Metallurgical accounting for gold operations

Gold accounting leaves little room for error. Head grades are measured in grams per tonne, so a small sampling or moisture bias on millions of tonnes can become thousands of unaccounted ounces. Gold can sit in the circuit on carbon and in solution for some time before it is poured. And the ounces a company reports depend directly on the plant’s numbers. MAX has been used on gold operations ranging from a single carbon-in-leach (CIL) plant to two parallel plants sharing one mine, and on heap leach.

From mill feed to doré

Flowsheets differ from plant to plant. A carbon-in-leach (CIL) or carbon-in-pulp (CIP) plant might crush and grind ore, recover free gold in a gravity circuit, leach the rest, strip the loaded carbon in an elution circuit, electrowin the gold from the eluate, and smelt the result into doré bars. In MAX each of those stages is modelled as measure points, so feed tonnes, moisture, head grade, solution and carbon assays, tails grade and gold-room figures each land in the right place, and formulas turn them into contained metal and recoveries.

Because every formula states how it rolls up from shift to month, the monthly recovery is calculated from the month’s totals rather than by averaging daily percentages, and contained ounces are summed rather than averaged. Averaging percentages is an easy mistake to make in a month-end spreadsheet; in MAX the rule is settled in the configuration rather than left to whoever builds the workbook.

Gold and silver, separately

Where an operation also produces silver, its recovery can differ from gold’s, and silver loading takes up carbon, elution and electrowinning capacity. MAX balances each metal in its own right, with its own assays, contained metal and recoveries through every stage, rather than inferring one from the other. Doré is recorded with both gold and silver assays.

Gravity and leach on the same basis

Gravity gold reaches the gold room by a different route from leached gold, through a concentrator and, at some plants, an intensive leach reactor. Reporting “gravity recovery” and “leach recovery” consistently needs the gravity concentrate’s mass and grade and the reactor solution assays as well as the leach circuit’s figures. The gravity pathway can be modelled as its own measure points, so the split is reported the same way every month. One site records gravity bars and solution bars separately, each from their own pours.

Gold in circuit and the gold room

One swing item between gold produced and gold poured is gold in circuit: on carbon in the leach tanks, in the elution circuit and in electrowinning cells. In MAX it can be calculated from tank volumes, carbon concentrations and solution and carbon assays, in formulas grouped in a category of their own, so the period-end figure is calculated the same way every time. Elution and electrowinning can be recorded strip by strip, with their own daily reports.

In the gold room, each pour is recorded as a batch of doré bars, each bar with its weight and gold and silver assays, and bars are grouped into shipments to the refiner. Batches are approved before their figures are relied on, and bar registers give the auditable record of what left site.

More than one plant

Where one mine feeds two plants, each plant needs its own feed, grade and recovery, and the site needs a combined view that reconciles back to the mine. One gold operation runs separate daily processing reports for each of its two parallel plants alongside a combined daily report and monthly reports, all calculated from the same database, so the plant-level and site-level figures always agree. When one plant runs below capacity, for example during a mill outage, downtime accounting separates the lost hours from metallurgical performance, so a recovery change is not blamed on the wrong cause.

Heap leach

Heap leach operations account for metal differently: ore is stacked on pads and irrigated, gold reports to pregnant solution ponds, and a recovery plant, such as an adsorption, desorption and recovery (ADR) plant, recovers it. Recovery from a pad builds up over time, so the inventory of gold on the pads and in the ponds is central to the balance. MAX has been configured for a heap leach operation with its own data categories for leach pads, irrigation, ponds and the ADR plant, alongside the usual weightometers, reagents and reconciliation.

Reporting to the market

Listed gold producers report production figures such as ounces, grades and recoveries to the market. With reconciled, approved monthly figures in one system, and original values kept beside adjusted ones, the numbers behind those disclosures can be traced back to the plant measurements that produced them, which is what the AMIRA P754 code asks for.