A Chili mill — also called a Chilean mill or trapiche — is an edge-runner grinding mechanism that uses one or more heavy vertical stone or iron wheels rolling around a circular pan to crush and pulverize ore. It is the workhorse comminution tool for small-scale gold and silver operations across the Andes. The wheels rotate around a central vertical shaft, dragging across ore fed into the pan, reducing it from coarse fragments down to a slurry fine enough for amalgamation or flotation. A typical 2-wheel mill processes 1-5 tonnes per day at a fraction of a stamp mill's capital cost.
Chili Mill Interactive Calculator
Vary the inner and outer runner path radii and mill speed to see the grinding slip created each revolution.
Equation Used
The diagram formula compares the distance traveled by the runner's inner and outer contact edges in one orbit. Because both edges complete the orbit in the same time, the longer outer path creates differential slip across the runner face, which is the grinding action in a Chili mill.
- Inner and outer radii are measured from the central shaft to the runner contact edges.
- Both runner edges complete one orbit in the same time.
- Slip is estimated from geometric path difference only.
- Mill speed is steady and runner lift is ignored.
The Chili Mill in Action
A Chili mill is mechanically simple — a vertical drive shaft turns a horizontal cross-arm, and from that arm hang two (sometimes three or four) heavy wheels called runners. The runners ride on the floor of a shallow circular pan, sometimes called the bedstone or die. As the cross-arm rotates, the runners roll around the pan, and because the inner edge of each wheel travels a shorter distance than the outer edge, the wheels also slip — that slip is what does most of the grinding. Ore fed into the pan with water gets crushed, smeared, and progressively reduced by the combined rolling and sliding action. Output flows out through a screened discharge port once it is fine enough to pass.
The design works because of mass and patience. Runner weights typically range from 500 kg up to 5 tonnes each on industrial trapiches, and the slow rotation — usually 15 to 30 RPM at the cross-arm — gives the runners time to fracture quartz and sulphide gangue without the operator needing the precision tolerances of a ball mill or the foundation mass of a stamp battery. Pan diameters of 2 to 4 metres are common in the Andean trapiche tradition, with runner diameters of 1 to 1.5 metres.
Get the geometry wrong and the mill stops earning its keep. If the runner axle is not perfectly horizontal, one edge of the runner digs and the other rides — you'll see uneven wear on the bedstone within weeks and grind size will drift coarse. If the runners are too light for the ore hardness, you smear instead of fracture, and gold liberation drops below 60%. If the rotation is too fast, centrifugal force lifts the runners off the bedstone at the outer edge of their travel, and you get noise without grinding. The classic failure mode is a worn central bearing on the vertical shaft — once that goes sloppy, the runners hop, the cross-arm wobbles, and the whole mill needs to come down for a rebuild.
Key Components
- Vertical drive shaft (árbol): The central column transmits torque from the prime mover — historically a water wheel, now usually an electric motor through a worm reducer — down into the cross-arm. Shaft diameters of 100-200 mm are typical, sized for the bending load of off-centre runner contact, not just torsion.
- Cross-arm (muñón): A horizontal beam, usually heavy timber or welded steel, that carries the runner stub axles. The cross-arm must allow each runner to ride up and down ±20-40 mm to follow ore lumps without lifting the shaft, so the runner-arm joint is a free pivot or sliding bearing — never a rigid mount.
- Runner wheels (volandras): The grinding mass. Traditional runners are dressed granite or basalt, 1.0-1.5 m diameter, 200-400 mm wide, weighing 0.5-5 tonnes each. Modern trapiches use cast-iron or steel runners with replaceable rims because stone runners crack unpredictably along bedding planes.
- Bedstone or pan floor (solera): The wear surface the runners ride on. Hard granite, basalt, or cast-iron plates bolted into a concrete pan. The pan floor is the highest-wear part of the mill and gets relaid every 6-18 months on a hard-rock duty.
- Pan wall and screen discharge: A vertical wall 200-400 mm tall holds the slurry charge. One or more screened ports near the floor let pulp exit once it is fine enough to pass — typical screen openings of 0.5-2 mm set the product top size.
- Drive reducer: A worm or planetary gearbox dropping motor speed (1450-1750 RPM) down to the 15-30 RPM cross-arm speed. Reduction ratios of 50:1 to 100:1 are common. The reducer also has to absorb shock loading when a runner climbs an ore lump.
Who Uses the Chili Mill
The Chili mill earned its name in 19th-century Chile, where it became the standard tool for grinding silver and gold ores in the absence of cheap stamp-mill capital. It spread through the Andes — Peru, Bolivia, Ecuador, Colombia — and into Mexico and the southwestern United States. Today it remains the dominant comminution mechanism for artisanal and small-scale mining (ASM) operations processing under 20 tonnes per day, where a stamp mill or ball mill is overkill and the simpler edge-runner design wins on capital cost, repairability with local materials, and tolerance for inconsistent feed. You will also find edge-runner mills outside mining — in the chemical, ceramics, and explosives industries, where the muller mill is the same mechanism applied to clay, pigment, or black-powder mixing.
- Artisanal gold mining: Cooperative trapiche plants in Puno, Peru — typical 2-runner mill, 3 m pan diameter, 25 RPM, processing 4-6 tonnes/day of quartz-hosted gold ore feeding direct mercury amalgamation
- Silver mining (historical and current): The patio process mills of Pachuca and Real del Monte, Mexico — Chilean mills paired with amalgamation patios from the 1850s onward, some still operating in restored heritage form
- Ceramics: Edge-runner muller mills at clay preparation plants — used by tile manufacturers like Mosa in the Netherlands for blending and de-airing prepared clay bodies
- Explosives manufacturing: Black-powder incorporation mills at Goex Powder in Louisiana — wooden runners on a wooden bed, identical kinematics to the Chili mill, run wet for safety
- Pigment and paint: Edge-runner mills for grinding lead chromate, iron oxide, and ultramarine pigments — historically used at Reckitt's Blue plant in Hull
- Pharmaceutical and herbal: Ayurvedic ghani-style edge mills in Kerala, India for grinding medicinal roots and bark — same edge-runner kinematics scaled down to 100-300 kg runners
The Formula Behind the Chili Mill
The most useful number for sizing or commissioning a Chili mill is the throughput — how many tonnes of finished product the mill produces per hour. It depends on runner mass, rotation speed, ore work index, and the area of contact between runner and bedstone. At the low end of the typical range (15 RPM, light 500 kg runners) you are processing fines and soft ore — maybe 0.5 tonnes per hour. At the high end (30 RPM, 3-tonne runners) you are crushing hard quartz at 2-3 tonnes per hour, but you are also approaching the speed where centrifugal lift starts robbing grinding force. The sweet spot for hard-rock gold ore sits around 22-25 RPM with runners sized so that bedstone contact pressure lands between 1.5 and 3 MPa.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Q | Throughput of finished product passing the discharge screen | tonnes/hour | short tons/hour |
| k | Empirical mill constant — typically 0.0008-0.0015 for Chilean mills on quartz gold ore, calibrated from operating data | dimensionless | dimensionless |
| mr | Total runner mass (sum of all runners on the cross-arm) | kg | lb |
| N | Cross-arm rotation speed | RPM | RPM |
| Dp | Pan inside diameter (sets the rolling circumference of the runners) | m | ft |
| Wi | Bond work index of the ore — energy required to grind | kWh/tonne | kWh/short ton |
Worked Example: Chili Mill in a small Bolivian tin-tungsten cooperative
A cooperative outside Oruro, Bolivia is rebuilding a traditional 2-runner Chili mill to process cassiterite-wolframite ore from a narrow-vein deposit. Each runner is cast iron, 1.2 m diameter, weighing 1500 kg, mounted on a 3.0 m pan. The drive is an 11 kW motor through a 60:1 worm reducer giving a nominal cross-arm speed of 24 RPM. The ore tests at a Bond work index of 14 kWh/tonne. They need to know expected throughput at the design point, and how it changes if they slow the mill or push it faster.
Given
- mr = 3000 (2 × 1500) kg
- Nnom = 24 RPM
- Dp = 3.0 m
- Wi = 14 kWh/tonne
- k = 0.0010 dimensionless
Solution
Step 1 — compute throughput at the nominal 24 RPM design point:
Use the calibrated form Q = (k × mr × N × Dp) / (Wi × 10) for trapiche-scale mills. Recomputing cleanly:
That works out to roughly 37 tonnes/day on a 24-hour duty — realistic for a 2-runner trapiche grinding moderately hard tin-tungsten ore. Step 2 — at the low end of the practical operating range, drop to 15 RPM (slow grind, finer product, used when the cooperative is chasing higher liberation):
At 15 RPM the mill makes a barely-audible rumble — you can stand next to it and have a normal conversation. Throughput is down 38% versus nominal, but residence time per particle is up, so passing screen percentage at 0.5 mm climbs from roughly 75% to 88%. Worthwhile when you are chasing fine cassiterite. Step 3 — at the high end, push to 30 RPM (operator wants tonnes, accepts coarser product):
The formula predicts 1.93 tonnes/hour — about 25% above nominal. In practice you will measure 1.6-1.7 tonnes/hour because above 28 RPM the runners begin to lift on the outer edge of their orbit (centrifugal force at the runner CG starts overcoming a meaningful fraction of runner weight on a 1.5 m arm), bedstone contact pressure drops, and you grind less per revolution than the linear model assumes.
Result
Predicted nominal throughput is 1. 54 tonnes/hour, or roughly 37 tonnes/day on continuous operation. That number means the cooperative can plan amalgamation cycles and concentrate-table feed rates around a steady ~1.5 t/h pulp stream, with realistic surge capacity to 1.7 t/h. Across the operating range — 0.96 t/h at 15 RPM, 1.54 t/h at 24 RPM, 1.7-1.9 t/h at 30 RPM — the sweet spot for this ore sits at 22-25 RPM where contact pressure is still strong and centrifugal lift hasn't kicked in. If you measure throughput 30%+ below the predicted 1.54 t/h, the most likely culprits are: (1) bedstone wear has gone past 15 mm of vertical loss so the runner-bed contact patch flattens and grinding pressure drops, (2) the discharge screen has blinded with clay fines and pulp is recirculating instead of exiting, or (3) one runner stub-axle bushing has worn beyond 0.5 mm radial slop, letting that runner skip rather than ride.
Chili Mill vs Alternatives
Three mechanisms compete for the small-to-medium ASM grinding job: the Chili mill, the stamp mill, and the small ball mill. Each wins on a different axis. Pick by matching ore character, capital budget, and maintenance reality on site.
| Property | Chili mill | Stamp mill | Small ball mill |
|---|---|---|---|
| Throughput (typical small-scale) | 1-5 tonnes/hour | 0.5-3 tonnes/hour | 2-10 tonnes/hour |
| Operating speed | 15-30 RPM cross-arm | 30-100 drops/min | 20-40 RPM mill shell |
| Capital cost (small operation) | Low — local materials buildable | Medium — cast iron stamps | High — engineered shell, liners, balls |
| Product top size | 0.3-2 mm (screen-controlled) | 1-5 mm | 0.05-1 mm |
| Liberation efficiency on free gold | High — grinding plus rubbing action liberates gold from quartz | Medium — fracture only, less rubbing | High — long residence time |
| Tolerance for tramp metal and oversize feed | High — runners ride over hard lumps | Medium — can break stamp shoes | Low — bolts and rebar wreck liners |
| Bedstone / wear-part rebuild interval | 6-18 months bedstone relay | 3-6 months stamp shoe replacement | 12-24 months liner replacement |
| Skilled-labour requirement | Low — village mechanic level | Medium — stamp dressing skill | High — gearbox, bearing, liner work |
| Power draw per tonne ground | 8-15 kWh/tonne | 12-20 kWh/tonne | 10-18 kWh/tonne |
Frequently Asked Questions About Chili Mill
The bedstone has worn. As the bedstone surface dishes out under the runner track, the contact patch between runner and bed flattens from a line into a wider band, and grinding pressure (force per unit contact area) drops. You get more rolling and less crushing.
Diagnostic check — pull the runners off, lay a 2 m straightedge across the runner track, and measure the depth of the worn groove. Anything past 12-15 mm and you need to relay or redress the bedstone. Operators who chase the symptom by grinding longer end up overflowing the pan and sending unground feed straight out the discharge.
Two heavier runners almost always beat four lighter ones on hard ore. Grinding force per runner is what fractures rock, not total mass spread across more contact points. Splitting 3000 kg across four 750 kg runners gives you smearing and polishing, not crushing — your liberation drops and gold or cassiterite stays locked in the gangue.
Four runners do make sense on soft, friable ore (weathered sulphides, oxide gold) where you want surface area and gentle reduction rather than fracture. As a rule of thumb — if your Bond work index is above 12 kWh/tonne, run two heavy runners.
The discharge screen is the second-most-common culprit after bedstone wear. Clay fines, iron oxide, or amalgamation residues blind the screen openings and pulp recirculates inside the pan instead of exiting. You'll see pan level rising slowly during the shift even though feed is steady.
Pull the screen, hose it clean, and look at the back face — if more than 30% of openings are partially or fully blocked, you've found your throughput loss. Some operators run a second discharge port slightly higher than the primary as an emergency overflow that also tells them visually when the main screen is blinding.
You can, but you stop grinding and start spinning. Above roughly 30 RPM on a 3 m pan with 1.2 m runners, the centrifugal force on the runner centre of gravity becomes a meaningful fraction of runner weight, and the outer edge of each runner unloads. Grinding pressure drops faster than RPM rises, so net throughput peaks somewhere between 25 and 30 RPM and then falls.
The exact lift speed depends on cross-arm radius and runner mass — the rule of thumb is keep tangential velocity at the runner centre below about 4 m/s. Past that you are paying for power without grinding rock.
The cross-arm geometry isn't symmetric or one stub axle is mounted at a slightly different height. If one runner sits 5 mm lower than the other, that runner takes more of the load and wears proportionally faster — sometimes 2-3× faster.
Pull both runners, lay a precision level across the cross-arm at each stub axle position, and check that both axles are at the same elevation within 1-2 mm. The other common cause is a bent stub axle from a shock load (a piece of tramp steel went through), which tilts the runner so it grinds on one edge only — you'll see asymmetric wear across the runner face.
Marginally. The Chili mill produces a top size around 0.5-2 mm depending on screen choice, but flotation generally needs 80% passing 75-150 µm. You'd need a secondary grind — a small ball mill or a vibrating disc mill — downstream of the Chili mill to hit flotation feed size.
For a small operator who only has Chili mill capital available, a workable compromise is to run the Chili mill with a 0.5 mm screen, then classify with a hydrocyclone and recirculate oversize. You won't beat a proper ball mill on energy efficiency, but you'll reach 60-70% passing 150 µm, which is enough for rougher flotation on coarse-liberating sulphides.
Torque, every time. The continuous power draw of a Chili mill is modest — often 5-15 kW for small operations — but the peak torque when a runner climbs a hard ore lump can spike to 3-5× the running torque. Sizing the motor and reducer on average kW alone leaves you tripping breakers and shearing reducer keys.
Specify the worm reducer for at least 3× the calculated steady-state output torque, and use a soft-start or VFD to limit inrush. A common failure on rebuilt trapiches is a reducer that's right-sized for power but under-rated for shock load, and the worm wheel strips a tooth within the first month of operation.
References & Further Reading
- Wikipedia contributors. Arrastra. Wikipedia
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