Chrome-Corundum Castables: Cr2O3 Content, Water Addition and Dry-Out Control

Chrome-Corundum Castables: Cr2O3 Content, Water Addition and Dry-Out Control

Chrome-corundum castable is often treated as a synonym for "premium castable", but what you actually pay for is the chrome oxide. That premium only pays back where the slag is high in lime and alkali, service temperature stays above 1400 C, and molten slag either sits on the lining or sweeps across it at speed. Put it in the wrong place and it cracks earlier than a plain low-cement high-alumina castable, because it is denser and far less thermal-shock resistant. Put it in the right place and the same lining thickness can run for two years instead of eight months.

1. First decide whether the location justifies chrome

Three numbers settle it: slag basicity, service temperature, and whether slag dwells on the surface. Basicity here means CaO/SiO2; above 1.5 the slag attacks an alumina matrix quickly. Sustained temperatures above 1400 C start dissolving the mullite bond. And where slag hangs on the wall or pools in a corner, attack is continuous rather than intermittent. Two out of three, and chrome-corundum is worth pricing.

Locations that usually justify it: the grate sidewalls and arch of waste-to-energy boilers, the secondary combustion chamber outlet, the slag tap and quench section of coal-water slurry and waste-liquid gasifiers, the lower shell and launder area of aluminium melting furnaces, and the 300 mm band above and below the slag line in steel ladles.

Locations that do not: anything above 1600 C for long periods, and any strongly reducing or high-CO atmosphere. Under low oxygen potential, Cr2O3 reduces to metallic chromium or lower oxides, and the volume change cracks the lining from the inside. Blast furnace stacks and coke oven chambers should stay on carbon or silica materials. Above 1600 C, Cr2O3 also volatilises as CrO3 at a measurable rate, so the lining thins year by year and needs periodic thickness surveys.

2. Splitting the 3%, 6% and 12% grades

The three grades are not a ladder of quality; they are three different balances between slag resistance and thermal shock resistance.

The 3–5% grade leans on slag penetration resistance while staying economical. Thermal cycling resistance (1100 C water quench) runs about 15–20 cycles. It suits medium-basicity slag and locations dominated by gas-borne particle erosion, such as the drying section of an incinerator or a flue gas turning section.

The 6–8% grade is the workhorse. It covers both slag and thermal shock, and fits incinerator grate sidewalls, gasifier slag taps and non-ferrous launders. Apparent porosity should stay under 16%; above 18%, slag travels through the open pores and reaches the anchors.

The 10–12% grade is for high-lime, high-alkali slag with long dwell times, such as the slag section of a secondary combustion chamber or an aluminium dross furnace. The trade-off is thermal shock resistance dropping to 8–12 cycles, so heat-up and cool-down rates have to be held down and emergency repairs cannot involve rapid quenching.

One practical warning: higher Cr2O3 pushes the sintering temperature up. At 10% and above, firing needs 1550–1600 C, or a small addition of a sintering aid such as TiO2 or MgO. If a supplier only says "chrome corundum", ask for the actual Cr2O3 figure and the test method. The price gap between 3% and 12% material can exceed 100%.

3. Water at 5.0–5.8%: beyond 6.0% the premium is wasted

These castables are ultra-low-cement systems: 1–2% cement, 4–6% microsilica, with dispersants carrying the flow. The water window is narrow, typically 5.0–5.8%, and the supplier's flow curve should define the exact figure.

Why the tolerance is so tight: every extra 0.5 percentage point of water raises apparent porosity after drying at 110 C by roughly 1–1.5 points, drops cold crushing strength by 8–10 MPa, and multiplies slag penetration depth. You bought chrome to resist slag; excess water gives half of that back.

Three field controls:

Weigh the water. Use a graduated container and calculate against dry weight. Do not judge by eye with a plastic scoop.

Measure flow. On a flow table, 140–160 mm is the working range. Above 180 mm means too much water or dispersant. Below 130 mm, do not fix it by adding water; check first whether the material has picked up moisture or the mixing time was short.

Control the mixer. Forced-action mixer, 1–2 minutes dry mixing so the chrome fines and microsilica pre-disperse, then 3–5 minutes wet mixing, and place within 20 minutes of discharge. Chrome concentrate is typically ground below 0.045 mm and will ball up if tipped straight into water, so it must be dry-blended with the microsilica first.

Size grading is built around a 5–8 mm top size, with 5–3 mm : 3–1 mm : 1–0 mm close to 35:30:35 and 30–35% of the mix below 0.074 mm. When fines run short, flow drops, the crew adds water, and porosity climbs. That loop is the single most common cause of a lining that underperforms its datasheet.

4. Anchors, expansion joints and formwork

Use 310S or 253MA anchors, 10–12 mm diameter, on a 200–250 mm staggered grid. Keep the tip 15–20 mm back from the hot face so the anchor sits in a cooler zone instead of standing in the flame. Wrap the tip with 1–2 mm tape or coat it with bitumen to allow axial expansion; without it, the anchor pushes the lining apart during dry-out. Weld both sides, 4 mm leg, chip the slag and inspect. A missed weld shows up as an anchor backing out after two or three months.

Where gas velocity is high, add a hex-mesh (tortoise shell) layer 15–20 mm deep, spot-welded to the anchors, with the cells fully compacted and no voids left behind.

Expansion joints go in every 1.5–2 m, 3–5 mm wide, with 3 mm ceramic fibre board or bitumen board. The critical detail is that the joint must run from the hot face all the way to the anchor tips. Many sites press fibre board only 20 mm into the surface; it looks like a joint and behaves like a crack initiator. Corners, pipe penetrations and burner openings need their own joints, widened to 5–8 mm.

Coat the formwork with release agent and mitre internal corners at 45 degrees. No standing water inside the formwork in the 24 hours before placing. Timber formwork draws water out of the mix, so the face against the board ends up with a distorted water-to-binder ratio and lower strength, which is exactly where corners spall when the form is struck.

5. Dry-out: never skip the 300 C hold

Chrome-corundum castings are dense and have low permeability, so steam from free and chemically bound water cannot escape easily. That is the whole reason this material explodes more readily than high-alumina castable.

A workable schedule for 200–300 mm lining thickness:

Ambient to 150 C at 15 C/h, hold 8 hours. 150 C to 300 C at 10 C/h, hold 6–8 hours. 300 C to 600 C at 20 C/h, hold 4 hours. Above 600 C, follow the furnace's own curve, never faster than 30 C/h.

The 300 C hold is the pivot of the whole schedule. Free water leaves between 100 and 150 C; gel and chemically bound water concentrates between 250 and 350 C. Run straight through that band and internal steam pressure has nowhere to go, leaving spall craters 50–200 mm deep. The hold is complete when the surface temperature is steady and no localised steaming remains.

Do not rely on the furnace thermocouple alone. Place measurement points every 100 mm through the lining thickness. If the surface runs more than 80 C ahead of the 100 mm depth, pause the ramp and let it equalise. For emergency work, use hot air or electric heaters rather than a flame on the surface: the face reaches 500 C while the interior is still wet, and the result is a sintered skin over a spalled core.

6. Incoming and installation checklist

Six items cover most of what goes wrong on site:

1. Packaging and dates. Check batch number, production date and shelf life. Cement-bonded material stores for six months; past three months, re-test flow at the specified water level.
2. Sampling. One set per 20 t or per batch, testing cold crushing strength after 110 C for 24 h, apparent porosity and flow value. Batches below 60 MPa or above 18% porosity do not go into critical zones.
3. Conditions. Clean mixer walls, water at 5–30 C, ambient at least 5 C, windbreak when wind exceeds force 5.
4. Placing. Layers of 300–400 mm, poker vibrator inserted fast and withdrawn slowly, spacing under 300 mm, each layer vibrated until mortar rises and bubbles stop. Over-vibration segregates the mix.
5. Curing. Strip forms after 24 hours at 20 C or above; keep the surface damp or covered until dry-out begins; re-wet if exposure exceeds seven days.
6. Records. Actual water addition, flow value, ambient temperature, anchor weld inspection and the dry-out curve, all filed together. When a lining fails later, this record is the fastest way to find out why.