Garnet for SX-EW electrolyte filters
Copper SX-EW plants clean their electrolyte in dual media filters of anthracite over garnet before it reaches the tankhouse. GRM Garnet supplies the garnet for the whole bed, from the fine coalescing layer to the coarse support, for first fills, top-ups and rebeds.
The last clean-up before the tankhouse.
Leachable copper ores, oxides and secondary sulphides, are heap or dump leached and the copper recovered by solvent extraction and electrowinning. The dual media filter treats the loaded (advance) electrolyte after stripping, just before it enters the tankhouse.3,4
- Heap or dump leachAcid dissolves the copper
- Solvent extractionOrganic loads the copper
- StrippingCopper moves into electrolyte
- Dual media filterAnthracite over GRM garnet
- TankhouseElectrowinning to cathode
What the filter removes
Electrolyte leaving solvent extraction carries entrained organic droplets and fine solids. Inlet organic is typically 20–30 ppm, and the filter’s target is below 2 ppm at the outlet.1 Filter makers link this clean-up directly to cathode purity: organic that reaches the cells can mark and contaminate the copper.2
How the bed is built
Anthracite on top, with an effective size of about 0.6–1.2 mm, catches most of the organic. Garnet below it is the fine filter and coalescing layer, sitting on coarser garnet support layers over the underdrain.3,4 Filters run at about 12 m³ per m² per hour and are backwashed regularly.3
The garnet has to stay below the anthracite after every backwash, survive years of washing without wearing down, and add as little as possible to the electrolyte. Those are the three things GRM Garnet is tested for.
A typical arrangement.3,4 Each filter maker sets its own media sizes and depths; GRM supplies every layer to that design.
Fine and support garnet from one source.
No public specification sets the garnet sizes for electrolyte filters; each filter maker designs its own bed. All ten GRM grades can be supplied as filter media, so the bed can be filled to the filter maker’s design. The highlighted grades are the usual choices.
| Grade | ES (d10) | UC | Typical place in an electrolyte filter |
|---|---|---|---|
| 120# | 0.11 mm | 1.45 | Very fine layer, by the filter maker’s specification |
| 80# | 0.16 mm | 1.43 | Very fine layer, by the filter maker’s specification |
| 50# | 0.26 mm | 1.38 | Finer filter layer where specified |
| 20/60 | 0.23 mm | 1.98 | Finer filter layer where specified |
| 14/40 Specialty Filter | 0.65 mm | 1.82 | Fine filter and coalescing layer under the anthracite |
| 36# | 0.38 mm | 2.63 | Filter layer with a broader grading, where specified |
| 8/80 Alpha Blast | 0.25 mm | 3.40 | Broad grading, by the filter maker’s specification |
| 8/12 | 1.73 mm | 1.17 | Garnet support layer |
| 4/8 | 2.58 mm | 1.55 | Coarse support · rarely offered |
| 2/4 | 5.30 mm | 1.45 | Coarsest support over the underdrain · rarely offered |
Coarse support garnet is hard to find. Published filter-garnet grades stop at about 2.5 mm. GRM grades hard-rock garnet up to 8 mm and beyond, so the whole support bed can be garnet rather than lighter silica gravel, which can mix into the garnet above it in backwash.
Tested in hot sulphuric acid.
Electrolyte is strong sulphuric acid, so buyers want to know what the garnet gives up to it. In an accelerated leach test, GRM garnet was held in 200 g/L sulphuric acid for 7 days at 50 °C, and the leachate was analysed by ICP. The solids were analysed by XRF, sized, and tested for density and relative hardness.5
| Element | mg/L |
|---|---|
| Iron (Fe) | 245.9 |
| Aluminium (Al) | 96.4 |
| Magnesium (Mg) | 8.14 |
| Calcium (Ca) | 7.67 |
| Manganese (Mn) | 3.67 |
| Potassium (K) | 2.07 |
| Sodium (Na) | 1.75 |
| Silicon (Si) | 1.25 |
200 g/L H₂SO₄, 7 days, 50 °C. ICP analysis of the leachate.
| Oxide | % |
|---|---|
| Fe₂O₃ | 39.4 |
| SiO₂ | 37.8 |
| Al₂O₃ | 20.7 |
| MgO | 3.1 |
| MnO | 1.3 |
| CaO | 0.7 |
| TiO₂ | 0.5 |
The iron–aluminium signature of almandine. The SiO₂ is bound in the garnet crystal; free (crystalline) silica is 0.5%.
| Solids specific gravity | 3.97 |
| Dry bulk density | 2,050 kg/m³ |
| Relative hardness | 1.29 × silica sand |
| Within 0.6–1.0 mm | 86.6% |
| Over 1.0 mm | 0.23% |
| Under 425 µm | 0.79% |
Sized on a 0.6–1.0 mm sample. Very little undersize, which is what a filter bed needs for good filtration efficiency.
About these results. The tests were run in 2022 on a 0.6–1.0 mm sample from an earlier GRM production run. Results for current production will be published here when they are available. Ask us for the latest data for your filter.
Garnet outlasts the anthracite above it.
Anthracite in electrolyte filters is typically changed every one to two years, while the garnet lasts considerably longer.3 So garnet is bought for three jobs.
New and expanded tankhouses
The full garnet bed for new filters, every layer from the fine garnet to the coarse support, to the filter maker’s design.
Replacing losses
Garnet lost in backwash or during maintenance, matched to the grades already in the filter.
Complete media changes
When a filter is stripped and refilled, garnet for every layer from a single source, with one chemistry and one set of test data.
25 kg bags, and 1 t and 2 t bulk bags. Minimum orders vary by region and are usually around 50 metric tonnes.
Most SX-EW copper comes from Chile, the DRC, the USA, Mexico, Zambia and Peru. Enquiries are welcome from plants, filter makers and their contractors in every region.
SX-EW filter garnet: common questions.
What size garnet goes in an SX-EW electrolyte filter?
Each filter maker sets its own bed. A typical bed has fine garnet under the anthracite as the filter and coalescing layer, for example 14/40, on coarser garnet support layers such as 8/12, 4/8 and 2/4. All ten GRM grades are available, so send us the filter maker’s media specification and we will match it.
Why use garnet in an electrolyte filter?
Garnet is about four times as dense as water and much heavier than anthracite, so it settles back below the anthracite after every backwash and keeps the bed in order. It is hard, so it resists wear over years of washing.
Does garnet dissolve in the electrolyte?
In our accelerated test, 7 days in 200 g/L sulphuric acid at 50 °C, the leachate contained 246 mg/L iron and 96 mg/L aluminium; every other element measured was below 10 mg/L. The full results are on this page; current-production results will be added when available.
How often is the media replaced?
Anthracite is typically changed every one to two years. The garnet lasts considerably longer, so it is mainly bought for first fills of new or expanded tankhouses, top-ups and full rebeds.
How is GRM filter garnet packed?
In 25 kg bags and 1 t and 2 t bulk bags. Minimum orders vary by region and are usually around 50 metric tonnes.
Can GRM supply every garnet layer for a rebed?
Yes. GRM supplies garnet from the fine 120# up to the coarse 2/4 support grade, so every garnet layer in the bed can come from one source, with one chemistry and one set of test data.
Specifications and further reading.
Other uses of GRM Garnet
Sources
- Kuosa et al. (2017). International Journal of Mineral Processing.
- Smith & Loveless. Electrolyte filtration for SX-EW.
- Metso. Dual media filter brochure.
- Sharpenn. Copper SX-EW filtration duties.
- GRM Garnet (2022). Acid leach, XRF, size, density and relative hardness tests on a 0.6–1.0 mm sample from an earlier production run.
GRM Garnet values are typical and do not constitute a specification. Third-party figures describe electrolyte filters in general, not GRM Garnet, unless stated.