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June 22th, 2017

WesTechEco²Tails: Ecological and EconomicalMethod to Improve Recovery of Water, Rich Solution and Safe Tails

Author : Jerold Johnson

Company : WESTECH ENGINEERING, INC.- SALT LAKE CITY UTAH USA

Email address: JJOHNSON@westech-inc.com

Summary

The purpose of this paper is to provide a comparison of CCD circuits using paste type thickeners compared to those using high rate type thickeners. The ability of the paste type thickener to produce significantly higher underflow density compared to a high rate thickener has profound benefits for CCD circuits. This higher density increases the efficiency of each CCD stagewhich will have the following effects:.

Reduce the wash water needed, and/or reduce the number of stages needed, Depending on the selected design, paste thickeners produce higher

concentration pregnant liquor. Paste type thickeners when used as the final stage thickener will recover more

water and send minimum amount of water to the tailings pond. Little or no free

water on the tailings, which permits the impoundment to have the many benefits of a surface stack.

Paste type thickeners when used as the final stage thickener will recover more water and send minimum amount of water to the tailings pond. Little or no free water on the tailings, which permits the impoundment to have the many benefits of a surface stack.

This paper presents a case study of a Gold Mine in México operating a CCD conventional circuit.

The gold plant in Mexico is operating with a 5 stage high rate thickener CCD circuit. The process is used as a Case Study.

WesTech Engineering, Inc. performed bench-scale thickener tests on-site at the plant. The test results provided sizing criteria for high rate type thickeners and paste type thickeners.

The study presents results for the following issues:

Optimum feed solids, flocculant dosage and mixing feed slurry flocculation and settling rate.

High rate thickener unit area sizing, underflow wt%

Rheology study to base paste underflow prediction upon.

CCD Circuit comparison

Process Predictions

Process Observations

High level CAPEX and OPEX review CCD options

Relative cost comparisons between paste thickeners and high rate thickeners for CCD circuits

BackgroundThe purpose of this paper is to provide a comparison of CCD circuits using paste type thickeners compared to those using high rate type thickeners. The ability of the paste type thickener to produce significantly higher underflow density compared to a high rate thickener has profound benefits for CCD circuits.

CCD circuits recovers or washes liquor value from the solids or tailings. The general process in depicted in the figure below as a four stage CCD circuit. The feed stream

enters the system and is blended with the stage 2 overflow (wash water stream). This combined stream is then thickened in the first stage thickener. The underflow from stage 1 is first combined with stage 3 overflow (wash water stream) and then thickened in the stage 2 thickener. The process is repeated for each stage. The final stage feed is the underflow from the previous stage combined with the fresh wash water. This CCD method is effective due to features like:

The last stage feed is blended with the fresh wash water providing maximum washing to achieve the low discharge concentration.

Each stage’s efficiency is a function of the underflow pulp density. As the underflow density increases the liquor forwarded to the next stage in the underflow is decreased.

Once a thickener type (defining the underflow density) is selected then the number of stages is balanced with the amount of wash water for the final CCD design. Fewer stages require more wash water to have the same recovery.

The significant advantage of high density or paste thickeners in CCD circuits is that these thickeners produce higher underflow density than other types of thickeners. This higher density increases the efficiency of each CCD stage.

The higher underflow density improves the efficiency of each stage which will have the following effects:

Reduce the wash water needed, and/orreduce the number of stages needed, Depending on the selected design, paste thickeners produce higher

concentration pregnant liquor. Paste type thickeners when used as the final stage thickener will recover more

water and send minimum amount of water to the tailings pond. Little or no free water on the tailings, which permits theimpoundmentto have the many benefits of a surface stack.

Figure 1 – Typical four stage CCD circuit

Case StudyThe goldplant in Mexico is operating with a 5 stage high rate thickener CCD circuit. The process is used as a Case Study. The established operating process is found in the following table.

Table 1 – Current 5 stage CCD at

Feed Stream Value UnitsDesign dry solids 365 Metric tphFeed wt% solids 25 – 35 Wt%Solids density 2.7 g/mlLiquor density 1.0 g/mlCCD stages 5 NumberThickener dia. 45 MeterThickener sizing 0.181 UA, m2/mtpdUnderflow wt% solids

~55 Wt%

Wash water ratio 2.07 – 2.71 Kg water / kg solidsReported wash water flow rate

752 m3/h

Sizing Test SummaryWesTech Engineering, Inc. performed bench-scale thickener tests on-site at the plant. The test results provided sizing criteria for high rate type thickeners and paste type thickeners. The study results are summarized as follows:

Optimum feed solids, flocculant dosage and mixing feed slurry flocculation and settling rate.

Optimum solid concentration for flocculation = 10 – 15 wt% Flocculant dosage = 35 – 50 g/t Settling rate = 20 – 28 m/h

High rate thickener unit area sizing, underflow wt%Table 2 – Sedimentation Results (used for high rate type thickeners)

Test High ratePlant flocculant, g/t ~50 Optimum solid concentration - wt% 15

Rise Rate:Observed Settling Rate – m/hDesign Rise Rate – m/h

20 – 28 10 – 14

Unit Area: Design Unit Area – m2/TPD At UF solid conc. – wt.%

0.1 – 0.1255 (53 –

56)

Rheology study to base paste underflow prediction upon.

The common non-Newtonian underflow target is in the range of 30 to about 80 Pa. For the material tested at this correlates to an underflow solids of 61 to approximately 63 wt%.

The solids residence time to achieve this type of underflow is approximately 4 – 8 times longer than the normal solids residence time in a high rate thickener. Simply put, it takes more time to dewater to these higher wt% solids.

Figure 2 – Yield stress results for sample

CCD Circuit comparisonThe current plant CCD circuit is a five stage circuit. In this section of the paper the current plant CCD is compared to the recommended WesTech options:

Option 0 – Current CCD circuit performance

Option 1 – WesTech sized high rate thickeners CCD

Option 2 – Current plant CCD with an added paste thickener at the end of the circuit

Option 3 – Replacing the last stage (5th stage) of the current high rate thickener CCD with paste thickener

Option 4 –Alternative total paste thickener CCD circuit (5 stage, 4 stage and 3 stage circuits)

The comparison between these options will include a process prediction of performance, relative CAPEX and OPEX

Process PredictionsA CCD Model was developed to calculate each option’s parameters to produce 99.5 % reduction in the value concentration in the liquor. This 99.5 % reduction was measured by assuming that the feed had 100 g/l value and the final tailings liquor had only 0.5 g/l.

Table 3 – CCD calculation for the design option consider in this case study

Option Opt 0 Opt 1 Opt 2 Opt 3 Opt 4Number stage

5 5 6 5 5 4 3

Thickener type

High rate

High rate High rate stage1 – 5 PasteStage6

High rate stage1 – 4 PasteStage5

Paste Paste Paste

Thickener diameter, m

45 HRCurrent

35 HRWesTech recommend

35 HR25 paste

35 HR25 paste

25 paste

25 paste

25 paste

Wash water ratio

2.21 2.21 1.43 2.03 1.64 2.56 5.04

WW m3/h 807 807 521 742 599 935 1841Water in 298.6 298.6 214.4 214.4 214.4 214.4 214.4

final tails, m3/hValue loss to final tails, value kg / kg tails

0.41 0.41 0.29 0.29 0.29 0.29 .029

Process Observations1. The Option 0 and Option 1 are equivalent circuits with only the thickener

diameter changing. The benefit will be in the installed costs.2. Option 2 gave the lowest wash ratio with the 6 stage (5 stage high rates with last

stage paste). The combined benefits of an added stage and the improved efficiency of the paste thickener produced wash water savings of over 280 m3/h.

3. Using a paste thickener as the last stage provide (Options 2, 3 and 4) several benefits;

a. Over 80m3/h less water loss to tailings.b. 40% Reduced value loss (benefit of reduced water loss)c. Tailings can be used for a surface stack

4. Replacing the last stage with a paste thickener (Option 3) benefits include;a. >80 m3/h less water loss, (298.6 m3/h down to 214.4 m3/h)b. 40% less value loss (0.41 kg/kg solids down to 0.29 kg/kg solids)c. Reduce wash water use by approx. 9 % (807 m3/h down to 742 m3/h)

5. The current 5 stage high rate circuit performs slightly better than a four stage paste circuit. The 4 stage paste circuit uses 16 % higher wash water to achieve the same washing. The benefit of reducing one stage must consider other factors than wash water volume. The paste thickener last stage will still have the benefits listed above in item 3. Full comparison needs to include CAPEX and OPEX discussed below.

The options are ranked in order of the lowest wash water, (see Figure 3). The 6-stage option #2 was the lowest wash water followed by the 5 stage paste option #4 – 5 stg. The lower the wash water volume the high the value concentration in the stage 1 overflow. Therefore the same ranking applies to pregnant liquor concentration.

Figure 3 – Minimum Wash water rankings for the options considered

High level CAPEX and OPEX review CCD optionsThis costing comparison is not a comprehensive study but only highlights some of the major costs involved. The reader is encouraged to perform a site specific review to fully evaluate the advantages of each option. Comparing costs of different CCD option will require comparing the whole system. This system includes not only the required equipment for the CCD stages but the cost of downstream operations. Downstream operations like filtering, pumping, and tailings disposal can be greatly affected by the type of thickener used as the last stage.

The higher underflow density of the paste thickener benefits the efficiency of the CCD as demonstrated above in this paper, reducing the wash water and/or the number of stages. This higher density is generally non-Newtonian with a yield stress and all the associated traits. These traits have effects on the downstream operations that include:

Table 4 – Downstream operations affected by thickener type selection

Downstream operation

Paste Properties Result of non-Newtonian underflow

Filtration Non-settling and non-segregation, reduced water volume

Decrease hydraulic load on filter, Improved efficiency, higher throughput of filter

Pumping Measurable yield stress, increased pressure drop in pipe line

Pumping should be designed by those familiar with non-Newtonian suspension transport. May need added booster pumps or positive displacement pumps

Tailings impoundment

Quick ‘capillary’ dry mechanism,Self-supporting, higher final density than slurry deposited material, little or no free water on stack.

Water retaining dams not required or reduced. Containment berms only. Rate of rise significantly less reducing dam raises frequency. Greatly reduced failure risk.

These downstream options must consider the site location, the life of mine, and permitting situation. This complex evaluation is best conducted by assembling a paste team of experts. WesTech Engineering, Inc. has been providing paste thickener for more than 15 years. During this time WesTech has worked with other experts in pumping and geotechnical impoundment experts to provide the full system review or design. WesTech and assemble the paste team for such studies. The success of any paste type project is greatly increased with the use of a paste team as most mines do not have expertise in these areas.

Relative comparisonsThe following table provides some relative cost comparisons between paste thickeners and high rate thickeners for CCD circuits;

Table 5 – Relative comparison for high rate and paste thickeners

Item High rate versus paste type thickeners

Advantage per thickener

Flocculent consumption

Same for high rate or paste type thickeners

Same

Inter-stage mixing Paste underflow may require more mixing.

High rate or similar

Operator supervision Similar SamePower for thickener drive

Paste requires twice or more times the power (motor size)

High rate

Inter-stage pumping Paste has higher yield stress and viscosity

Likely same pump for the short distances of a CCD circuit, potential larger motor for paste

CCD footprint Paste type thickener generally have smaller diameters

Paste (see below)

Flocculant system, inter-stage mixing, inter-stage pumping – these costs, for either a high rate or a paste thickener is relatively the same. Therefore any advantage would be a result of the reduction of the number of stages in the paste circuits.

Power for thickener drive – the paste type thickener must be designed to first produce the non-Newtonian underflow and then be able to discharge it. The mechanism is a unique design and the drive torque must be 2 – 3 times that needed for the high rate thickener.

CCD Footprint – A critical factor at many mines is the footprint of the CCD and that savings in this area requirement are valuable. For the site plant case the following footprint relations can be stated. For this comparison, we assume that there is 5 meters between thickeners so a simple estimate of the total CCD footprint is given in the table. The space savings is dramatic between high rate and paste thickeners. The 4 paste thickener stages of Option 4 only require 29% of the footprint of the current 5 stage from the mine CCD. Option 1 only requires 64 % of the footprint of Option 0. Despite the fact that paste requires more foundation than a high rate thickener, there should be an advantage to Paste for earthworks and foundation for these multiple unit CCD circuits.

Table 6 – Footprint comparison for the CCD Options considered in this study

Option Number of stages

Thickener diameter, m

Area per thickener, m2

Total m2

Option 0 5 HR 45 HR 2500 12500Option 1 5 HR 35 HR 1600 8000Option 2 5 HR

1 Paste45 HR

25 Paste2500900

13400

Option 3 4 HR1 Paste

45 HR25 Paste

2500900

10900

Option 4 5 Paste 25 Paste 900 45004 Paste 25 Paste 900 36003 Paste 25 Paste 900 2700

Thickener CostTo achieve the higher underflow density of the paste thickener requires a unique design. The type of thickener recommended for this site is the WesTech Engineering, Inc. HiDensityTM paste. Based on the sizing tests conducted on-site the HiDensity unit is a 25m diameter by 8 meter sidewall thickener.

There are three thickener sizes discussed in this study, the current 45m high rate thickeners, WesTech recommended 35m high rate thickener and the 25m WesTech HiDensityTM . The budget pricing for these three options (thickener only) are:

45m diameter high rate = US$ 2.1 million/unit 35m diameter high rate = US$ 1.2 million/unit 25m diameter HiDensity thickener = US$ 1.3 million/unit.

The thickener only cost comparisons based on these pricing is given in table 7.

Table 7 - WesTech Engineering, Inc. recommendations for the CCD plant

Current thickener WesTech recommendation5 stage current CCD circuit = US$ 10.5 million

5 Stage 35m high rate CCD circuit = US$ 6 million4 stage 25m HiDensity CCD circuit = US$ 5.2 million4 stg 35m HR + 1 stg 25m Paste = US$ 4.9 million

The Westech recommendations are based on the bench-scale sizing tests conducted at the mine site.

ConclusionThe use of paste type thickeners (WesTech HiDensity paste thickener) in CCD circuits provides significant benefits. The benefits include;

Savings in plant water. The water in the final tailings is reduced >80 m3/h which is approximately 26% reduction in the tailing.

Increase recovery. The case study showed that producing the same final liquor concentration, the last stage paste thickener will send 40% less value to the tailings.

Reduction in the number of stages needed (reduced from 5 to 4 stages) Reduction in the CCD circuit footprint Ability to deposit the tailings as a surface stack

As shown by the case study these benefits due to using paste thickener can be accomplished even with a decrease in capital costs.

Successful paste systems require a holistic approach to design. The paste production, paste transport and the paste deposition method must be designed as a single system. WesTech can help assemble the experts for this paste team to work with the end-user.