The short answer
Oversize is any piece of blasted rock too large for the primary crusher, in practice anything over about 80 per cent of the jaw's feed opening. Most of it comes from four places: the uncharged collar at the top of the bench, burden or spacing set too wide, holes that wandered off line, and natural joints that cut the rock into blocks. Each boulder is paid for twice, once in the blast and again under a breaker.
A shot can look good from the crest and still be a bad shot. The muckpile has thrown forward, the face behind it is clean, and under the fines sits a layer of blocks nobody can load. They surface one bucket at a time over the next week.
01Where boulders come from
| Source | What you see in the muckpile | Usual cause |
|---|---|---|
| Collar zone | Slabs on top of the pile, flat on one side | Stemming column too long, or a hard cap rock |
| Between holes | Blocks through the body of the pile | Spacing too wide, or two holes that converged |
| Front row | Large pieces at the toe of the pile | Uneven face leaving excess burden at the bottom |
| Back and sides | Boulders along the new face | Back-break along joints, loose rock from the last shot |
| Geology | Blocks with weathered, stained faces | Joints had already cut the rock before it was drilled |
The stained faces are the useful clue. A boulder with fresh broken rock on every side was made by the blast and can be designed out. One with brown, weathered faces was a block in the ground before anyone arrived, and the explosive gases escaped along its joints instead of breaking it. Those need a tighter pattern across the joint spacing or they need accepting.
02Putting a number on it
Oversize gets managed when it gets counted. Two measures are enough. The first is breaker hours per thousand tonnes fed to the crusher, which costs nothing to record. The second is a count of pieces set aside per shot. Track both against the pattern that produced them and after a dozen shots the bench is telling you which change paid.
03What one boulder costs
- The excavator stops loading to push it aside, and the tipper under the bucket waits.
- A breaker, on a second excavator, spends several minutes on it. That machine, its operator and its diesel are doing nothing else.
- The pieces are loaded a second time.
- If it is missed and tipped into the hopper, it bridges across the jaw. The feeder stops, somebody brings a hook or a breaker to the hopper, and the whole plant behind it runs empty until it clears.
The fourth is the expensive one. A 250 TPH plant standing for twenty minutes has lost about eighty tonnes of production, and bridging rarely happens once in a shift. Oversize looks cheap at the face and expensive at the hopper, where everything behind it waits.
04The fixes, in the order they usually pay
Shorten or split the stemming
The top two or three metres of every hole carry no explosive. In a hard cap that zone breaks only by being lifted and dropped. Reducing the stemming length is the first thing to try, within what fly rock control allows. Where it cannot be reduced, a small separate charge placed in the stemming zone breaks the collar rock without unloading the hole.
Check the holes are where the pattern says
Blocks through the body of the pile, appearing in some shots and not others, point at drilling accuracy before they point at design. The arithmetic of how little angle it takes is in blast hole deviation.
Then tighten the pattern
Closing burden and spacing by ten per cent raises drilling and explosive per tonne by nearly a quarter. That is a real cost and it should be weighed against the breaker hours it removes, which is the calculation in reading a drilling pattern.
Look at the timing
Delays that are too short fire neighbouring holes almost together, and the rock has not started to move before the next charge goes. Each hole needs a free face to break towards. Giving it one is a matter of milliseconds, and it changes fragmentation more than most adjustments to charge weight.
05Where to stop
A shot with no oversize at all has been over-drilled and over-charged, and it has made more fines than the plant can sell. The target is the lowest total cost at the stockpile, which usually leaves a few per cent of the pile for the breaker. Finding that point takes one firm looking at the drill log, the explosive issue and the crusher's downtime sheet together.