Crushing Circuit Mass Balance

What a crushing circuit mass balance is

Every belt, every screen deck and every stockpile cone in a plant carries a specific tonnage per hour. A crushing circuit mass balance is the exercise of writing down how the tonnage entering from the quarry distributes itself across those streams: what goes in must equal the saleable products plus whatever keeps circulating inside the circuit. If the arithmetic does not close, either a stream was left off the sheet or there is a spillage, an overflow or a quietly growing stockpile that nobody has accounted for.

The value of the exercise shows up twice. Before a plant is built it is the only defensible basis for choosing belt widths, drive power and stockpile area. In a running plant it settles arguments about where the bottleneck sits, because lost capacity usually appears not in the crusher itself but in a narrow stream immediately upstream or downstream of it.

Defining the streams

The feed stream

The starting point is the fresh feed passing through the feeder. That figure comes either from truck counts or from the feeder's set discharge rate. Moisture and the proportion of oversize lumps after blasting act directly on this stream: wet, clayey feed reduces scalping capacity, so the share reporting to the scalper undersize shrinks.

Undersize and oversize

The main screen divides whatever reaches it according to its apertures. Material finer than the aperture reports to the product streams; material coarser than it returns to a crusher for a second pass. That returning stream raises the total tonnage the crusher actually sees above the fresh feed figure, which is exactly why sizing a crusher or its feed belt on fresh feed alone is a mistake.

Stockpile and loadout

Each product fraction builds its own cone. If the stockpile is not treated as a stream in its own right, nobody knows how many tonnes and how many cubic metres an hour of production occupies by the end of the shift, and the loader and truck plan has no basis.

A worked example at 200 tonnes per hour

Take a three-fraction circuit running closed. Fresh feed is 200 t/h and the oversize stream returning to the crusher is 60 t/h. The main screen therefore sees 260 t/h in total, while the tonnage leaving the circuit as product must still add up to 200 t/h. Distribute the products as 40 t/h of 0-5 mm, 60 t/h of 5-15 mm and 100 t/h of 15-25 mm; the sum equals the fresh feed and the balance closes.

In the last column each belt is selected for roughly 1.25 times the tonnage it carries. That margin absorbs swings in feeder rate and differences in moisture; sizing a belt for the exact figure means spillage at the first surge.

How screening efficiency distorts the balance

The table above assumes the screen passes all of the fine material presented to it. In reality some of it is dragged along with the oversize. Screening efficiency expresses how much of the material that should have passed the aperture actually did: fines through the deck divided by total fines in the feed. When efficiency drops, the circulating stream grows, the crusher spends energy re-crushing material that was already fine enough, and product belt tonnage falls. Deck aperture, deck slope and mesh type therefore belong inside the mass balance, not in a separate discussion.

From tonnes to cubic metres

Stockpile area is measured in cubic metres, not tonnes. Loose bulk density for crushed stone generally sits between 1.5 and 1.7 t/m³, so the 800 tonnes that the 15-25 mm stream produces over an eight-hour shift occupies roughly 470 to 530 m³. Cone volume follows from base radius and height, and height is limited by the angle at which the material stands. Crushed aggregate typically rests at an angle of repose between 35° and 45°, and a cone cannot be forced steeper than that.

Turning the sheet into equipment selection

Once the table exists, equipment selection becomes mechanical: the tonnage on each stream sets the belt width serving it and the screening area required. With the sheet in hand, the capacity needed at each stage of a stationary crushing plant configuration falls out on its own. The same logic applies to mobile installations, where only belt lengths and stockpile distances shorten. For the wider context, the mining engineering articles cover neighbouring ground.

Three recurring mistakes

First, sizing the crusher on fresh feed: in a closed circuit it works on part of a 260 t/h flow, not 200. Second, writing undersize percentages from product demand instead of from the feed gradation; demand does not drive the arithmetic, gradation does. Third, omitting the stockpile stream altogether and then being surprised when the yard fills up by the end of the shift.

Questions

Why does the balance never close exactly?

Fines drawn into the dust extraction system, carryback lost along the belts and moisture loss are small but genuine streams. A discrepancy of a few percent is normal; ten percent points to a measurement error or a stream that was never written down.

What measurements are enough?

The feeder rate, a belt weigher reading on each product belt and one sieve analysis taken from the screen feed. Without those three the table rests on guesswork.