# Packed Bed Voidage: How 6x12 Mesh Carbon Behaves in a Gold Column
> Source: https://www.yelicarbon.com/blog-activated-carbon-packed-bed-voidage-gold.html
Voidage is the space between carbon particles - and it is what the pregnant solution actually flows through. Here is how to estimate it from bulk density, what values to expect for 6x12 mesh coconut carbon, and why falling voidage shows up as climbing pressure long before any assay changes.
**By the YELI Technical Team - Updated October 2026 - 7 min read**
> **Short answer:** Bed voidage (void fraction) is the share of a carbon column's volume that is open space rather than particle. For randomly packed 6x12 mesh coconut shell granular activated carbon it typically falls in the **0.36-0.44 range**, and you can estimate it from bulk density: voidage = 1 − (bulk density / particle envelope density). It matters because voidage sets how much pregnant solution the bed holds, what residence time that solution sees, and how pressure drop behaves as the bed fouls. Two carbon lots with identical iodine and hardness can pack differently if their sieve distribution or platelet content differs.
Column behaviour in a CIC circuit is usually discussed in terms of the carbon: mesh cut, iodine number, hardness. Voidage is the property that describes the space between the particles, and it is what the solution actually travels through. It is rarely written on a data sheet and is worth understanding before you size a column or diagnose a pressure problem.
## 1. What bed voidage means
A carbon bed is not solid. Between the granules there is a connected network of channels, and that network carries the pregnant solution. Bed voidage (ε, also called void fraction or bed porosity) is the fraction of the total bed volume occupied by those channels:
**ε = void volume / total bed volume**
For a bed of 10 m³ with a voidage of 0.40, roughly 4 m³ of that volume is solution and roughly 6 m³ is carbon particle. Two distinct voids exist in activated carbon and they are often confused:
- **Inter-particle voidage** - the channels between granules. This is what governs flow, pressure drop and the solution volume held in the bed.
- **Intra-particle porosity** - the pore network inside each granule. This is what the iodine number measures, and where gold cyanide adsorbs.
Both matter, but they are not interchangeable, and a supplier quoting "porosity" may mean either.
## 2. Typical voidage for 6x12 mesh coconut GAC
Randomly packed, near-spherical granules typically settle between about 0.36 (dense random packing) and 0.44 (loose random packing). Coconut shell carbon, which is hard and close to granular rather than spherical, generally lands in that band.
| Material state | Typical bed voidage | Comment |
|---|---|---|
| Loosely poured, dry granules | about 0.42-0.44 | Highest voidage; settles after wetting |
| Settled and water-soaked bed | about 0.36-0.42 | The working condition in a column |
| Bed with excess fines or platelets | below 0.36 | Fines fill channels; channelling and pressure rise |
| Bed with scale or biofilm | falls further in service | Fouling reduces voidage over time |
Engineering-typical ranges. Measure your own bed rather than assuming a value.
## 3. Estimating voidage from bulk density
The practical route uses two numbers you can obtain:
**ε = 1 − (bulk density / particle envelope density)**
- **Bulk (apparent) density** of gold-grade 6x12 coconut carbon is typically 0.45-0.52 g/cm³, measured by ASTM D2854 / GB/T 12496.1. This is the mass divided by the volume the carbon occupies when poured - it already includes the inter-particle voids.
- **Particle envelope density** is the density of a single granule including its internal pores but excluding the space between granules. For coconut shell GAC this is commonly in the region of 0.8-0.9 g/cm³.
**Worked example.** With bulk density 0.48 g/cm³ and particle envelope density 0.85 g/cm³:
ε = 1 − (0.48 / 0.85) = 1 − 0.565 = **0.435**
Illustrative arithmetic with stated assumptions. Both densities vary with lot, moisture and how the bed was filled, so treat the result as an estimate and confirm it on your own column.
## 4. Why voidage matters in a gold column
- **Solution held in the bed.** A 10 m³ bed at ε = 0.40 holds about 4 m³ of solution. That volume, divided by the flow rate, is the contact time the pregnant solution sees - and contact time is what decides whether the carbon reaches its loading potential.
- **Pressure drop.** Pressure drop through a packed bed rises steeply as voidage falls. This is why a bed that has accumulated fines, platelets or scale shows a climbing pressure differential long before any assay changes.
- **Flow distribution.** Uniform voidage gives uniform flow. Local regions of low voidage - from segregated fines, flaky particles or a poorly filled column - create channelling, where solution shortcuts through part of the bed and bypasses much of the carbon.
- **How much carbon you actually bought.** Voidage plus bulk density is what converts a tank or column volume into tonnes. A 10 m³ column holds roughly 4,500 kg at 0.45 g/cm³ and 5,200 kg at 0.52 g/cm³ - a 15% difference in inventory for the same vessel.
## 5. What reduces voidage in service
- **Fines.** Fragments below the screen aperture migrate downward and pack the channels. This is a hardness and handling issue, not a voidage property of the product.
- **Platelets.** Flat, flaky particles that survive screening pack badly and create regions of low voidage. Water-polished, de-flaked carbon with platelet content held below 1.5% packs more uniformly.
- **Sieve distribution.** A wide distribution lets small particles sit in the gaps between large ones, lowering voidage. Asking for the sieve distribution, not just the nominal mesh, is how you control this.
- **Scale and organics.** Calcium carbonate from lime dosing, oils and flotation reagents all occupy void space over time.
## 6. What to measure and what to ask
**On site:** bed depth and column diameter (to give bed volume), flow rate, pressure differential across the bed, and the fines content of the screen undersize. A rising pressure differential at constant flow is the clearest early warning that voidage is falling.
**From your supplier:** bulk density with its method, full sieve distribution, and platelet content. Those three, plus hardness, predict how a lot will pack.
## Frequently asked questions
**What is a normal voidage for a 6x12 mesh activated carbon bed?**
Randomly packed 6x12 mesh coconut shell granular carbon typically sits around 0.36-0.44, with a settled, water-soaked bed toward the lower end. Treat it as an engineering-typical range and measure your own bed.
**How do you calculate bed voidage?**
Voidage = 1 − (bulk density / particle envelope density). For example, bulk 0.48 g/cm³ with particle envelope density 0.85 g/cm³ gives approximately 0.435. Both densities vary by lot, so confirm on site.
**Does voidage affect gold recovery?**
Indirectly but materially: voidage sets the solution volume held in the bed, the contact time, the pressure drop and the flow distribution. Poor flow distribution (channelling) means part of your carbon never sees the solution it was bought to treat.
**Why does pressure drop increase over time in a carbon column?**
Because voidage falls. Fines migration, platelet packing, calcium scale and organic fouling all fill channels. Check fines generation and scale before assuming the column itself is at fault.
**Does 6x12 mesh carbon pack better than 8x16?**
Coarser cuts generally give slightly higher voidage and lower pressure drop for the same flow. The bigger practical factors are sieve distribution uniformity, platelet content and the fines fraction - which is why those should be specified alongside the mesh.
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