Chemistry & Assay Methods
The science behind MineGuard's on-site gold measurement — from wet chemistry extraction to electrochemical detection.
1. Why Chemistry?
There is no affordable gold concentration sensor on the market. The alternatives are prohibitively expensive for small-scale mining operations:
$25,000+
XRF analyzers — fragile, need calibration, too expensive
$50+/sample
Fire assay — accurate but needs a lab and furnace, 1-2 hours per batch
$0.15-0.35
MineGuard — simple wet chemistry + electrochemical measurement
MineGuard's approach uses common commodity chemicals and disposable electrodes to achieve ±5% accuracy for concentration ranges relevant to alluvial mining. No specialized lab, no expensive instruments — gold dissolves passively in a jar of bleach+HCl while workers do their normal duties, then SWV measurement takes seconds. The cost per test is under $0.15.
2. Bleach + HCl Method (Daily Assay)
The primary method for daily gold content measurement, using sodium hypochlorite (bleach) and hydrochloric acid — both cheap, commonly available chemicals.
Weigh concentrate sample (10-20g) on precision scale
Add sodium hypochlorite (bleach, NaOCl) to dissolve gold
Add hydrochloric acid (HCl) to form soluble gold chloride
Filter to remove undissolved solids
Measure gold concentration in solution using SWV
Dissolution reaction
2Au + 3NaOCl + 6HCl → 2AuCl₃ + 3NaCl + 3H₂O
Note: This is a simplification. The actual dissolution involves hypochlorous acid (HOCl) as the oxidant, forming tetrachloroauric acid (HAuCl₄) in solution.
3. Iodine + KI Method (Drum Extraction)
An alternative method using potassium iodide and iodine solution. More selective than the bleach method with fewer side reactions, especially useful for samples with high sulfide content.
Dissolution reaction
Au + I₃⁻ + I⁻ → [AuI₄]⁻
Gold dissolves as the tetraiodoaurate(III) complex anion, which is stable in solution and can be measured electrochemically.
Advantages
- More selective — fewer interfering reactions
- Better for sulfide-rich samples
- Lower safety risk (no HCl fumes)
Trade-offs
- Higher cost ($0.35 vs $0.15 per test)
- Longer dissolution (8-12 hrs vs 4-8 hrs for bleach)
- Reagents are light-sensitive (shorter shelf life)
4. When to Use Which Method
| Factor | Bleach + HCl | Iodine + KI |
|---|---|---|
| Cost per test | $0.15 | $0.35 |
| Dissolution time | ~4-8 hrs (passive) | ~8-12 hrs (overnight) |
| Measurement time | Seconds (SWV) | Seconds (SWV) |
| Best for | Raw ground (daily assay) | Sulfide-rich / drum extraction |
| Accuracy | ±5% | ±3% |
| Interferences | Sulfides, organics | Fewer |
| Shelf life | Months (sealed) | Weeks (light-sensitive) |
| Safety | Moderate (HCl fumes) | Low |
Decision Tree
5. Square Wave Voltammetry (SWV)
After dissolving gold into solution, the concentration is measured using Square Wave Voltammetry — an electrochemical technique that produces a characteristic signal proportional to gold content.
Working Electrode
Screen-printed carbon electrode (disposable, $0.50, ~20 uses)
Potentiostat
3-electrode cell: working, reference (Ag/AgCl), counter
Sensitivity
Detects down to ~0.1 ppm gold in solution
How SWV Works
The potentiostat sweeps voltage across the working electrode while measuring current. Gold(III) ions in solution produce a characteristic reduction peak at approximately +0.4V to +0.6V (vs Ag/AgCl reference). The peak current is directly proportional to gold concentration — higher current means more gold.
Think of it like scanning a radio dial — each element has its own “frequency” (voltage). Gold produces a clear, distinct peak that the software identifies and quantifies using a pre-established calibration curve.
Conceptual Voltammogram
6. Field Challenges & Solutions
Laboratory conditions are ideal. Mining sites in tropical Guyana are not. Here are the real-world complications and how MineGuard handles each one.
Preg-robbing
Problem: Carbonaceous material in sample absorbs dissolved gold, reducing measured concentration.
Solution: Pre-roast carbon-rich samples at 500°C or add activated carbon competitor before dissolution.
Organic Matter
Problem: Plant material and humic acids interfere with the electrochemical signal.
Solution: Add hydrogen peroxide (H₂O₂) to destroy organics before measurement.
Clay Interference
Problem: Fine clay particles clog filters and trap gold, leading to underreporting.
Solution: Use flocculant (aluminum sulfate) to settle clay before filtering.
Temperature
Problem: Tropical heat (35°C+) affects chemical reaction rates and electrode response.
Solution: Calibrate at ambient temperature. Software applies temperature compensation automatically.
Humidity
Problem: Electronics corrode quickly in humid jungle conditions.
Solution: Conformal coating on all PCBs plus desiccant packets inside the sealed enclosure.
Power
Problem: No reliable grid power at remote mining sites.
Solution: Solar panel + battery system. The Raspberry Pi draws only 5W, easily sustained off-grid.
7. Calibration Procedure
The electrochemical system must be calibrated to convert peak current readings into accurate gold concentrations. Calibration is straightforward and uses commercially available gold standard solutions.
Prepare 5 standard solutions with known gold concentrations: 0, 0.5, 1.0, 2.0, 5.0 ppm
Measure each standard using SWV and record the peak currents
Plot calibration curve (concentration vs peak current) — should be linear
Software stores the calibration coefficients automatically
Re-calibrate monthly or whenever electrodes are replaced
Quality control: measure one known standard before each batch of field samples
Note: Calibration standards can be prepared from commercial gold standard solution — approximately $15 for 100mL at 1000 ppm, enough for hundreds of calibration runs.
8. Cost Per Test
Detailed cost breakdown for both extraction methods. These are commodity chemicals available worldwide.
Bleach + HCl Method
| Item | Unit Cost | Tests Per Unit | Per Test |
|---|---|---|---|
| Bleach (NaOCl 12%) | $3/gallon | ~200 tests/gallon | $0.015 |
| HCl (32%) | $8/liter | ~100 tests/liter | $0.08 |
| SPE electrode | $0.50/electrode | ~20 uses | $0.025 |
| Filter paper | - | - | $0.01 |
| Total per test | ~$0.13-0.15 | ||
Iodine + KI Method
| Item | Unit Cost | Tests Per Unit | Per Test |
|---|---|---|---|
| Potassium iodide (KI) | $25/500g | ~200 tests | $0.125 |
| Iodine crystals (I₂) | $15/100g | ~200 tests | $0.075 |
| SPE electrode | $0.50/electrode | ~20 uses | $0.025 |
| Filter + water | - | - | $0.01 |
| Total per test | ~$0.25-0.35 | ||
Monthly Cost Summary
For daily testing (one composite assay per day, ~30 tests/month):
$4-10
Chemicals per month
$8
Electrodes per month
~$12-20
Total per month
9. Research References
The chemistry and electrochemistry behind MineGuard is grounded in peer-reviewed research. Key papers supporting the methods described above:
Hypochlorite/Chloride Leaching
Baghalha, M. (2007)
“Leaching of an oxide gold ore with chloride/hypochlorite solutions”
International Journal of Mineral Processing, 82(4), 178-186
Key finding: 90% gold extraction in 30 min under lab conditions with fine particles
Kozin, L.F. & Melekhin, V.T. (2005)
“Kinetics and mechanism of gold corrosion dissolution in hypochlorite solutions”
Protection of Metals, 41(2), 105-111
Key finding: Activation energy 53.43 kJ/mol; rate constants measured at multiple temperatures
Quijada-Noriega et al. (2020)
“Dissolution of silver and gold with NaOCl and HCl in refractory minerals”
Mining, Metallurgy & Exploration, 37, 1709-1719
Key finding: Multi-stage leaching achieves 96% gold extraction from concentrate
Iodine/Iodide Leaching
Qi, P.H. & Hiskey, J.B. (1991)
“Dissolution kinetics of gold in iodide solutions”
Hydrometallurgy, 27(1), 47-62
Key finding: Gold dissolution rate comparable to cyanide; first order in triiodide
Baghalha, M. & Sadegh Gh, H. (2012)
“Leaching kinetics of an oxide gold ore with iodide/iodine solutions”
Hydrometallurgy, 113-114, 42-50
Key finding: 89% Au extraction in 24h from ore at room temperature
Wang, H.X. et al. (2013)
“Study on gold concentrate leaching by iodine-iodide”
Int. J. Minerals, Metallurgy and Materials, 20(4), 323-328
Key finding: >85% gold recovery in 4h at 25°C with optimized conditions
Electrochemical Detection
Hall, G.E.M. et al. (1992)
“Determination of gold in geological samples by ASV at field locations”
Chemical Geology, 102(1-4), 41-52
Key finding: 10 ppb detection limit for gold using field-portable anodic stripping voltammetry
Mpinga, C.N. et al. (2019)
“Kinetic investigation and dissolution behavior of cyanide alternative gold leaching reagents”
Scientific Reports, 9, 7191
Key finding: Dissolution rate ranking: aqua regia > iodine > bromine > cyanide > thiourea > thiosulfate
Note on dissolution times: Academic papers report dissolution times under ideal lab conditions (fine pure gold particles, controlled temperature, clean solutions). In field conditions with raw ground containing clay, organics, and coarser particles, dissolution takes significantly longer — typically 4-8 hours for bleach+HCl and 8-12 hours for iodine+KI. MineGuard's protocol accounts for this by dissolving the composite sample passively throughout the work day.
See the Full System
Learn how the verification math and hardware tie it all together.