Mako Project Commissioning

Mako Project Commissioning

Mako Project Commissioning – The Mako Gold Project is located in south-eastern Senegal in the Kedougou region, close to the Gambia river and township of Mako.  Toro Gold Ltd awarded the EP contract to Lycopodium in August 2016 and site construction with Petowal Mining Company (PMC) commenced in September 2016.

 

Ramp Up Curve

The Mako processing plant was designed to treat 1.8 Mtpa of some of the hardest and most abrasive ore in OMC’s database (Axb in the low 20s and Ai above 1). The ore consisted of felsic and basalt ore types with only a small amount of softer oxidised and transitional material to be treated during the early months of operation.

OMC was involved in the project from design through to commissioning, providing input into the grinding circuit selection and configuration.  OMC proposed that the comminution circuit, best suited to this feed profile, was a single toggle jaw crusher followed by single stage SAG milling incorporating recycle pebble crushing (Primary Crush SS SAC circuit).  This circuit was selected to reduce operational complexity, capital costs, ongoing wear-related operating costs and overall risk of achieving design.  It was noted during modelling that, due to the ore hardness, the power split in a conventional SAG and Ball milling circuit was heavily skewed towards the SAG mill.  Implementing this design would result in installation of a large SAG mill and small ball mill.  By combining these into a single stage SAG and adding contingency power, the overall risk to achieving throughput and grind size was reduced.

The installed mill is an Ø8.50m x 6.10m EGL (28’ x 20’) Outotec grate discharge SAG mill, complete with an 8 MW variable speed drive (VSD).  Having a variable voltage variable frequency (VVVF) drive for VSD control allows the mill to be run at any speed up to 78% of critical, giving flexibility during the commissioning and ramp up phases, and to adjust performance for changes in future ore hardness and liner wear.  This grinding circuit is followed by a pre-leach thickener, carbon in leach circuit, Zadra elution electro-winning circuit and cyanide detoxification circuit.

First Ore Crushed To The Stockpile

The Mako commissioning team comprised Lycopodium, ECG Engineering and OMC Staff, integrated into the construction and operations teams.  As part of the team, OMC provided process support during the commissioning and ramp up period, particularly around the milling circuit.

Felsic ore, close to design hardness, was specifically reserved by the Toro mining department in the lead up to commissioning, allowing both ramp up of the milling circuit and performance testing under design ore conditions.  First ore was crushed to the stockpile on 18th November 2017 following successful dry plant commissioning.  Pushing forward into the wet plant with the bulk of equipment to be commissioned, first ore was treated through the grinding circuit on 26th December 2017 and continuous operations achieved on the 3rd of January 2018.  During this phase there was a trade-off between construction completion, energization and commissioning tasks to be worked through.  The commissioning phase proceeded safely, ahead of plan and all equipment was handed over by early February.  First gold, a respectable 42kg of doré from the CIL/elution circuit, was poured a month later on the 26th January 2018, making Australia day celebrations in Senegal an event to remember!

Following these milestones, the commissioning team focused all efforts on understanding, controlling and ramping up the grinding circuit in order to achieve design throughput and product grind size.  Being a single stage design (closed with hydrocyclones) the Mako mill performs the combined duties of a traditional SAG and ball mill in one mill.  As a result, one of the biggest challenges was associated with waiting for the initial ball charge to season in, which is required to achieve efficient grinding at both coarse and fine rock sizes.  A decision not to pebble crush during the initial period of operation provided more autogenous grinding media at smaller size fractions in the absence of smaller balls.  The mill power/load response initially proved to be very sensitive to feed water addition and cyclone underflow flow rates returning to the mill, this sensitivity was reduced and control improved after changing to a smaller spigot size.  Even so, finding the correct mill load, speed and ball charge to avoid both ‘sanding’ or restricting total throughput was a fine balancing act.  Stabilisation of control was initially difficult, necessitating a robust overall control philosophy including:

 

  • Feed water control in a ratio to new feed, adjustable based on the current mill power/load relationship.
  • Cyclone control at a constant pressure setpoint with make-up water addition tied loosely to discharge hopper level. This allowed a stable circulating load and self-regulation of the circuit product size.  This control method has the additional benefit of being able to use cyclone feed density as a direct indicator of cyclone overflow product grind size (P80).
  • Mill load control in cascade to new feed rate, which required a lot of work to tune.
  • Mill speed was not implemented as a primary form of control, however maximizing power using speed for a given ore type was left as an operator controlled parameter. Speed was also used to allow protection of the SAG load during periods of lost feed, and to allow initial build-up of an operating charge following shutdowns or grind out events.  Having a VSD also allowed inching of the mill (breaking of locked charge) to be automated and carried out without conventional swapping to a dedicated inching drive.
  • The commissioning team undertook hands-on operator training during the ramp up phase, and once all parties were more confident in the performance of the grinding circuit a week-long performance trial was initiated.  The key performance criteria of availability, throughput and grind size were achieved without the need for the pebble crusher.

 

Close Up Of Hard Rock Feed

Ultimately, the ramp up of the Mako grinding circuit from first ore to design throughput occurred within 6 weeks on very hard and competent feed rock.  Milling circuit control proved to be robust, resulting in steady operation of the circuit.  Importantly the control philosophy was well understood by the control room operators (many being inexperienced) and allowed them to make measured changes to achieve desired production targets.

Four OMC team members assisted with the Mako startup, meaning things were a lot quieter back at the Perth office.  The entire team bonded and worked well together on site, achieving the goal of safely commissioning the plant within the project time frame.  Even though it is a hard slog commissioning a remote African project, some great times were had by the team along the way, particularly given much of the team spent Christmas and New Years on site.

 

 

OMC would like to extend a sincere thanks to both Lycopodium Minerals and Toro Gold for having us on the Mako project.  We would like to recognize the efforts of the Lycopodium design and PMC construction teams, particularly those that first mobilized to site in late 2016 and wrapped up the performance testing to return home in late February 2018.  We would also like to thank the PMC production, maintenance and project teams for their support.  Working and interacting with the local Senegalese workforce was a great experience, with their eternal positivity and helpfulness spurring us on throughout the project.  A large contributor to the project success was the positive and supportive team effort from all involved.

We wish Toro all the best moving forward and believe that they are well positioned to do so based on the early performance of the project.  Onwards to the next challenge!