Study of the behavior of iron ore particles in spiral
concentrators
Mémoire
Maryam Sadeghi
Maîtrise en génie des mines- génie mines et minéralurgie
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Résumé
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Abstract
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Table of Contents
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3-7 Estimation of assays of size fractions in the streams of the spiral ... 22
3-8 Solution to the data reconciliation problem ... 23
3-9 Evaluation of the data reconciliation results ... 24
3-10 Evaluation of spiral performance indices from the reconciled data ... 24
3-11 Typical partition curves and spirals partition curves ... 25
4 Chapter 4: Effects of operating conditions on the performance of a WW-6E spiral ... 29
4-1 Test set-up ... 29
4-2 Test conditions and the 23 factorial design ... 31
4-3 Results analysis ... 32
4-4 Partition curves for iron and silica carriers ... 34
4-5 Factors influencing the Fe2O3 content and recovery to the concentrate ... 37
4-6 Grade-recovery curve of Fe2O3 carriers ... 38
4-7 Discussion ... 38
5 Chapter 5: Preliminary investigation on the effect of the number of turns on spirals performance ... 41
5-1 Introduction ... 41
5-2 Description of the spiral test rig ... 42
5-2-1 Test rig ... 42
5-2-2 Validation of the feed distributor ... 43
5-2-3 Wash water addition ... 46
5-2-4 Concentrate cutters ... 48
5-2-5 Dewatering hydro cyclones ... 48
5-2-6 Sampling the divided slurry flow at the last turn of the spirals ... 49
5-2-7 Main sampling system ... 49
5-3 Experimental procedure ... 50
5-3-1 The ore and the tests conditions ... 50
5-3-2 Sampling procedure ... 51
5-4 Data reconciliation ... 52
5-5 Reproducibility of the tests and feed characteristics ... 54
5-5-1 Slurry solids concentration ... 54
5-5-2 Slurry mass flow rate of the feed ... 55
5-5-3 Wash water flow rate ... 55
5-5-4 Chemical composition ... 55 viii
5-5-5 Size distribution ... 57
5-5-6 Size distribution of the chemical species ... 57
5-5-7 Summary ... 59
5-6 Effect of the number of turns on global performance indices ... 59
5-6-1 Reproducibility of the tests ... 59
5-6-2 Number of turns and spiral concentrate grade ... 60
5-6-3 Number of turns and mass split to the concentrate ... 61
5-6-4 Slurry solids concentration and flow rate in the concentrate stream ... 62
5-6-5 Number of turns and Fe2O3 carriers recovery ... 63
5-6-6 Number of turns and Fe2O3 grade-recovery curves ... 65
5-6-7 Number of turns and spiral selectivity ... 65
5-6-8 Multivariable analysis of the test results ... 66
5-6-9 Summary ... 68
5-7 Effects of the number of turns on the species partition curves ... 69
5-7-1 Effects of the number of turns on partition curves ... 69
5-7-2 Possible use of partition curves to associate the species with minerals ... 73
5-8 Particles stratification along the trough of the spirals ... 74
5-8-1 Solids characteristics within the reject flows ... 74
5-8-3 Distribution to the reject flows of the spiral discharge ... 82
5-9 Discussion ... 85
6 Chapter 6: Conclusion ... 87
Bibliography ... 91
Appendix ... 93
A-1 Method of standard deviation estimation for the measurements in a data reconciliation procedure 93 A-2 Measurements and the reconciled values from test 9 ... 94
A-3 Data related to the Figures presented in Chapter 5 ... 95
List of Tables
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Table 5-17: Independent and dependent variables used for the analysis (reconciled values) ... 67
Table A- 0-1: Measurements and reconciled values from test 9 conducted on the WW6E rougher spiral ... 94
Table A- 0-2: Spirals characteristics ... 95
Table A- 0-3: Hydro cyclones characteristics ... 95
Table A- 0-4: Fe2O3 carriers partition factors to the concentrate, tests 7 to 12 ... 95
Table A- 0-5: SiO2 carriers partition factors to the concentrate, tests 7 to 12 ... 95
Table A- 0-6: Solids split to the reject flows, tests 5 to 10 ... 96
Table A- 0-7: Solids concentration in the reject flows, tests 5 to 10 ... 96
Table A- 0-8: Volumetric flow rate of the reject flows, tests 5 to 10 ... 96
Table A- 0-9: SiO2 content within the reject flows, test 5 to 10 ... 96
Table A- 0-10: Fe2O3 content within the reject flows, test 5 to 10 ... 97
Table A- 0-11: Average size distribution of solids within the output flows, tests 5 to 10 ... 97
Table A- 0-12: Average size distribution of Fe2O3 carriers within the output flows, tests 7, 8 and 9 ... 97
Table A- 0-13: Average size distribution of SiO2 carriers within the output flows, tests 7, 8 and 9 ... 98
Table A- 0-14: SiO2 carriers distribution within the reject flows, tests 5 to 10 ... 98
Table A- 0-15: Fe2O3 carriers distribution within the reject flows, tests 5 to 10 ... 98
Table A- 0-16: Average partition factors of SiO2 carriers from the feed to the concentrate and the 4 reject flows, tests 7, 8 and 9 ... 99
Table A- 0-17: Average partition factors of Fe2O3 carriers from the feed to the concentrate and the 4 reject flows, tests 7, 8 and 9 ... 99
List of Figures
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Figure 5-6: Wash water distribution scheme along the channel of the spiral, spigots and cutters ... 47
Figure 5-7: Wash water flow rate via the spigots; total flow rate 16.6 L/min (spigots in the middle position) ... 48
Figure 5-8: Automatic sampler, 24 samples at the same time ... 50
Figure 5-9: Sampling configuration ... 50
Figure 5-10: Spirals network for the data reconciliation ... 52
Figure 5-11: Average SiO2 content in the feed of the three spirals, tests 5 to 12 ... 57
Figure 5-12: Average Fe2O3 content in the feed of the three spirals, tests 5 to 12 ... 57
Figure 5-13: Fe2O3 distribution within the size intervals, tests 9 to 12 ... 58
Figure 5-14: Concentrate grade as a function of the feed grade, for the three spirals, tests 5 to 12 ... 61
Figure 5-15: Upgrading ratio of the three spirals, tests 5 to 12 ... 61
Figure 5-16: Variation of the mass split as a function of the Fe2O3 content in the feed ... 62
Figure 5-17: Mass split as a function of the upgrading ratio ... 62
Figure 5-18: Solids concentration in the concentrate stream in the 3 spirals, tests 5 to 12 ... 63
Figure 5-19: Slurry flow rate to the concentrate stream in the three spirals, tests 5 to 12 ... 63
Figure 5-20: Recovery of the Fe2O3 to the concentrate as a function of the Fe2O3 content in the feed ... 63
Figure 5-21: Fe2O3recovery as a function of the upgrading ratio ... 64
Figure 5-22: Grade-recovery curve for the three spirals ... 65
Figure 5-23: Fe2O3 recovery versus SiO2 recovery (selectivity curves) for the three spirals ... 66
Figure 5-24: Selectivity factors as a function of the number of turns ... 66
Figure 5-25: Partition curves of Fe2O3 and SiO2 carriers for tests 7 and 8 (operating conditions: 35%solids, 2.2 t/h solids flow rate) ... 69
Figure 5-26: Partition curves of Fe2O3 and SiO2 carriers in tests 9 and 10 (28 L/min wash water flow rate) ... 70
Figure 5-27: Partition curves of Fe2O3 and SiO2 carriers for tests 11 and 12 (operation conditions: 37%solids, 2.6 t/h solids flow rate) ... 71
Figure 5-28: Fe2O3 partition curves obtained from the 3-turn-spiral, tests 7 to 12 ... 72
Figure 5-29: SiO2 partition curves obtained from the 3-turn-spiral, tests 7 to 12 ... 72
Figure 5-30Figure 5-30: Partition curves obtained from the 5-turn-spiral, tests 7 to 12 ... 72 xiv
Figure 5-31: Partition curves obtained from the 7-turn-spiral, tests 7 to 12 ... 73
Figure 5-32: Similar partition curves for species that may belong to the same mineral ... 73
Figure 5-33: Spiral channel division at discharge and the rejects numbering ... 74
Figure 5-34: Average solids split to the reject flows, tests 5 to 10 ... 75
Figure 5-35: Average slurry solids concentration in the reject flows, tests 5 to 10 ... 75
Figure 5-36: Average volumetric flow rate in the reject flows, tests 5 to 10 ... 75
Figure 5-37: Average chemical composition of the solids in the reject flows at the spirals discharge ... 76
Figure 5-38: Size distribution of the solids in the reject flows of the 3-turn-spiral ... 77
Figure 5-39: Size distribution of the solids in the reject flows of the 5-turn-spiral ... 77
Figure 5-40: Size distribution of the solids in the reject flows of the 7-turn-spiral ... 77
Figure 5-41: D80 of the solids within the reject flows of the three spirals, tests 5 to 10 ... 78
Figure 5-42: Size distribution of solids -0.106 mm in the reject flows of the three spirals, tests 5 to 10 ... 78
Figure 5-43: Average size distribution of Fe2O3 carriers in the output flows of the 3-turn-spiral ... 79
Figure 5-44: Average size distribution of SiO2 carriers in the output flows of the 3-turn-spiral ... 79
Figure 5-45: Average size distribution of Fe2O3 carriers in the output flows of the 5-turn-spiral ... 79
Figure 5-46: Average size distribution of SiO2 carriers in the output flows of the 5-turn-spiral ... 79
Figure 5-47: Average size distribution of Fe2O3 carriers in the output flows of the 7-turn-spiral ... 79
Figure 5-48: Average size distribution of SiO2 carriers in the output flows of the 7-turn-spiral ... 79
Figure 5-49: Size distribution of solids and SiO2 and Fe2O3 carriers in the concentrate ... 80
Figure 5-50: Size distribution of solids and SiO2 and Fe2O3 carriers in reject 1 (R1) ... 80
Figure 5-51: Size distribution of solids and SiO2 and Fe2O3 carriers in reject 2 (R2) ... 80
Figure 5-52: Size distribution of solids and SiO2 and Fe2O3 carriers in reject 3 (R3) ... 81
Figure 5-53: Size distribution of solids and SiO2 and Fe2O3 carriers in reject 4 (R4) ... 81
Figure 5-54: Average SiO2 and Fe2O3 distribution to the reject flows, tests 5 to 10 ... 82
Figure 5-55: SiO2 distribution to the reject flows in the three spirals, average of tests 7, 8 and 9 ... 83
Figure 5-56: Fe2O3 distribution to the reject flows in the three spirals, average of tests 7, 8 and 9 ... 84
Acknowledgements
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1 Chapter 1: Introduction
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1989) but also in the coarse size fraction (larger than 0.25 mm). The complex behavior of gangue minerals as a function of their size is also highlighted in the memoir.
Particle classification in spirals depends also on the spiral geometry and operating variables. Spiral geometry is mainly defined by diameter of the channel and the number of turns (Bouchard, 2001). The main operating variables are the feed rate, slurry solids concentration, wash water addition and cutters position (Wills and Napier-Munn, 2006). Dallaire et al. (1978) examined the effect of feed rate and slurry solids concentration on a Humphrey spiral for iron ore. Their results indicate that increasing the slurry solids concentration is beneficial for the recovery of valuable minerals but deteriorates the concentrate grade. Wash water addition is an important variable that washes away light minerals from the concentrate band. It may also enhance the recovery of valuable minerals probably by breaking some build-up barrier of gangue particles in the inner part of the spiral trough (Ramotsabi et al., 2015). Significant benefits, however, were reported after the removal of wash water from spirals treating light minerals (Burt and Mills, 1984; Richards and Palmer, 1997; Bouchard, 2001), while higher concentration grades of heavy minerals are obtained at higher wash water flow rates (Sadeghi et al., 2014). Although wash water addition is recognized as a strategic operating variable for spirals, it was not possible to find any detailed analysis of the action of this variable in the literature. Results of a test work conducted on a pilot spiral show that wash water addition mainly influences the recovery of coarse particles. These results are published (Sadeghi et al., 2014) and are explained in details in Chapter 4.
Work is also carried out in this memoir to study the effect of the number of turns on spirals performance. Intuitively it is expected that recovery of the valuable minerals increases with the number of turns, as the residence time of the slurry in the spiral increases providing more opportunities for particles to be collected by the cutters. However, this was not observed from the conducted test work using a test rig consisting of three parallel spirals with 3, 5 and 7 turns, as the results show a maximum recovery from the 3-turn-spiral. Each spiral of the test rig is equipped with a splitter installed at discharge of the last turn. The splitter divides the slurry flow into 6 streams. The streams can be individually sampled to provide a further understanding of particles stratification along the spiral trough. These results are described in Chapter 5.
The project was financially supported by COREM that allows the test work on the pilot plant spirals and also provides the technical assistance to operate the equipments. Chemical assays of the collected samples were performed by COREM personnel. The ore used for the test work is provided by the Mount Wright mine of ArcelorMittal in Quebec.
All measurements collected during the sampling campaigns are reconciled using BILMAT (Hodouin and Everell, 1980). The reconciled data are used to assess performance indices and mineral partition curves. The data reconciliation algorithm is programmed in Ms. Excel. This algorithm is prepared following a demand from COREM to be able to balance the measurements collected through the sampling campaigns and to 2
characterize the optimal operation of the equipment. A special iterative method is used to ensure that the constraints of consistency for the size distributions, assays of size fractions and head assays are verified as well as the mass conservation constraints. The algorithm is described in chapter 3. The use of reconciled data facilitates the estimation of performance indices, particularly partition curves. Partition curves provide an excellent tool to study the effect of operating and design variables.
The memoir is divided into 6 chapters including the introduction and the conclusion. Chapter 2 presents a literature review of the design and operation of spirals. Chapter 3 describes the algorithm used for the data reconciliation and the calculation of the species partition curves. Chapter 4 reports the results of the investigation on the effect of wash water flow rate, feed rate and slurry solids concentration on the performance of a commercial spiral. Chapter 5 studies the effect of number of turns on spirals performance as well as the species classification along the spiral trough.
2
Chapter 2: Literature review
2-1 Spiral concentrators
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2-2 Spiral geometry
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2-2-1Profile of the spiral trough
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2-2-2Spiral diameter
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2-2-3Pitch
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2-2-4Cutters
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2-2-5Number of turns
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2-3 Slurry flow in the spirals
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• The outer zone, where dense particles move into the lower layers to return to the inner zone by the secondary flow as the concentrate.
The spiral feed flow rate directly affects the secondary flow and could contribute to a loss of classification capacity (Sivamohan and Forssberg, 1985; Holland-Batt, 1990; Holtham, 1990).
Figure 2-5: Flow behavior in a cross-section of a spiral channel (Richards and Palmer, 1997)
Particles classification results from a balance between a combined action of gravity, centrifugal, buoyancy, drag, and Bagnold forces (Atasoy and Spottiswood, 1995). Except the gravity force, all other forces depend on spiral geometry and operating variables. For instance, the circular movement of the slurry through the turns induces the centrifugal force which makes the light particles migrate outward (Bouchard, 2001). Friction force plays a role in low feed rate situations since in the laminar flow solids deposit quickly that leads to an inefficient separation of particles (Holland-Batt, 1995a). Bagnold effect occurs in high solids concentration and has an important effect on the classification of coarse particles in the vicinity of the inner part of the spiral (Matthews et al., 1998). The Bagnold effect is discussed in the following paragraph. Figure 2-6 shows a sketch of forces acting on the particles passing through the spiral. The combined action of the forces pull the heavy (coarse/dense) particles toward the inner part of the trough, while the lighter and finer ones migrate to the outer part of the trough (Wills and Napier-Munn, 2006). Figure 2-7 shows a spiral concentrating an iron ore. The large and heavy iron carrier particles form the gray area. Lighter and finer particles, mostly silica carriers, are transported to the outer part of the trough with the water.
Several authors (Holtham, 1992; Atasoy and Spottiswood, 1995; Bouchard, 2001; Mishra and Tripathy, 2010) tried to mathematically describe the effect of the Bagnold force in a spiral. The amplitude of the force depends on the slurry density which varies along the trough. Bagnold (1954) estimates that the Bagnold force for the particles finer than 0.2 mm is negligible. Bouchard (2001) used the shearing stress produced by the slurry to describe the force in a very simplistic way, although the explanation remains unclear on how the Bagnold force is generated. In a spiral the Bagnold force drives coarse particles to the outer part of the spiral trough while in 10
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2-4 Operating variables
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2-4-4Feed characteristics
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2-5 Performance indices used to assess spirals operation
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2-6 Summary
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Variable Typical value Variables to be considered for the selection
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3 Chapter 3: Data reconciliation
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3-1 Mass conservation network for the spiral
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3-2 Mass conservation and coherency constraints
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𝐷�𝐹 = 𝐷�𝑅+ 𝐷�𝐶 Eq.3-1 The variable D stands for a solids flow rate while the subscripts F, R and C indicate the feed, reject and concentrate streams of the spiral. The ^ indicates an estimate of the variable that verifies the constraint. The variable without the hat is used for the measurements. The slurry mass conservation equation is written as:
𝐷�𝐹 𝑆̂𝐹+ 𝑄�𝑊= 𝐷�𝑅 𝑆̂𝑅 + 𝐷�𝐶 𝑆̂𝐶 Eq.3-2 The variable S stands for the solids concentration of the slurry and QW for the flow rate of wash water. Mass
and volumetric flow rates of the slurry in any stream j of the spiral are given by:
𝑊�𝑗 =𝐷�𝑆̂𝑗𝑗 Eq.3-3 𝑄�𝑗= 𝐷�𝑗�𝑆𝐺�1𝑗+1−𝑆̂𝑆̂𝑗𝑗� Eq.3-4 Where W denotes the mass flow rate and Q the volumetric flow rate of the slurry in stream “j”; the specific gravity of the solids in stream “j” is noted as SGj. The specific gravity of wash water is assumed as 1 g/cm3.
The mass conservation of any chemical species “i” is written as:
𝐷�𝐹𝑋�𝑖;𝐹 = 𝐷�𝑅𝑋�𝑖;𝑅+ 𝐷�𝐶𝑋�𝑖;𝐶 𝐹𝑜𝑟 𝑖 = 1, 2, … 𝑁𝑒 Eq.3-5 The concentration of species “i” in a stream “j” (j stands for the streams of F, R and C) is noted 𝑋𝑖;𝑗 and the number of chemical species in the samples is noted Ne. The uppercase “X”, i.e. 𝑋𝑖;𝑗 is used to indicate the
head assays. Similarly the mass conservation of the solids within a size interval “r” is written:
𝐷�𝐹𝑦�𝑟;𝐹 = 𝐷�𝑅𝑦�𝑟;𝑅+ 𝐷�𝐶𝑦�𝑟;𝐶 𝐹𝑜𝑟 𝑟 = 1, … 𝑁𝑠 Eq.3-6 The weight retained within the size interval “r” in the stream “j” is noted 𝑦𝑟;𝑗 and the number of size intervals is noted Ns. The retained mass fractions of any stream should sum up to 1.0, i.e.:
1.0 = ∑𝑁𝑟=1𝑠 𝑦�𝑟;𝑗 𝐹𝑜𝑟 𝑗 = 𝐹, 𝑅 𝑎𝑛𝑑 𝐶 Eq.3-7 The mass conservation of a chemical species “i” within the size interval “r” is given by:
𝐷�𝐹𝑦�𝑟;𝐹𝑥�𝑖;𝑟;𝐹 = 𝐷�𝑅𝑦�𝑟;𝑅𝑥�𝑖;𝑟;𝑅+ 𝐷�𝐶𝑦�𝑟;𝐶𝑥�𝑖;𝑟;𝐶 𝐹𝑜𝑟 𝑟 = 1, … 𝑁𝑠 𝑎𝑛𝑑 𝑖 = 1, 2, … 𝑁𝑒 Eq.3-8
The variable 𝑥𝑖;𝑟;𝑗 stands for the concentration of chemical species “i” within the size interval “r” in branch “j”. The chemical assays within the size intervals should be consistent with the head assays, i.e.:
𝑋�𝑖;𝑗 = ∑𝑁𝑟=1𝑠 𝑦�𝑟;𝑗𝑥�𝑖;𝑟;𝑗 𝐹𝑜𝑟 𝑖 = 1, 2, … 𝑁𝑒, 𝑟 = 1, … 𝑁𝑠 𝑎𝑛𝑑 𝑗 = 𝐹, 𝑅 𝑎𝑛𝑑 𝐶 Eq.3-9
3-3 Data reconciliation criterion
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