Grizzly Feeder Design and Scalping Before the Primary Crusher

Vibrating grizzly feeders are used as feeders that feed run-of-mine (ROM) material, generally obtained by blasting, to the primary crusher in front of them. The maximum lump size of ROM material can reach 1500 mm. Depending on the size of the primary crusher they feed, their feed capacities can reach 1500 t/h, and the feeder must be able to deliver this capacity.

This article covers what a vibrating grizzly feeder does, why fine and clayey material is bypassed before the primary crusher, the hopper above the grizzly, and the design variables that set capacity, including the VSMA method for grizzly area. Feeder selection across all feeder types is covered in a separate guide.

What a Vibrating Grizzly Feeder Does in a Crushing Plant

What is a grizzly feeder in a crushing plant? It is a feeder with two jobs. The first is feeding: as a grizzly feeder for crusher duty at the primary stage, it feeds ROM material to the primary crusher in front of it. The second is pre-screening: thanks to the grizzly screening section with one or more steps at the front of the feeder, fine and dirty material that is not wanted in the crusher is bypassed and kept out of the crusher. This is the grizzly feeder working principle in short: feeding and scalping in one unit.

Figure 1 shows the general view of a MEKA vibrating grizzly feeder body.

Scalping: Why Fines and Clay Are Bypassed Before the Primary Crusher

In primary crushers, material smaller than the crusher discharge setting entering the crusher is generally not desired, because of crusher performance. It is also desired that clay and similar material in the ROM feed is separated from the plant, that is bypassed, before it enters the crusher. For this reason, thanks to the grizzly screening section with one or more steps at the front of the feeder, the fine and dirty material that is not wanted in the crusher is bypassed and prevented from entering the crusher. In this role the unit works as a scalping feeder.

Where the feeder and the grizzly screen are built as separate units, the grizzly section should have at least two steps. In this way, the material drops and turns as it passes from one step to the next, and clay and other foreign material stuck to it separates and goes to the bypass line. By designing the grizzly screen as a double-deck unit when needed, it is possible to remove unwanted dirty material from the plant without using a separate bypass screen.

Grizzly feeders and scalpers built as separate units are presented on the pan feeder with grizzly scalper page, and a MEKA vibrating and grizzly feeder installation in Serbia is described in a separate case study.

The Grizzly Hopper: Truck Size, Impact Loads and Wear

ROM material can be brought to the grizzly feeder hopper by dump trucks with capacities of up to 50 m³. For this reason, the hoppers of these feeders must have a high capacity. Feeder bodies and main frames must also withstand the high impact loads that large lumps create during dumping. If the material is abrasive, the necessary measures against wear must be taken both in the hopper and in the feeder body.

Vibrating Grizzly Feeder Design Variables That Decide Capacity

The capacity formulas are given in a separate guide on vibrating feeders. This section gives the parameter ranges used in grizzly feeder design and the VSMA method for sizing the grizzly area.

Deck Width and Grizzly Length

The width of the feeder part of the vibrating feeder–grizzly screen combination is determined in the same way as for vibrating grizzly feeders. If the size distribution curve of the feed is known, the area of the grizzly screen to be used in the combination, and therefore the length of a screen whose width is already set, can be calculated with the VSMA method. For a sound and correct selection, the size distribution curve of the feed must therefore be known.

VSMA formula for grizzly screens:

IE_a = 0.907 · U / (Factor A · Factor B · Factor C)

• IE_a: grizzly section area, m²
• U: capacity of material to pass under the grizzly section, t/h

Factor A, Factor B and Factor C values can be taken from Table 2, depending on the grizzly opening and the size distribution of the feed.

Factor B — half-size factor

Factor C — oversize factor

Table 2. Factor A, Factor B and Factor C values

Amplitude, Speed and Slope

For vibrating grizzly feeders, values of 2.5–3.5 are suitable for the throw number Γ. In vibrating grizzly feeders, n, the revolutions per minute of the exciter element, is selected between 650 and 1000 rpm according to the type of exciter mechanism used. The throw angle α, the angle of the line of vibration to the horizontal, is selected between 35° and 55°; 35° is the most widely applied throw angle.

• β: angle of the trough plane to the horizontal
• μ_β: slope factor, taken as follows:

For vibrating grizzly feeders, the theoretical travel speed can be determined approximately from Graph 1, which gives it for a throw angle of α = 45° at different amplitude values. Figure 2 shows a sketch of a vibrating grizzly feeder.

Measuring amplitude on an installed feeder is covered in a separate service article.

Material Size, Moisture and Bed Depth

The material size factor μ_s is taken, based on experience, as:

• μ_s = 1 for sand

• μ_s = 0.8–0.9 for material sizes up to 150 mm

• μ_s = 0.6 for material larger than 150 mm

The coefficient μ_m, related to the moisture content and contamination of the material, is taken as:

• μ_m = 1 for dry material

• μ_m = 0.8 for moist material

• μ_m = 0.6 for moist and clayey material

In the calculations, the bed depth at the feeder discharge, H, is taken on certain assumptions based on experience, where B is the inner width of the feeder:

• H ≤ 0.2 B for sand and fine gravel

• H ≤ 0.3 B for ROM material up to 150 mm

• H ≤ 0.5 B for material larger than 150 mm

Vibrating Grizzly Feeder or Separate Feeder and Grizzly Screen?

In crushing and screening plants, in some cases the feeding and grizzly groups are separated from each other and applied as separate units, to screen out the fines in the feed more efficiently than a vibro grizzly feeder does, raising crusher performance and reducing wear costs. Driving the grizzly screen independently of the feeder allows variables such as the throw angle, vibration amplitude and angular velocity to be set independently of the feeder, so screening becomes more efficient.

Figure 3 shows a view of a MEKA pan feeder with grizzly scalper.

MEKA vibrating grizzly feeders are presented on the grizzly feeder product page. Other feeder types, including the wobbler feeder, are listed on the wobbler feeder page.

FAQ

What is the difference between a vibrating grizzly feeder and a feeder with a separate grizzly screen?

In a vibrating grizzly feeder, the grizzly screening section, with one or more steps, is at the front of the feeder. In a pan feeder with grizzly scalper, the feeding and grizzly groups are separated and applied as separate units. Driving the grizzly screen independently of the feeder allows the throw angle, vibration amplitude and angular velocity to be set independently of the feeder, so screening becomes more efficient.

How is moist, clayey feed taken into account in a vibrating grizzly feeder?

In the capacity calculation, the moisture and contamination factor μ_m is taken as 1 for dry material, 0.8 for moist material and 0.6 for moist and clayey material. It is desired that clay and similar material in the ROM feed is bypassed before it enters the crusher; the grizzly screening section at the front of the feeder bypasses this dirty material. In a grizzly section with at least two steps, the material drops and turns as it passes from one step to the next, and clay stuck to it separates and goes to the bypass line.

Why is fine material kept out of the primary crusher?

In primary crushers, material smaller than the crusher discharge setting entering the crusher is generally not desired, because of crusher performance. In some cases, the feeding and grizzly groups are separated into separate units to screen out the fines in the feed more efficiently, raising crusher performance and reducing wear costs.