For a successful sand producing circuit, the material flow should be as shown in the flow sheet and the plant diagram. An explanation of the devices and their purpose in the circuit are given in their functional sequence.
A. Pre-screening – In a typical sand producing circuit, new feed is pre-screened on a No. 4 opening screen . Feed should be prepared before it enters the Gyradisc crusher surge bin. As much fee water as practical should be removed. There are several Nordberg screens which are well suited for dewatering.
B. Surge Bin – The prepared feed enters a surge bin of sufficient capacity to sustain continuous full load operation. The size of the surge bin is related to the efficiency or continuity of the new feed rate entering the sand circuit. Normally, however, the bin sizes should be at least 20 ton live storage for the 36″ and 48″, and 40 ton for the 66″ and 84″ sizes.
C. feeder – A vibrating feeder or variable speed conveyor belt takes new feed from the surge bin and delivers it to the crusher.
D. Gyradisc Crusher – The feed enters the crusher where it is reduced into cubical shaped particles. To minimize the reaction time between the control point (feeder) and the Gyradisc crusher, it is recommended that the crusher be located as close to the bin and feed as practical. This circuit arrangement is well suited for automation or efficient manual control.
E. Screening – The crushed product should be screened on a double deck dry screen like the Nordberg GP screen. The exact sizes of the screen openings will vary and are dependent on the crushing characteristics of the materials and the final specification. Usually a cloth having a No. 4 opening is used on the top deck and No. 8, 10 or 12 is used on the bottom deck.
F. Flop Gate – The Flop gate regulates the amount of between deck material that is returned to the Gyradisc crusher. The correct percentage of material passing the top or bottom deck screen can be controlled by adjusting the flop gate to increase or decrease the recirculating load. Note the method of returning the circulating load to the Gyradisc crusher. Bypassing the surge bin eliminates size segregation in the bin and increases the effective surge capacity.
G. Classifier – Excess fines (passing No. 100) are removed through the use of a hydraulic classifier.
H. Separator – An air separator can be employed when the disposal of tailing is difficult, when water is unavailable, or where a market exists for excess fines in the form of mineral filler.
An alternate sand circuit would involve producing a screen undersize consisting entirely of a minus No. 6 or No. 8 Gyradisc crusher product. This arrangement, however, does not have the built-in flexibility available in the specification sand circuit shown in the typical flow sheet.
These two charts show the results of a test conducted, with the Gyradisc Crusher circuit in balance, comparing minus No. 4 screen undersize with ASTM C-33 Specification Sand.