Crushing Circuit Design for a Gold Ore That Doesn’t Behave Like a Textbook Example

Process circuit design and equipment selection for a gold ore concentrator with complex mineralogy, variable ore hardness, and a requirement for early recovery of free gold ahead of the main grinding circuit.

The Ore and Its Challenges

Gold deposits of this type — greenstone belt-hosted, with subsidiary quartz vein and shear zone mineralisation — present a specific combination of crushing challenges that a standard circuit design does not fully address.

Ore hardness and variability. The host rock is predominantly granite, greenstone, and quartzite with a Protodyakonov hardness coefficient (f) in the 12–18 range — upper-medium to hard by the classification relevant to crushing circuit design. Compressive strength of the ore ranges from 100 to 200 MPa depending on the proportion of competent quartz vein in the sample. This hardness range is workable in a cone crusher circuit, but the variability — softer altered wallrock interbedded with competent quartz-dominant ore — creates uneven liner wear patterns if liner grade and chamber geometry are not selected for the feed blend rather than a single representative hardness.

Gold occurrence. The gold occurs in two distinct modes. The majority is locked within sulphide minerals — primarily pyrite and arsenopyrite — and silica, requiring fine grinding to below 74 μm (200 mesh) for liberation. A secondary but economically significant fraction occurs as free (native) gold or coarse electrum visible at coarser sizes. The natural ductility of free gold creates a specific problem during crushing: compressive crushing forces can flatten free gold particles into thin sheets rather than fracturing them, which increases the risk of loss to the reject fraction and complicates subsequent gravity recovery. Free gold that has been plastically deformed in the crushing circuit is harder to recover than free gold that has been liberated cleanly.

Clay minerals in oxidised zones. Oxidised ore from near-surface mining panels contains clay minerals that have an affinity for the crushing chamber surfaces and screen apertures. Clay adhesion reduces effective screen opening size, lowers throughput, and in some operating conditions creates blockages that require manual clearing.

Circuit Design and Equipment Selection

The circuit adopted for this installation runs: run-of-mine ore → primary jaw crushing → secondary cone crushing → tertiary high-pressure grinding roll or cone crushing → ball mill grinding with hydrocyclone classification. This is a standard circuit architecture for hard gold ore, but the implementation decisions at each stage reflect the specific ore characteristics described above.

Primary stage: jaw crusher. A deep-chamber jaw crusher with Mn18Cr2 liners, selected for the upper end of the ore hardness range. The jaw crusher is the appropriate primary stage for this feed size and hardness combination — robust, mechanically simple, and tolerant of the variable feed hardness that characterises this ore type.

Free gold recovery at primary stage. For ore panels identified as containing significant free gold — typically shallower oxidised zones and quartz-dominant vein intersections — a portable jigging table was incorporated into the primary circuit to process the primary crusher discharge before it enters the secondary circuit. Jigging exploits the high specific gravity of native gold (SG ~19) relative to gangue minerals (SG 2.6–3.0) to separate free gold by density at relatively coarse sizes. This early recovery step prevents free gold from entering the grinding circuit where the risk of plastic deformation and smearing is highest, and reduces the total amount of gold that must be recovered from the fine fraction after leaching.

Secondary and tertiary stages: hydraulic cone crushers with automated control. The cone crushers selected for this circuit incorporate single-cylinder hydraulic systems that serve three functions: discharge setting adjustment, tramp iron protection (automatic release and reset when uncrushable material enters the chamber), and chamber clearing after planned and unplanned stops. The automated adjustment capability addresses the variable product size requirement directly — the concentrator takes ore from multiple mining panels with different liberation size requirements, and being able to adjust the secondary cone CSS without stopping the circuit and without manual mechanical adjustment reduces the operational overhead of managing the variable feed.

The automated control system added to the hydraulic cones in this installation extends the standard functionality to include programmable crushing modes — coarse, medium, and fine settings pre-configured for the three main ore types encountered at this deposit. The crusher operator selects the mode based on the ore panel being processed; the hydraulic system adjusts the CSS to the stored setting automatically. The blockage rate under normal operating conditions has been negligible.

Liner grade for the cone crushers. The secondary cone liners are specified in Mn18Cr2, consistent with the upper ore hardness range. For the tertiary stage operating at tighter CSS and lower impact energy, the wear mechanism shifts toward abrasion-dominant, and high-chromium iron liners are used for the tertiary mantle and concave where the feed is predominantly fine and abrasive after two prior crushing stages. This split liner grade strategy — manganese for the impact-loaded secondary stage, high-chromium iron for the abrasion-dominant tertiary stage — reflects the different wear environments within the same circuit rather than a single material applied throughout.

Occupational Health: Dust Control at the Crushing Circuit

Arsenopyrite is a common gold-bearing mineral in greenstone belt deposits of this type. When arsenopyrite is fractured during crushing, fine particles of arsenic-bearing dust are released into the crushing circuit atmosphere. In an enclosed or semi-enclosed crushing plant — which is the norm for dust and noise management — these particles concentrate in the working environment of the crusher operators and maintenance personnel.

A dust suppression and extraction system was specified as an integral part of the crushing circuit design for this installation, not as an afterthought. The system covers the primary jaw crusher feed point, the primary discharge to the secondary circuit, and the secondary cone crusher discharge — the three points of highest dust generation in the circuit. Wet suppression (water misting) at feed and discharge points reduces airborne particle generation; extraction ventilation removes residual fine dust from the enclosed crusher building.

At project completion and handover, the site supervisor responsible for the crushing circuit operations acknowledged the dust control design directly. His comment was that very few equipment suppliers had considered the working conditions of the people actually operating the plant — that the attention to this aspect of the design represented a form of care and respect for the site’s operational workforce that was not typically part of the technical specification conversation.

We record this not as a marketing point but because it reflects something we believe about how engineering projects should be conducted. The people who operate and maintain crushing circuits in remote mining operations work in physically demanding conditions. Occupational dust exposure — particularly arsenic-bearing dust in sulphide gold operations — is a serious and preventable health hazard. Including adequate dust control in the circuit design is an engineering obligation, not an optional extra.

Results and Observations

The circuit has operated across the full range of ore types encountered at the deposit. The automated CSS adjustment has allowed the concentrator to respond to ore variability without circuit downtime for mechanical adjustment. Early free gold recovery at the primary stage has improved overall gold recovery for ore panels with significant free gold content — the specific recovery improvement varies with the free gold proportion in the feed, which changes with mining panel. The blockage rate across the circuit is negligible under normal operating conditions.

The combination of pre-programmed crushing modes, hydraulic tramp iron protection, and the discharge clearing function has reduced the frequency and duration of unplanned stops compared with a mechanically adjusted circuit, which is relevant for a remote operation where downtime has high opportunity cost and maintenance support is not immediately available.

What This Case Illustrates

Complex gold ores with variable hardness, multiple gold occurrence modes, and sulphide mineralogy require circuit design decisions that a standard specification does not capture. The key decisions in this case — liner grade differentiation within the circuit, early free gold recovery at primary stage, automated hydraulic control for variable CSS, and integral dust suppression — each addressed a specific characteristic of this ore and this operating environment. None of them is unusual in isolation; the value is in combining them into a circuit design that reflects the actual ore rather than a generic hard-rock gold ore template.

For technical discussion of crushing circuit design for specific ore types and operating conditions, contact our engineering team. We respond to technical enquiries within 1–2 working days.