TONGO MINE MINING OPTIONS DESKTOP STUDY FOR December 2014 Confidentiality Notice This document is strictly confidential and is the property of Paradigm Project Management (Pty) Ltd. No part of this document may be reproduced, translated, stored in a retrieval system or transmitted in any form to any third party by any means, without prior written permission of Paradigm Project Management (Pty) Ltd.
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TONGO MINE
MINING OPTIONS DESKTOP STUDY
FOR
December 2014
Confidentiality Notice This document is strictly confidential and is the property of
Paradigm Project Management (Pty) Ltd. No part of this document may be reproduced, translated, stored in a retrieval system or transmitted in any form to any third party by any means, without prior written permission of Paradigm Project Management
Paradigm Project Management (Pty) Ltd (PPM) has completed a Conceptual Economic Scoping Study (CESS) for Stellar Diamonds Plc (Stellar) for the Tongo Mine in Sierra Leone. The CESS indicated a viable project that could proceed into the next phase of development. However, the underground shrinkage mining method proposed in the CESS, whilst producing positive NPV and IRR metrics, does not support early cash flow generation. Stellar believes that there may be surface mining options that could realise early financial returns when operated in parallel to the development of access to the underground mine. This led to PPM being appointed to undertake a desktop study to consider various options for surface mining. Following its appointment, PPM conducted a brainstorming session. The purpose of the session was to identify and evaluate, at high level, the merits of various mining and hoisting methods. The surface mining options included: • Mechanised open pit mining options • Slot mining options – these include manual mining, Bauer trench cutting, raise boring, and
long hole drilling options The desk top study identified 13 possible mining methods. These were subjected to a qualitative assessment, to determine their relative technical feasibility and economic viability. The evaluation criteria that were used in the qualitative assessment included: • Suitability for kimberlite dyke mining • Practicality in the context of mining in Sierra Leone • Mining Track record • Capex and Opex burden • Support infrastructure requirements • Applicability to both surface and underground mining • Production Potential (Tonne/day) • Early production potential
The outcome of this qualitative assessment indicated that the most technically feasible and economically viable mining methods were: • Trench or slot - Underhand stoping • Trench or slot - Bench stoping
The suitability of these two manual mining methods was borne out by the indicative financial modelling that was conducted.
Indicative financial modelling was done on the following five selected mining methods, informed inter alia by the outcome of the qualitative assessment.
Mining method modelled Comment 1 Trench or slot underhand stoping Named underhand stoping in financial model 2 Trench or slot manual drill and blast Named Bench stoping in financial model 3 Trench or slot drill and blast from surface Named Long hole drilling stoping in financial model 4 Trench or slot Bauer trench cutter
Included because initial work was done at Koidu using this technology. Revisited taking on board the learning from this earlier work
5 Valley open pit mechanised mining
A hybrid of shallow open pit and shallow trench mining excavated from the open pit floor – initial calculations suggested this to be a viable method
The outcome of the financial modelling indicated that the most attractive surface mining option would be: Slot mining utilising the bench stoping mining method It is recommended that Stellar consider undertaking additional work to confirm the viability of this mining method. It is further recommended that the valley open pit mechanised mining method, being a hybrid comprising shallow trench mining and shallow slot mining be explored during a next study phase. The hybrid open pit mine would also provide a useful benchmark against which to measure the attractiveness of the slot mining option.
8. ANNEXURES 28 Discussion on mining method 12 28 Communication with Bauer: the Trench Cutting Investigation 29 Communication with Koidu 31 Trip report to Mauritius to view the Bauer technology 32 Note for the record – brainstorming session 37 Updated Tongo Resource 49 Financial model (Base Case) 54 Financial model Option A 81 Financial model Option B 87
Background Paradigm Project Management (Pty) Ltd (PPM) completed a Conceptual Economic Scoping Study (CESS) for Stellar Diamonds Plc (Stellar) for the Tongo Mine in Sierra Leone during June 2013. The CESS indicated a viable project that could proceed into the next phase of development. However, the underground shrinkage mining method proposed in the CESS, whilst producing positive NPV and IRR metrics, does not support early cash flow generation. Stellar believes that there may be surface mining options that could realise early financial returns when operated in parallel to the development of access to the underground mine. This led to PPM being appointed to undertake a desktop study to consider various options for surface mining.
The Brief The strategic driver as defined by Stellar is to generate positive cash flow as soon as possible. PPM has been requested to consider other potential mining options, specifically open pit options which could decrease the lead time to full production levels and allow for continued production whilst access to underground workings is completed.
Structure of the study The following overall structure was adopted for this desktop study. It reflects a logical progression towards the recommended way forward. • Brainstorming session to identify mining and material handling methods • Qualitative assessment of mining methods • Indicative financial modelling of preferred mining methods • Conclusions • Recommendations
2. BRAINSTORMING SESSION A brainstorming session was conducted on Monday 09 June 2014. The purpose of the session was to identify and evaluate, at high level, the merits of various mining and hoisting methods. The outcome of this brainstorming session is documented in a note for the record (NFTR) which is appended to this report. The NFTR was submitted to Stellar on 22 June 2014. The NFTR was structured to address the following aspects of surface mining: • Mining concepts – open pit or slot mining? • Mining method – manual or mechanised? • Hoisting and material handling methods • Support requirements
The outcome of this brainstorming session resulted in twelve possible mining methods to be subjected to a qualitative assessment. One additional method was included following a discussion with Maxem and Sandvik as relates to the opportunity for surface drilling combined with underground ore extraction.
X section Mining method Material Handling Options 1 Valley open pit
Mechanised Mining Trucks and shovel
1a Valley open pit
Mechanised Mining - shallow open pit with shallow trench
Trucks and shovel, excavator
2 Vertical wall open pit
Manual drill and blast Manual lifting, scraper winch, Blondin
3 Trench or slot Under hand stoping Scraper winch, Blondin, Headgear and Kibble
4 Trench or slot
Manual drill and blast – bench stoping
Scraper winch, Blondin, Headgear and Kibble
5 Trench or slot
Drill and blast from surface – long hole drilling stoping
Scraper winch, Blondin, Headgear and Kibble
6 Trench or slot
Bauer Trench Cutter Mechanical - cutter
7 Trench or slot Bauer Grab Mechanical – grab 8 Trench or slot Large diameter holes from
surface Mechanical – drilling
9 Trench or slot Raisebore Mechanical – raise bore machine 10 Trench or slot Barge and cutter Mechanical - cutter 11 Trench or slot Road cutter Mechanical - cutter 12 Trench or slot Shallow vertical shafts with
The mining methods that were assessed are briefly described: A Mining Method Description 1 Valley open pit Mechanised
Mining Typical open cast mine. Shovels and trucks, with benching and vehicular ramps (60deg slopes). The width of the pit at base to accommodate truck access. Depth of pit to full depth as required by the mine planning
1a Valley open pit Mechanised Mining - hybrid
As for 1, except pit is shallow and the fissure is mined utilising slot mining from the base of the shallow pit
2 Vertical wall open pit - Manual drill and blast
Steep sided pit with extensive rock anchors. Hand drilled blasted and lashed. There is precedent for this in Sierra Leone
3 Trench or slot - Underhand stoping
Similar mining method as to what is proposed for the U/G mine. Drill and blast on the incline. Blasted rock is scraped to the base of the excavation and hoisted vertically to surface using one of the suggested hoisting methods (preferred is the headgear and kibble on tracks on the foot wall)
4 Trench or slot - Manual drill and blast - bench stoping
Benches are created in the slot from which the drilling and blasting is done on a vertical face. Multiple benches and faces possible. Blasted rock is scraped to the lowest wide bench from where it is hoisted to surface
5 Trench or slot - Drill and blast from surface - long hole drilling stoping
Long hole drill is done in the slot from surface and rock blasted into the slot. Cleaning is done on the slot floor. Material is hoisted to surface
6 Trench or slot - Bauer Trench Cutter
Bauer Trench Cutter cuts 800mm wide contiguous slots from surface. Inclination of 80deg can be accommodated. Chips from the cutting process are pumped to surface and sent to a dewatering and screening plant
7 Trench or slot - Bauer Grab As for 6, except the slot is “pre-conditioned” – blasted and the material loaded using an hydraulic grab. The cross section of the grab similar to the cutter, resulting also in a contiguous slot
8 Trench or slot - Large diameter holes from surface
Proposal received from DeWet Drilling. Large diameter holes are drilled from surface (1.2m possible) at suitable intervals. The dyke material between the holes drilled using long hole drilling and blasted
9 Trench or slot - Raisebore Decline is constructed with an off reef drive, cross cuts and a reef drive. A raise bore machine is used to cut to surface and the chippings removed via the decline. Pilot hole drilled from surface before raise boring can occur
10 Trench or slot - Barge and cutter
Submersed dyke using “bund walls” to create a dam to float a cutter unit
11 Trench or slot - Road cutter Continuous miner approach – limited reach 12 Trench or slot - Surface
drilling with U/G extraction Shallow vertical shafts or pits (25m deep) constructed at about 150m intervals. Vertical shafts constructed using thermal spalling and drill and blast (refer method described in the annexure). Reef drive constructed between shafts. Long hole drilling from surface, blasting and cleaning from U/G. Material hoisted via the vertical shaft pits
Practicality in the context of mining in Sierra Leone
Access to site, logistics, manpower, expertise requirement
Track record Previous history in mining operations Capex and Opex Impact on viability Support infrastructure Equipment, power, diesel Applicability to both surface and underground mining
Can the technology, method be relocated from surface to underground
Production Potential (Tonne/day) Production as a function of profitable production and sustainability while waiting for the U/G mine to be established
Early production potential The primary reason for surface mining Table 3-1: Evaluation Criteria
Ranking and weighting was applied to both the mining methods and evaluation criteria used. Weighting – this was applied to the evaluation criteria (number 1-5). A high number indicates a dominant impact of the criteria on the mining method under review Ranking – this was applied to the mining methods (number 1-5). A high number reflects the “attractiveness” of the mining method for the criteria under review For each mining method and criteria, a product of the weighting and ranking was returned in the following table. These products were summed for each mining method. Therefore the mining method with the highest “sum of product” number was adjudged the most attractive option. The following table reflects the outcome of this process.
Notwithstanding the results of the assessment, the following mining methods were disqualified as having little or no potential.
Mining method disqualified Reasons
Valley open pit – Mechanised Mining Large strip ratio Vertical wall open pit – Manual drill and blast
With trench mining considered feasible, there would be no reason to hand blast and lash large amounts of country rock to create an open pit (albeit steep sided)
Trench or slot - Raisebore No early ore to plant possible – a decline is required to get underground to allow the setup of the raise boring equipment
Trench or slot - Barge and cutter No early ore to plant possible – extensive set up and time required to submerse the workings and start production
Trench or slot - Road cutter Very costly equipment and very limited reach / depth possible
Table 3-2: Mining Methods Disqualified
Following the qualitative assessment the following mining methods were taken forward into the indicative financial modelling work.
Mining method modelled Comment 1 Trench or slot underhand stoping Named underhand stoping in financial model 2 Trench or slot manual drill and
blast Named bench stoping in financial model
3 Trench or slot drill and blast from surface
Named Long hole drilling stoping in financial model 4 Trench or slot Bauer trench cutter
Included because initial work was done at Koidu using this technology. Revisited taking on board the learning from this earlier work
5 Valley open pit mechanised mining
A hybrid of shallow open pit and shallow trench mining excavated from the open pit floor – initial calculations suggested this to be a viable method
Trench or slot - Drill and blast from surface – long hole stoping
Trench or slot - Bauer Trench Cutter In order to ensure that there is alignment on terminology, the following definitions are used in the report and the financial modelling: • Pit: A mining module on strike • Stope: Entire side of the Pit being advanced (each pit has two stopes) • Face: Working area – part of a Stope where drilling is done • Panel: Another name used interchangeably for Face
For the underhand stoping and long hole drilling methods, the face is the stope.
The assumptions made regarding the practical depth of mining for each selected method are tabulated:
Mining method Depth
(m) Constraint
Trench or slot underhand stoping 30 Restricted by safe operation Trench or slot manual drill and blast 40 Benching is safer method Trench or slot drill and blast from surface 30 Restricted by drilling control Trench or slot Bauer trench cutter 25 Severely restricted by cutter control Valley open pit mechanised mining 25 + 5 Constrained by economics
Table 3-4: Assumptions regarding depth These depths have been adopted in the financial model calculations. It should be possible with adequate support and ventilation to extend the bench stoping method deeper should this be required. This should be explored in the next phase of the project.
4. SLOT MINING – BENCH STOPING METHOD Slot mining with bench stoping has been selected for further development as, at face value, it provides the most attractive mining method. Therefore, the associated services have been detailed for this particular mining method. It is accepted that these services will be applicable to all manual slot mining methods as outlined in this report. This work on services therefore enhances the pricing as reflected in the financial model.
Mine Scheduling and Planning It is currently proposed that the mine will consist of 3-4 mining pits each covering a strike length of 500m. Each pit will start mining at the centre of the 500m strike and as the benches develop to either side of the pit, it will be deepened by 2.5m and the next bench elevation will be created. It is proposed that 6 benches will be advanced simultaneously in opposite directions. This provides 12 working faces at any point in time. This will mean that a single pit can have 32 faces during steady state of which 6 will be mined simultaneously. This method provides good flexibility within each pit.
4.1.1. Face Naming Convention For the ease of scheduling each face will have a certain identification number. Example: Face 2East7 The identification proposed: • 2 refers to Pit 2 • East refers to mining advancing in an easterly direction • 7 refers to bench 7
This will assist with mine scheduling and planning.
The bench mining concept accommodates a working platform for the mining crews to drill from. The current proposal is for a 5m horizontal section sloped slightly upwards towards the face to allow for water to drain under gravity to the lowest established point in the pit for pumping to surface. The mining width is 1.5m and the face height is 2.5m. The expected advance per blast is 1.5m drilling 2m-long holes.
Mining Unit Value Strike Length m 2 000 Dyke Width m 0.45 Mining depth m 40 Total fissure volume in situ m³ 36 000 Ore Density t/m³ 2.79 Total fissure tonnes in situ t 100 440 Dyke Grade cpht 120 Total carats in situ cts 120 528 Stope width m 1.50 Waste Density t/m³ 2.70 Total waste in situ t 226 800 Total ore mined t 327 240 Monthly horizontal face advance m 12 Ideal stope face angle ° to horizontal 90 Average Stope Length m 40 Mining Recovery % 98% Fissure tonnes per stope per month t 591 Tons waste per stope per month t 1 361 Mining Efficiency % 95% Fissure tonnes per stope per month t 561 Total tonnes per stope per month t 1 854 Total tonnes per stope per annum t 22 246 Number of stopes per pit No 2 Fissure tonnes per pit per annum t 13 465 Total tonnes per pit per annum t 44 492
Mining Cycle The mining cycle consist of three shifts per day namely: • Morning Shift – Drill & Blast • Afternoon Shift – Cleaning and Hoisting • Night Shift – Underground support and services
4.2.1. Drilling and Blasting
Drilling will start on day shift and 6 faces will be drilled simultaneously with hand held rock drills, air legs and compressed air. One drilling machine will be used to drill the 1.5m×2.5m face. Once the drilling is complete the miner will prep and charge-up the faces. All people will then move to surface and the blast activated towards the end of the shift. The re-entry period would be 30 minutes.
4.2.2. Cleaning The afternoon shift will clean the blasted ore from the benches. They will start at the top bench manually moving the material to the bench below. The two top benches will be cleaned manually and the 3rd bench will be equipped with a mechanical scraper winch. This winch will scrape the material towards the hoisting system. The ore will then be loaded from the 3rd bench into the kibble and hoisted to surface, where it is tipped into the headgear chute and into the dump truck.
4.2.3. Support and extending mining services Night shift will be responsible for supporting the blasted area and also to make the area safe of any loose rock. They will extend any service water and compressed air piping and ventilation brattices to prepare for morning shift and the drilling operation.
Hoisting
Hoisting of ore will take place just below the third bench of that section of benches being mined. The headgear will be a steel structure that will straddle the excavation opening and will run on rails along the length of the dyke. The rail gauge is estimated to be between 2.3m to 3m. The headgear structure will be equipped with a winch and a discharge chute. The winch will hoist the 1 or 2 tonne kibbles. The kibble will discharge ore into the chute, which will transfer the ore into a dump truck parked below the chute. Rails will be installed at 50m intervals on the footwall to support and guide the kibble running on wheels.
Mine Dewatering Submersible pumps will be installed at the lowest point of the pit, most likely positioned in the centre of the pit. Water will be discharged into a horizontal settler for water treatment. The suspended solids will be loaded out with a front end loader and transported to the plant for processing. The clean water overflow pumped to the service water tank for re-introduction into the service water system. Excess water will be pumped to an environmental dam for further treatment and discharge back into the environment if the water balance is positive. It is assumed that the mine will produce water at 100% of the consumption and therefore the pumping system will be designed to dewater at double the rate of the service water required for drilling.
Services
Services such as service water and compressed air will be supplied by a main line running along the length of the strike. Take-offs from the centre of each pit will extend down the pit and towards the faces for service water and compressed air supply. It is assumed that water consumption for drilling is 1m³ water/tonne of rock mined. It is assumed that each pit will have a dedicated mobile compressor, which will supply compressed air at 5 - 7 bar. The proposed mining plan suggests that 6 rock drills will be used during drilling and each will use approximately 130cfm. This means that a small 800cfm compressor unit will suffice.
Each pit will have its own ventilation system. An upcast ventilation column will be installed at the centre of each pit, equipped with 2 fans (one running and one standby). Ventilation brattices and curtains will allow the air to be sucked from the outer extremities of the pit into the top benches and allow it to flow down each face to the lowest bench and towards the centre of the pit where it will upcast through the ventilation column to surface. The challenge will be to have sufficient ventilation during the cleaning shift when hoisting takes place. It is proposed that a cover will be opened directly above the bench used for loading. These covers will be closed once the specific set of benches has been cleaned.
5. INDICATIVE FINANCIAL MODELLING The financial modelling for this Desk Top Study (DTS) was based on the results of the Tongo Mine Conceptual Economic Scoping Study (CESS) completed by PPM on 18 June 2013. The “Stellar Acquisition” option in the CESS has been used as the base case for this DTS. In addition, the revised Resource statement of 14 November 2014, as provided in Table 5-1, has been included in this DTS (Annexure 8.6). The Lower Grade model of 120cpht with a value of US$270/ct has been adopted for this DTS.
Tongo Dyke-1 Resource Dec-12
Resource Nov-14
Lower grade model
Resource Nov-14
Higher grade model
Tonnes 895 000 895,000 895 000 Grade (cpht) +1.0mm cut-off 120 120 165 Carats in resource 1 074 000 1 074 000 1 447 000 Diamond Value (US$/ct) 248 270 145 $ per tonne in-situ at average value 297 324 239 Contained Value US$ million 266 290 214
Table 5-1: Revised Resource This DTS has assumed that the shaft access and underground mining from this option will be retained. However, the access decline proposed in the CESS has been removed from the model and replaced by one of the five selected surface mining options. Whilst this may not be entirely accurate since, for example, the issue of a second access once the mine has gone underground would need to be addressed in any future feasibility study, it does provide a basis for comparison of the various surface mining options. In addition, the operating costs for the plant, engineering, manpower, and logistics have been amended to cater for the earlier production arising from the surface mining. It was necessary to make some assumptions as to the level of the support functions such as power generation, accommodation and messing, and logistics required for the early start of surface mining operations and these have been incorporated into the financial model. The following parameters were therefore retained as per the CESS: • Shaft waste • The schedule to complete the shaft • Underground ore mined via the shaft • Mined grade • Diamond value • Capital expenditure for all sections except the development of the decline • Working costs for all sections except the decline operations
The financial model for each of the five surface mining options has their capital expenditure and working costs added to the base costs for underground mining provided for in the CESS. This is to determine the effect on the Notional Profit and hence the cash flows. In order to ensure the compatibility of the comparisons, the same exchange rate that was used in the CESS (ZAR10.25 : US$1) has been retained. Three financial models have been prepared: • The base case – reflecting the financial modelling for the 5 selected mining methods • Bench stoping Option A – the bench stoping method included in the base case but
with the surface mining commencing 3 years earlier than assumed in the base case • Bench stoping Option B – the bench stoping method included in the base case but
with the surface mining commencing 2 years earlier than assumed in the base case Table 5-2 is the input sheet associated with the financial model, and reflects the most pertinent assumptions made.
Table 5-2: Input assumptions to the financial model
The summarised results from this model are indicated in Table 5-3. The comparison indicates the preferred option is for Slot 2 Bench Mining. The financial model assumes that the surface mining and underground development will commence simultaneously in Year 1. However two other alternatives have also been modelled whereby the surface mining commences either 2 or 3 years prior to the
ITEM UNIT VALUECOMMERCIALRevenue per carat $/ct 270Exchange Rate ZAR/US$ 10.25Diesel Price US$/l 1.31Diesel Price: Duty Exemption US$/l 0.93Capex contingency % 15%Opex contingency % 15%MANPOWERBurden % 10%TECHNICALIn-situ kimberlite grade ct/100t 120Recovery Efficiency % 100%Manned Hours per day Hours 16Manned Hours per month Hours 320Manned Hours per year Hours 3 840
commencement of development for the underground mine. These options are indicated in Table 5-4. Whilst it is not shown in this report in detail, reductions in the estimates used for the percentage of up-front capital required for the surface mining options, prior to commencement of the underground development, do not make significant changes to these results.
YEAR -3 YEAR -2 YEAR -1 YEAR 1 YEAR 2 YEAR 3 YEAR 4 YEAR 5 YEAR 6 YEAR 7 YEAR 8 YEAR 9 YEAR 10 YEAR 11 YEAR 12 YEAR 13US$ US$ US$ US$ US$ US$ US$ US$ US$ US$ US$ US$ US$ US$ US$ US$
It was concluded that surface mining to supplement the underground mine is both technically feasible and economically viable. However, economic viability is ultimately determined by Stellar setting thresholds, as to what NPV and IRR are required. The financial model indicates that the most economically viable surface mining options are the three manual slot mining techniques. The following table provides the more qualitative comparisons of the five options considered in the financial model. Mining method
modelled Technical Feasibility Economic Viability
1 Trench or slot – underhand stoping
This is a similar mining method to that proposed for the underground mine. As the trench or slot mine is down to 25m depth, there is a safety concern for both the drilling and cleaning cycles.
This method is economically viable though it lacks the scalability and flexibility that the bench stoping method offers.
2 Trench or slot manual drill and blast – bench stoping
This is the most attractive slot mining method. It offers scalability and flexibility in that a large number of faces are available for mining. The number of mining benches and pits can be easily increased to increase production. This method provides the greatest degree of dilution control as the contact zone is evident in the vertical mining face.
This method appears to be the most profitable of the three manual slot mining methods.
3 Trench or slot drill and blast from surface – long hole drilling stoping
This mining method lacks flexibility and scalability as it is assumed that a single drill rig will provide for the long hole drilling from surface. But it is likely that the drill rig will be a bottleneck to production. There is a concern that secondary blasting may be required before cleaning can occur. This method will also be subject to the risk of higher dilution considering the lack of control on the long hole drilling.
In terms of viability scalability and flexibility, this mining method is very similar to underhand stoping.
This method is limited. A single machine is utilised which significantly restricts production. The method requires significant technical support and is subject to low levels of availability. Production levels are lowest of all of the surface mining methods considered.
This method is not economically viable in so far as that positive cash flow is realized only in year four of a seven year operating period.
5 Valley open pit mechanised mining
To this method being a hybrid of shallow open pit and shallow trench mining is technically feasible
This method has an NPV nominally less than what can be achieved from the CESS only. It is therefore economically unattractive.
Table 6-1: Qualitative comparisons of options The Slot 2 Bench Mining option is considered to be the preferred solution from the perspectives of financial returns, ease of mining, safety, and the ability to potentially continue to a deeper level if required. The potential to accelerate this option was looked at in the financial model. Table 6-2 indicates the effect on the NPV and IRR.
Option Years earlier NPV @10% IRR % Based Case 0 $33 340 000 43% Option A 3 $20 628 000 28% Option B 2 $24 165 000 32%
Table 6-2: Accelerated option effect on NPV and IRR
There is clearly no reason to try to start the surface mining earlier since, whilst it is economical within itself and generates early production, it still requires a large percentage of the total project capital early in the project and does not provide adequate cash flows to make this option financially attractive.
7. RECOMMENDATION The financial modelling and the qualitative assessment of the various potential surface mining options indicate that the preferred solution is manual slot bench mining. The valley open pit option is considered to be the easiest technical solution, but is effectively rendered unattractive by the large mining equipment capital expenditure. If this can be alleviated by the purchase of second hand or utilising existing equipment, then this may merit further investigation as well. This option, being the only mechanised one, could also be used as a benchmark for comparison against the manual slot bench mining proposed. The best financial returns are obtained by commencing with the surface mining and the underground development at the same time.
Discussion on mining method 12 As this method was not reflected in the NFTR of the brainstorming session, it is reproduced here.
Reef drive
Crown Pillar
Vertical Shaft
(Thermal Spalling)
Surface
Drill and blast from
surface
Fissure
150m25
m
2m The method involves the following sequences: • Create shallow vertical shafts at about 150m centres using a combination of thermal
spalling and blasting. The thermal spalling is a method advanced by Maxem and utilises heat from diesel fired burners lowered in pre-drilled holes
• Create reef drives linking the vertical shafts (pits) • Drill and blast from surface • Transport to the vertical shafts • Hoist using a method already explored (frame with kibble)
Communication with Bauer: the Trench Cutting Investigation Discussions were held with Lars Roesler, Area Manager (Africa) for Bauer in South Africa on the 10th of June 2014. The discussions concerned the use of the Bauer Trench Cutter and Grab technologies for mining of the kimberlite dyke. Technical documentation in the form of brochures is available for these technologies. Bauer currently utilises these systems in the civil engineering environment for the excavation of cut off and diaphragm walls, and for intersecting groundwater and constructing foundations. There has to date been very limited application of the technology in the mining space. Where it has been used, it has been for bulk sampling and not for primary mining activity. Bauer also offers RC sampling drilling, utilised exclusively for exploration purposes.
8.2.1. General notes The following notes refer to both the Cutter and grab systems: • Cutting or grabbing at an inclination is possible. The body of the Cutter has 12
hydraulic pads that can be adjusted to force the Cutter off vertical as required • If inclination is required, then the steel cables that support the Cutter as well as the
pumping pipe will have to be supported at surface to prevent them snagging on the hanging wall when the Cutter reaches a specific depth
• 800 X 2800 is the most popular cross section. A range is available (refer Bauer brochures on this topic)
• The first three metres of the slot should be excavated by alternative methods. This will provide a guide for the body of the Cutter as it commences operations. Alternatively concrete guide walls can be constructed on surface
• For hard country rock such as granite, a specific Cutter head should be used. Alternatively the rock could be pre-fractured by blasting from surface and excavated using the grab system
• Under normal conditions the size fraction that results from cutting is less than 50 mm (the suction pipe is 150mm)
• Contiguous mining is possible as the Cutter wheels rotate in opposite directions towards the middle. The Cutter wheels are able to be independently rotated which means that lateral movement of the Cutter body can be achieved by differential speeds being applied to the Cutter wheels
• The plant can be dismantled for transportation purposes. Disassembled sizes and loads are available for in various models of Cutter or Grab
• Trackless and mobile support equipment would include: o Mobile crane to assist in the assembly of the Cutter or Grab o Excavator for cleaning out the blasted starter trench o Front-End loader to move the material from the Desander plant (screening
and dewatering) to the conveyor or bin feeding the plant • As the Cutter or Grab moves along the dyke the Desander plant remains static up to
a distance of 500 m from the Cutter before it requires relocation (a mobile Desander plant is available)
• The advantages/disadvantages of the Grab system o Rate of advance is faster than the Cutter system o The grab system is dry but yields a larger fraction size then the Cutter System o The equipment is smaller which would result in associated lower setup costs o Side wall support – need to check whether the blasting required for the grab
system means more support than using the Cutter System o With pre-fracturing, a greater rate of advance can be achieved with the Grab o The expected advance rate for the Cutter System is 40 metres per day based
on a 16 hour day o Using the two systems together, alternate panels can be cut with the Cutter
System, and the remaining panels blasted and cleaned using the Grab. This means that the blasting is done with two open faces available, and cleaning is facilitated (however, the cutter or grab units are very expensive pieces of equipment, so testing the viability of using both together would be important)
8.2.2. Sierra Leone Learnings
Bauer has utilised its Cutter Technology on a diamond project in Sierra Leone. This deployment, during 2011, was considered unsuccessful. Over a six month period, only 40 metres of advance was achieved – at an average rate of cut of 0.5m per hour. From a Bauer perspective, some of the reasons why this endeavour was not successful include:
A second hand machine (CBC 33) was sourced from Singapore. Availability of the machine was poor due to frequent breakdowns The type of machine used was inappropriate. The base carrier was a compact version more suitable for urban deployment – movement around the dyke was limited. The carrier almost stood on the dyke, restricting access Geological mapping was poor – the limited work performed by the Cutter unit was predominantly in the granite country rock with little intersection of the kimberlite – sometimes up to 90% of the cut was in country rock
The Cutter unit (RSC C403 chisel) was not suitable for the country rock encountered – wear was excessive – the Roller Bit Cutter should have been used
Site preparation was not suitable. The cutting machine requires a level platform which was not provided The infrastructure support was not adequate in terms of power, water supply and pumping etc. These facilities were to be provided by the client. It is Bauer’s view that in future deployment, it should be on a contract mining basis, where Bauer will supply all the supporting services and infrastructure as relates to water and power and logistics – split responsibility should be avoided. The client would still be depended on for staffing support, such as the accommodation camp and its facilities. Logistics had been problematic – “obtaining spare parts was a nightmare”
Communication with Koidu The following is a response received from Dino Coutinho, previously employed by Koidu.
Looking at the information provided by Lars, the summary is pretty accurate. There were a number of problems with the programme; 1. The reconditioned CBC 33 sourced from Singapore proved to be the wrong option, the unit itself had some mechanical issues and although being compact it lacked flexibility in that it had a short reach between the machine platform itself and the cutting head. Taking into account the weight of the machine and having to position it so close to the collar of the trench for one. I felt that an operating platform with a lattice boom arrangement would offer more flexibility, for example the Bauer MC128HD. 2. There were a number of logistical issues related to both coordination of split responsibilities as well as securing short lead times on spares replacement.
3. With regard to the level of geological information required, we provided what we could at the time but the more info the better. It is important to have accurate data on the geological emplacement of the dyke/s (accurate mapping of the vertical dip for example), the in-situ dilution, compressive strengths and cut-abilty characteristics of the country rock and kimberlite. The more core sampling and delineation drilling the better. A generous amount of country rock and kimberlite samples (say +10mm to -150mm) should also be secured for accurate testing. 4. I would agree that a contract mining approach with performance guarantees would be most likely to succeed and will do away with the tendency for parties to engage in finger pointing when results are not achieved. The important thing to note is that although there were numerous problems, in my mind it has been proven that the trench cutter technology can be applied to kimberlite dyke/fissure mining. We have identified a number of possible solutions to the problems encountered with the trial, amongst others for example; pre-conditioning of the ore body (drill & blast) and subsequent extraction by mechanical or hydraulic grab.
Should it be decided that the Bauer Trench Cutter or grab technologies should be taken into the next phase of study, Dino Coutinho, now a free agent and no longer working for Koidu, is available to participate as required. The following paragraph extracted from his e-mail illustrates this:
I am happy to provide you with this oversight off the top of my head. As I mentioned on the phone, I have a wealth of knowledge, experience and access to detail in respect of this subject matter and I am in a unique position to add value to your study and any further developments thereof, should you require a more detailed analysis.
Trip report to Mauritius to view the Bauer technology A site visit was conducted at the Bagatelle dam in Mauritius where the Bauer Trench Cutter technology is being applied. The application on this dam site is for a 800mm wide concrete cut off wall approximately 25 -30 m deep under the full length of an earth embankment dam with a concrete spillway section in the middle. The purpose of the wall is to ensure that water from the dam once complete and filled, does not permeate through the substrates under the dam and results in water losses.
The approximate total length of the cut off wall is 2000m. In the central section, where the hard (200mpa) basalt is encountered, the cut off wall is approximately 800m long in plan. The reason for investigating this particular site where the Bauer Trench Cutter technology is being employed is because it reasonably replicates the Sierra Leone conditions that will be encountered. The similarities are summarised: • Hardness of country rock (200Mpa basalt – 170Mpa granite in Sierra Leone) • Contiguous cutting for a width of 800mm • Depth of cutting is similar (25-30m) • Wet conditions – high rainfall • “Strike” length – about 2km • Cutting machine – BC40 with RSC
The fundamental differences of this application as opposed to mining are summarised: Item Dam Construction Mining Inclination Consistent Verticality Inclined at 80deg to the horizontal Linearity Minimum deviation from the vertical Rate of change of inclination with
depth is a factor Use Cut off wall – slot filled with “waterproof”
concrete. Material emanating from the cutting operation is waste
Material emanating from the cutting operation is ore
Material Single Material - Basalt Dual Material – granite and Kimberlite
Driver Minimum cost but no business case required – no revenue applicable
Business case important - revenue /cost relationship
The construction sequence broadly entails: • Construct Guide walls • Cut three panels (2 X 3.2m outer panels first with center cut next) to yield 8m
primary cut • Repeat for adjacent primary 8m panel • Clean both panels under “working” bentonite, replacing with fresh bentonite • Trammie concrete in both panels displacing bentonite • Cut (secondary) between primary panels (in the process cutting through edges of
primaries) • Trammie concrete into the secondary cut slot
A similar construction sequence (without the concreting) has been adopted for the Bauer Trench Cutting mining option. Cut material is sent to the Desander Plant where dewatering occurs yielding the following “waste” products: • Coarse waste +65mm • Fine waste +15 -65mm • “Sand” +.6 -15mm • “Rock powder” – dust is generated when the cutter malfunctions • Water – principally contaminated bentonite – treated and returned
Bentonite is used: • In areas where side wall stability is required when cutting through less competent
material (as will be encountered in the construction of the cut off wall for the earth embankment sections)
• To facilitate flowability of the rock chips in suspension. Bauer contends that without Bentonite (water only) the lines transporting the waste rock chips to the desander plant would get blocked (particularly in the area where the waste line is coiled on the cutting machine
Note: it will be important to ascertain the impact on the processing plant of the presence of Bentonite in the rock chips reporting to the plant. The current view held by Bauer (South Africa) is that Bentonite will not be required. It is not a view held by the Bauer site team. Observations on site: • The production rate on average is 15 000tpa – extrapolated based on actual
performance achieved in the central hard rock section. • The rule of thumb production rate is 300mm/hour and includes for plant availability,
set up times etc. availability is estimated to be no more that 50%. • Work on site is 24/6, with two 12 hour shifts. Shift change over is on a “hot seat”
basis • Staff complement is 54 day shift and 28 night shift. Ratio of skilled (Bauer
personnel) to unskilled (principally local labour is 1:1 as negotiated with the Mauritius government)
• At times, when advance rate is slow, impact chisels are judiciously used to weaken the rock before the cutter is again utilised
• The following is a table developed off the site records and reflects the anticipated production in the hard rock condition
• Fuel consumption – 1000litres diesel/machine/shift • The Desander machine – 1000kva – allowance for all pumping and plant operation • Considerable number of chissels being broken (50 in 1.5m advance while on site)
The following was offered by Thomas Stocker gleaned from his experience on the Koidu project in Sierra Leone, as a suitable site setup for this mining application. • BC40 with MC96 base carrier (HTS 50) • Use bentonite to prevent blockage in the slurry lines • RBH35 – agitator box • A liberal supply of spares and tools (a well-stocked workshop) • Comprehensive preparation associated with the guide walls • Desander on trailer (BE475) • The guide frame to be mechanically bolted to the guide walls (not hydraulic)
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YEAR 1 YEAR 2 YEAR 3 YEAR 4 YEAR 5 YEAR 6 YEAR 7 YEAR 8 YEAR 9 YEAR 10 YEAR 11 YEAR 12 YEAR 13 TOTALZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR
MINING UNIT VALUEStrike Length m 2 000 Dyke Width m 0.45 Mining depth m 30 Total fissure volume in situ m³ 27 000 Ore Density t/m3 2.79 Total fissure tonnes in situ t 75 330 Dyke Grade cpht 120 Total carats in situ cts 90 396 Stope width m 1.5 Waste Density t/m3 2.70 Total waste in situ t 170 100 Total ore mined t 245 430 Monthly horizontal stope advance m 10 Ideal stope face angle ° to horizontal 45 Average stope length m 42 Mining Recovery % 98%Fissure tons per stope per month t 522 Tons waste per stope per month t 1 203 Mining Efficiency % 95%Fissure tons per stope per month t 496Total tons per stope per month t 1 639 Total tons per stope per annum t 19 663 Number of stopes per pit No 2 Fissure tons per pit per annum t 11 902 Total tons per pit per annum t 39 325
YEAR 1 YEAR 2 YEAR 3 YEAR 4 YEAR 5 YEAR 6 YEAR 7 YEAR 8 YEAR 9 YEAR 10 YEAR 11 YEAR 12 YEAR 13 TOTALZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR
MINING UNIT VALUEStrike Length m 2 000 Dyke Width m 0.45 Mining depth m 40 Total fissure volume in situ m³ 36 000 Ore Density t/m³ 2.79 Total fissure tonnes in situ t 100 440 Dyke Grade cpht 120 Total carats in situ cts 120 528 Stope width m 1.50 Waste Density t/m³ 2.70 Total waste in situ t 226 800 Total ore mined t 327 240 Monthly horizontal face advance m 12 Ideal stope face angle ° to horizontal 90 Average Stope Length m 40 Mining Recovery % 98%Fissure tons per stope per month t 591 Tons waste per stope per month t 1 361 Mining Efficiency % 95%Fissure tons per stope per month t 561Total tons per stope per month t 1 854 Total tons per stope per annum t 22 246 Number of stopes per pit No 2 Fissure tons per pit per annum t 13 465 Total tons per pit per annum t 44 492
STELLAR DIAMONDSTONGO - MINING OPTIONS DESKTOP STUDY (BASE CASE)MINING CAPEXSURFACE SLOT 3: LONG HOLE DRILLING STOPING
YEAR 1 YEAR 2 YEAR 3 YEAR 4 YEAR 5 YEAR 6 YEAR 7 YEAR 8 YEAR 9 YEAR 10 YEAR 11 YEAR 12 YEAR 13 TOTALZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR
STELLAR DIAMONDSTONGO - MINING OPTIONS DESKTOP STUDY (BASE CASE)MINING OPEXSURFACE SLOT 3: LONG HOLE DRILLING STOPING
MINING UNIT VALUEStrike Length m 2 000 Dyke Width m 0.45 Mining depth m 30 Total fissure volume in situ m³ 27 000 Ore Density t/m³ 2.79 Total fissure tonnes in situ t 75 330 Dyke Grade cpht 120 Total carats in situ cts 90 396 Stope width m 1.5 Waste Density t/m³ 2.70 Total waste in situ t 170 100 Total ore mined t 245 430 Monthly horizontal stope advance m 15 Ideal stope face angle ° to horizontal 90 Average stope length m 30 Mining Recovery % 98%Fissure tons per stope per month t 554 Tons waste per stope per month t 1 276 Mining Efficiency % 95%Fissure tons per stope per month t 526Total tons per stope per month t 1 738 Total tons per stope per annum t 20 855 Number of stopes per pit No 2 Fissure tons per pit per annum t 12 624 Total tons per pit per annum t 41 711
PER MODULE UNIT VALUEPrimary Panels No 4Number of cuts per primary panel No 3Secondary panels No 3Number of cuts per secondary panel No 1Pillars No 1Number of cuts per pillar No 0Length of primary panel m 7Length of secondary panel m 2.4Length of pillar m 1.4Total length of module m 36.6Depth of cut m 25Width of cut m 0.8total length of cut m 375Rate of cutting m/hour 0.4hours per day hr 24Time to cut days 39Set ups No 15Time per set up hrs 1.5Total set up days No 0.9Total days No 40Advance on strike m/day 0.915Strike length m 2 000Total operating days No 2 186Working days per annum days 300Years of operation No 7.3Ore SG t/m³ 2.79Tonnes/day t 51Tonnes /annum t 15317Cutter width m 0.8Overbreak % 5%Mined width m 0.84Tonnes/annum t 16 083Dyke width m 0.45Mined grade cpht 64
YEAR 1 YEAR 2 YEAR 3 YEAR 4 YEAR 5 YEAR 6 YEAR 7 YEAR 8 YEAR 9 YEAR 10 YEAR 11 YEAR 12 YEAR 13 TOTALZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR
YEAR 1 YEAR 2 YEAR 3 YEAR 4 YEAR 5 YEAR 6 YEAR 7 YEAR 8 YEAR 9 YEAR 10 YEAR 11 YEAR 12 YEAR 13 TOTALZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR
YEAR 1 YEAR 2 YEAR 3 YEAR 4 YEAR 5 YEAR 6 YEAR 7 YEAR 8 YEAR 9 YEAR 10 YEAR 11 YEAR 12 YEAR 13 TOTALZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR
Deepening rate 30 mpa Interlevel spacing 40 m 33 55.6 40 71 564 tpa Stability pillar 5 m 66 Progressive face positions5 964 tpm Face angle 30 degrees
Stope working height 1.80 m 6 SIDE ELEVATIONStoping width 0.85 m Face length 66 Fissure grade 120 cpht Linear advance per month 12 m Month 1 M2 M3 M4 M5 30
Volume fissure per month 150 m3
Width Height Area Tons/m Tons fissure per face per month 419 t 20 15 12.5 12.0 12.0 Fissure drives 1.5 2.0 3.0 8.18 Tons waste per month 360 t LevelRock drives 2.0 2.0 4.0 10.80 Stope tons per month 779 t Crosscut Decreasing monthly face advance with increasing face lengthCrosscuts 2.0 2.0 4.0 10.80 Swell remaining at end of level #REF! tRaises 0.85 1.5 1.3 3.50 Vent Raiese 1.5 1.5 2.3 6.08 Mining recovery 95% Fissure
Level Depth, mbs0L, Top access 20 45kW fan 30 000R 1L, production 60 2L, production 100 3L, production 140 4L, production 180 5L, production 220 6L, production 260 7L, production 300 8L, production 340 9L, conveyor 360
Check Shaft bottom 370 Strike 1 900 mFissure width 0.45 m Decline to 0L 105 mDepth 300 m Decline to 1L 215 m additional (past 0L)SG ore 2.79 t/m3 Total decline length 319 mResource tons in situ 715 635 t, in situ 1L productionResource carats in situ 858 762 cts, in situ Access on 1L is 206 m from centre of strike
Allowances for mining Orepasses between levelsCrown pillar, subtract 20 m Drop raising 1 m per blastStability pillars, subtract 5 for each of 8 Assume 1 blast per day
ie 40 m Thus 30 m per monthMiing recovery, losses 95%Mineable tons (fissure only) 543 883 t Drop raising 2 000$ /m"Mineable" carats 652 659 ctScheduled carats 682 338 ct
STELLAR DIAMONDSTONGO - MINING OPTIONS DESKTOP STUDY (OPTION A)INPUTS
ITEM UNIT VALUECOMMERCIALRevenue per carat $/ct 270Exchange Rate ZAR/US$ 10.25Diesel Price US$/l 1.31Diesel Price: Duty Exemption US$/l 0.93Capex contingency % 15%Opex contingency % 15%MANPOWERBurden % 10%TECHNICALIn-situ kimberlite grade ct/100t 120Recovery Efficiency % 100%Manned Hours per day Hours 16Manned Hours per month Hours 320Manned Hours per year Hours 3 840
Currency Heading 1 ZARCurrency Heading 2 US$Currency Total Line 1 TOTAL ZARCurrency Total Line 2 TOTAL US$
STELLAR DIAMONDSTONGO - MINING OPTIONS DESKTOP STUDY (OPTION A)FINANCIAL MODELUS$
YEAR -3 YEAR -2 YEAR -1 YEAR 1 YEAR 2 YEAR 3 YEAR 4 YEAR 5 YEAR 6 YEAR 7 YEAR 8 YEAR 9 YEAR 10 YEAR 11 YEAR 12 YEAR 13US$ US$ US$ US$ US$ US$ US$ US$ US$ US$ US$ US$ US$ US$ US$ US$
YEAR -3 YEAR -2 YEAR -1 YEAR 1 YEAR 2 YEAR 3 YEAR 4 YEAR 5 YEAR 6 YEAR 7 YEAR 8 YEAR 9 YEAR 10 YEAR 11 YEAR 12 YEAR 13 TOTALZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR
MINING UNIT VALUEStrike Length m 2 000 Dyke Width m 0.45 Mining depth m 40 Total fissure volume in situ m³ 36 000 Ore Density t/m³ 2.79 Total fissure tonnes in situ t 100 440 Dyke Grade cpht 120 Total carats in situ cts 120 528 Stope width m 1.50 Waste Density t/m³ 2.70 Total waste in situ t 226 800 Total ore mined t 327 240 Monthly horizontal face advance m 12 Ideal stope face angle ° to horizontal 90 Average Stope Length m 40 Mining Recovery % 98%Fissure tons per stope per month t 591 Tons waste per stope per month t 1 361 Mining Efficiency % 95%Fissure tons per stope per month t 561Total tons per stope per month t 1 854 Total tons per stope per annum t 22 246 Number of stopes per pit No 2 Fissure tons per pit per annum t 13 465 Total tons per pit per annum t 44 492
STELLAR DIAMONDSTONGO - MINING OPTIONS DESKTOP STUDY (OPTION B)INPUTS
ITEM UNIT VALUECOMMERCIALRevenue per carat $/ct 270Exchange Rate ZAR/US$ 10.25Diesel Price US$/l 1.31Diesel Price: Duty Exemption US$/l 0.93Capex contingency % 15%Opex contingency % 15%MANPOWERBurden % 10%TECHNICALIn-situ kimberlite grade ct/100t 120Recovery Efficiency % 100%Manned Hours per day Hours 16Manned Hours per month Hours 320Manned Hours per year Hours 3 840
Currency Heading 1 ZARCurrency Heading 2 US$Currency Total Line 1 TOTAL ZARCurrency Total Line 2 TOTAL US$
STELLAR DIAMONDSTONGO - MINING OPTIONS DESKTOP STUDY (OPTION B)FINANCIAL MODELUS$
YEAR -2 YEAR -1 YEAR 1 YEAR 2 YEAR 3 YEAR 4 YEAR 5 YEAR 6 YEAR 7 YEAR 8 YEAR 9 YEAR 10 YEAR 11 YEAR 12 YEAR 13US$ US$ US$ US$ US$ US$ US$ US$ US$ US$ US$ US$ US$ US$ US$
YEAR -2 YEAR -1 YEAR 1 YEAR 2 YEAR 3 YEAR 4 YEAR 5 YEAR 6 YEAR 7 YEAR 8 YEAR 9 YEAR 10 YEAR 11 YEAR 12 YEAR 13 TOTALZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR ZAR
MINING UNIT VALUEStrike Length m 2 000 Dyke Width m 0.45 Mining depth m 40 Total fissure volume in situ m³ 36 000 Ore Density t/m³ 2.79 Total fissure tonnes in situ t 100 440 Dyke Grade cpht 120 Total carats in situ cts 120 528 Stope width m 1.50 Waste Density t/m³ 2.70 Total waste in situ t 226 800 Total ore mined t 327 240 Monthly horizontal face advance m 12 Ideal stope face angle ° to horizontal 90 Average Stope Length m 40 Mining Recovery % 98%Fissure tons per stope per month t 591 Tons waste per stope per month t 1 361 Mining Efficiency % 95%Fissure tons per stope per month t 561Total tons per stope per month t 1 854 Total tons per stope per annum t 22 246 Number of stopes per pit No 2 Fissure tons per pit per annum t 13 465 Total tons per pit per annum t 44 492