How to Choose a Cyanide Analyser for Gold Processing

You need to measure cyanide. But a quick search turns up bench-top laboratory systems, continuous online analysers, portable field kits, and half a dozen detection technologies. The specifications blur together. The price range spans an order of magnitude.

The good news: if you understand what you're measuring, where you're measuring it, and why, the right choice usually becomes clear. Here's how to work through the decision.

Quick Answer

For gold mill process control: An online continuous analyser using gas diffusion amperometry (GDA) can pay for itself quickly through cyanide savings alone.

For environmental compliance reporting: A laboratory flow injection analysis (FIA) system delivers the detection limits and method traceability that regulators expect.

For field screening and backup: A portable analyser covers you when the online system is down or when you need a quick check at a remote monitoring point.

Most operations need at least two of these three. The online analyser handles day-to-day process optimisation. The lab system provides regulatory-grade confirmation. Trying to cover both jobs with one instrument means compromising on something.

Start with the Application, Not the Instrument

Before comparing spec sheets, ask three questions:

What cyanide species do you need to measure? Free cyanide, WAD cyanide, and total cyanide require different analytical approaches. Process control in a CIL circuit typically needs available or WAD cyanide. ICMC compliance monitoring requires WAD cyanide. Environmental discharge permits may specify free cyanide, WAD, or both. If you haven't already, read our guide to cyanide analysis methods for the full picture on which measurement matters where.

What concentration range are you working in? Leach circuit concentrations run 50 to 500 mg/L. Tailings discharge after cyanide destruction sits around 1 to 50 mg/L. Environmental water quality triggers under the ANZECC guidelines are as low as 7 µg/L un-ionised HCN for freshwater ecosystems, with the actual concentration depending on pH and temperature. That's a five-order-of-magnitude span, and no single instrument covers all of it well.

How fast do you need the result? If you're adjusting cyanide dosing in a leach circuit, a lab result that arrives four hours after the sample was collected is already ancient history. If you're running monthly groundwater compliance, same-day turnaround from a lab is fine.

The answers to these three questions will point you toward the right category of instrument before you've looked at a single product.

The Three Analyser Categories

Online Continuous Analysers

These are the workhorses of process control. An online analyser draws sample directly from your process stream, filters it, and delivers a result every 3 to 6 minutes. That result feeds back to your control system via 4-20 mA or Modbus, enabling automated cyanide dosing.

The OI Analytical CNSolution 9310 (distributed in Australia and New Zealand by Walker Scientific) is a good example of what a modern online unit looks like. It uses gas diffusion amperometry per USEPA OIA-1677 to measure available cyanide, which serves as a practical proxy for WAD cyanide in gold leach solutions. Three selectable ranges (0.2 to 50, 2 to 500, or 20 to 2,000 mg/L) cover everything from environmental monitoring to leach circuit optimisation, with analysis completing in under 3 minutes. The NEMA 4X / IP-56 enclosure handles the dust, humidity, and temperature swings of a typical processing plant. Total cyanide capability is available with an optional UV digestion module.

Online analysers represent a larger upfront investment, but for many operations the payback through reagent savings and improved recovery comes faster than expected. More on that below.

Best suited for: CIL/CIP leach tank monitoring, cyanide destruction outlet, TSF compliance where continuous data is required.

Laboratory Bench Systems

Lab instruments generally deliver the best accuracy, lowest detection limits, and broadest species coverage. A flow injection analysis system with gas diffusion amperometry can measure free, WAD, and total cyanide (with UV digestion) down to sub-microgram-per-litre detection limits. The OI Analytical FS3700 FIA platform supports methods including OIA-1677, ASTM D6888, D7237, D7511, and D7284, and processes 30 to 40 samples per shift with a 90-position autosampler.

The trade-off is turnaround time. By the time a sample is collected, preserved, transported, queued, and analysed, hours to days have passed. That's fine for regulatory reporting but useless for real-time process adjustment.

Lab systems also provide the reference data against which online analysers are validated. When an auditor asks to see method traceability, the lab result is your anchor.

Best suited for: Environmental compliance at trace levels (ANZECC trigger values, drinking water limits), ICMC audit documentation, method validation for online instruments.

Portable and Field Instruments

Portable analysers range from simple colorimetric test kits (AUD 50 to 500) to digital handheld photometers (AUD 500 to 5,000) and newer smartphone-connected devices. They provide results in minutes at the sample point, require minimal training, and go where online analysers can't.

The accuracy trade-off is real. Most portable instruments measure free cyanide only, with precision of ±10 to 30% compared to ±5% for a well-maintained online system. That's adequate for screening and emergency response, but not for regulatory reporting or fine process control.

Portable testing is also more important than many operations acknowledge. When your online analyser goes down, and it will go down, a portable instrument is the difference between flying blind and maintaining basic process awareness. One mine site reported that none of their multiple online cyanide analysers worked properly in a high-copper environment, highlighting why backup capability matters.

Best suited for: Emergency response, backup during online analyser maintenance, field screening at remote monitoring bores, spot checks at multiple process points.

Why the Detection Method Matters

Not all analysers measure cyanide the same way. The detection technology determines what interferences you'll face, what species you can measure, and how much maintenance you're signing up for.

Gas diffusion amperometry (GDA) converts cyanide to HCN gas, which diffuses across a hydrophobic membrane into an alkaline receiving solution and is detected electrochemically. The membrane is the key advantage: it largely excludes non-volatile interferences like metals, thiocyanate, and sulfide. This is the technology behind OIA-1677 and the CNSolution 9310.

Direct amperometry measures cyanide at a silver electrode without gas diffusion separation. It's simpler mechanically and can be reagent-free for free cyanide measurement. The trade-off is less selectivity, particularly in the presence of sulfides, which require pre-treatment.

Potentiometric titration automates the traditional silver nitrate titration. It's well-established and familiar to lab staff, with wide measurement ranges suitable for high-concentration process streams. A key limitation is copper interference, which we'll cover next.

Colorimetric/photometric methods use colour-forming reagents and optical detection. Excellent sensitivity at low concentrations for water utility and environmental applications, but generally less suited to the high-concentration, high-interference matrices typical in gold processing.

For most gold processing applications, gas diffusion amperometry offers the best balance of interference immunity, regulatory acceptance, and practical reliability.

The Copper Question

If your ore contains copper minerals, this section matters more than anything else in this article.

Copper-cyanide complexes are common in gold processing and they wreak havoc on traditional measurement methods. Silver nitrate titration, still the default at many sites for quick process checks, responds to both free cyanide and copper-cyanide complexes. Research from CSIRO has shown that the rhodanine indicator method can overestimate free cyanide by over 100% in copper-bearing solutions. Other studies have found that even potentiometric endpoints overestimate by up to 56%.

This isn't an academic concern. If your titration tells you there's 200 mg/L of available cyanide when the true figure is 100 mg/L, you won't add more reagent when you actually need to. Or conversely, you might think your levels are fine when they've dropped below what's needed for efficient leaching.

Gas diffusion amperometry largely sidesteps this problem. The membrane excludes copper and other metals, measuring only the cyanide that diffuses as HCN. The CNSolution 9310's product literature confirms immunity to copper and metallic sulfide interferences, along with tolerance to common mining matrix components including thiosulfate and thiocyanate.

If you're processing copper-gold ore, a GDA-based analyser isn't a nice-to-have. It's one of the most reliable ways to get numbers you can trust.

Practical Deployment: What the Brochure Won't Tell You

Sample Conditioning Is Everything

The most sophisticated analyser in the world produces garbage results if it can't get a clean, representative sample from your 40% solids slurry. Sample conditioning—specifically filtration and sample transport—is one of the most common points of failure in online analyser installations.

Proven approaches include pipe samplers with compressed air purge cycles to break up filter cake, stainless steel filter probes with automatic backflush, and self-cleaning sampling systems. The CNSolution 9310 includes an integrated filtration system designed for mining slurry applications.

Whatever system you use, plan for blockages. They will happen. The question is whether your system clears them automatically or requires a technician to drive out at 2 AM.

Maintenance in Remote Australia

Online analysers need regular attention: electrode cleaning, calibration checks, reagent replenishment, pump tubing replacement, and filter maintenance. In a controlled lab environment, this is routine. At a remote Goldfields site with 45°C ambient temperatures, limited technical staff, and the nearest spare parts in Perth, the maintenance burden becomes a critical selection factor.

Consider reagent shelf life in extreme heat. Consider whether the unit needs air-conditioned housing or can handle outdoor installation in a rated enclosure. Consider how much of the maintenance can be done by your existing operators versus requiring a specialist.

SCADA Integration

Most major online analysers output 4-20 mA analog signals for SCADA/DCS integration. This is standard and shouldn't be a differentiator. What varies is the additional connectivity: Ethernet data export, Modbus RTU/TCP, USB data logging, and web-based remote access. If you're implementing closed-loop cyanide dosing control, confirm the output update rate and signal stability with your control system integrator before committing.

Variable Ore and Changing Matrices

Your ore body isn't uniform. As you mine through different zones, copper content changes, sulfide levels fluctuate, and the analytical matrix shifts. One case study documented four different ore sources with combined metal concentrations varying from 500 to 1,500 mg/L, where no online analyser produced reliable results without adaptation.

The solution isn't to avoid online analysis. It's to select a method with proven interference immunity (GDA or amperometric), build in a robust correlation program between online and lab results, and plan for recalibration when ore characteristics change significantly.

The Business Case

With sodium cyanide representing a significant operating cost for any gold processing operation, the financial case for continuous cyanide monitoring is straightforward.

Published case studies from online analyser deployments have reported cyanide consumption reductions of 5 to 20% through tighter process control. For a mid-size gold mill spending several million dollars annually on cyanide, even the conservative end of that range translates to substantial savings.

Add reduced detoxification costs from not over-treating discharge, improved gold recovery from maintaining optimal leach conditions, and lower compliance risk, and the investment case builds quickly. Operations that have deployed continuous monitoring often report payback periods of less than 12 months for the analyser system alone.

In one published example, cyanide represented 25% of total operating cost for a large gold processing region. Even a modest improvement in cyanide management at that scale translated to significant annual savings. Operations processing high-copper ore, where manual control is least accurate, tend to see the largest gains.

Decision Framework

The Bottom Line

For most Australian gold processing operations, the right analyser strategy is a combination: an online continuous analyser using gas diffusion amperometry for process control and compliance monitoring, supported by a laboratory FIA system for regulatory reporting at trace concentrations. A portable instrument rounds out the toolkit for backup and field screening.

The detection method matters. Gas diffusion amperometry provides the interference immunity, regulatory acceptance, and dual lab-online availability that gold processing demands. The practical factors—sample conditioning, maintenance burden, and SCADA integration—determine whether the instrument performs in a brochure or performs on your plant.

And the business case is clear. At current cyanide prices, accurate continuous monitoring isn't a luxury. It's one of the faster-returning capital investments a gold operation can make.

Frequently Asked Questions

Can one analyser handle both process control and environmental compliance?
Generally, no. Process control requires continuous, fast-turnaround measurement at mg/L concentrations. Environmental compliance often requires sub-µg/L detection limits and formal method traceability. An online analyser handles the first job. A laboratory system handles the second. Trying to do both with one instrument means compromising on speed or sensitivity.

How do I validate my online analyser against lab results?
Run parallel grab samples at regular intervals (weekly initially, then monthly once correlation is established). Analyse the grab samples on your lab FIA system using the same method chemistry (ideally OIA-1677 for both). Maintain a correlation log. Differences of ±10% are typical for well-maintained systems. Larger discrepancies warrant investigation of sample conditioning, calibration, or matrix changes.

What maintenance does an online cyanide analyser actually require?
Expect electrode cleaning (automated on most modern units), calibration verification (twice monthly initially, monthly once stable), reagent replenishment (weekly to monthly depending on sample rate), pump tubing replacement (quarterly to semi-annually), and filter maintenance (frequency depends on your slurry). Consumable and maintenance costs vary by site conditions and sample rate, so it's worth getting a specific estimate based on your application.

Do I need ICMC-specific methods?
ASTM D7728 is a guide to method selection for ICMC implementation. It helps operations identify appropriate analytical methods (such as OIA-1677 or ASTM D6888 for available/WAD cyanide) based on matrix and purpose. OIA-1677 and D6888 are among the most widely accepted test methods for WAD cyanide under ICMC. The key is documenting your chosen method, maintaining QA/QC records, and demonstrating correlation with reference methods. Auditors want to see a robust measurement program, not just a specific instrument.

What about newer technologies like ISE probes or test strips?
Ion-selective electrodes offer low cost and simplicity but suffer from interference problems in mining matrices and require frequent calibration. Test strips provide rough screening only. Both can serve as backup or screening tools, but neither replaces a properly validated GDA or amperometric system for process control or compliance.

Further Reading

• International Cyanide Management Code: cyanidecode.org
• USEPA Method OIA-1677: Available Cyanide by Ligand Exchange and Amperometric Detection
• ASTM D6888: Standard Test Method for Available Cyanide with Ligand Displacement and Flow Injection Analysis
• Understanding Cyanide Analysis Methods: Free vs. WAD vs. Total Cyanide
• WAD Cyanide Analysis: What You Need to Know for ICMC Compliance
• Cyanide Management in Western Australian Gold Mining