6 _ 0.1 led to better recoveries than operating outside this range, pointing to extreme sensitivity of fine particles to the chemical environment. Recovery, enrichment ratio and zinc yield per cell increased with increasing Zn grade of main plant tailings, as expected. When the main plant exhibited operational difficulties FASTFLOT zinc yield was 20 times the yield when the main plant operated at its very best. This suggests that FASTFLOT, besides its super-scavenging duty, can be used as a 'safety net' to contain value metal losses during main plant instability. Considering that the stream had ca 25 minutes residence time in the main plant Zn circuit, the recoveries and grades achieved during a mere one minute residence time in a single FASTFLOT cell are certainly remarkable by any standard. The technique used for determining average monthly Zn yield and full plant availability was found to be adequate for economic analysis of the project.
ACKNOWLEDGEMENT Authors indebted to Pasminco Mining and MD Research for their courageous support of the difficult and extensive study.
REFERENCES 1.
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Chudacek, M.W., Marshall, S.H., Estrina, H. & Fichera, M.A., FASTFLOT process - a unique tool for effective recycling. Filtration and Separation, 32(4), 293-303 (1995). Chudacek, M.W., Estrina, H, Marshall, S.H., Fichera, M.A, Burgess, J. & Burgess, F.L., Super-scavenging of zinc residue by the FASTFLOT process at Pasminco Mining, Broken Hill. A u s l M M Annual Conference, 207-211 (1995).