The term “Primary Crusher” implies that there is more than one stage of crushing involved in the operation. It could properly be indicated as a crusher handling quarry or pit run material. As this crusher can well be the largest and most expensive piece of equipment in the plant, the most careful study should be made as to its selection.
The capacity of the primary crusher should be chosen to exceed the average capacity of the plant, since in most quarries the only surge capacity between the quarry and the primary crusher consists of rock being loaded in crushers and cars.
Operating delays will result in subsequent delays at the primary crusher. If the plant is to maintain its rated tonnage, it will have to work at better than average capacity when it starts up again. The reserve capacity could range between 25 to 50%.
Product specifications are important. If a cubical particle is required, an impact crusher may be recommended over the compression type crusher if the material is compatible to being reduced by the impact type crusher.
If a minimum of fines is important, the jaw crusher , gyratory, or cone type might be chosen over the impact type crushers. The desirability of fines would recommend the hammer mill or the Gyradisc.
There are always exceptions to the generalized rules. The seemingly infinite considerations of rock characteristics, feed size, product required, tonnage requirements, local conditions, initial and operating costs, and in each case the conditions will have to be weighed differently where a choice of crushers is to be made. The generalized rules would be used only as a preliminary guide. The final decision would have to be arrived at by thoroughly testing the rock.
As shown in the table describing the various crusher types and their ranges of application, the primary crusher may be a jaw crusher in plants designed for approximately 600-1000 TPH. When referring to the capacity table, the discharge setting is considered to be the closed side setting. The product analysis for jaw crushers is shown on the pages.
When sizing law crushers, it is considered sound practice to use a crusher setting that is near the center of the range of settings shown in the catalog. For example, operating the crusher at the tighter settings decreases capacity and creates high wear. When operating at the wider settings, a large quantity of stabs could be generated causing delays at transfer points. In addition, the stabs require a large feed opening in the secondary crushing stage, causing a loss of efficiency in the crushing cavity.
For plants larger than 600 TPH the primary crusher is likely to be a gyratory. The terminology for describing a gyratory is by first showing the feed opening at the top of the crusher followed by the mantle diameter at the discharge. For example, a 42-70 Nordberg primary gyratory crusher has a 42″ feed opening and a 70″ mantle diameter. The description of a gyratory crusher product and capacity is based on the open side setting rather than the closed side as normally used on the high speed jaw and Symons cone crushers.
The reason for this is that the primary gyratory operates at a much slower speed and consequently the topsize of the rock in influenced by the open side setting particularly during periods when the crusher is not choke fed. Generally, the topsize of the primary gyratory will have a slab size with approximately a 1-2-3 ratio. The “1” being the open side setting.
For example, a primary gyratory crusher operating at 6″ open side setting may develop slab sizes measuring 6″ x 12″ x 18″. These dimensions become important when selecting the cavity for the secondary crusher. When sizing primary gyratory crushers, it is considered sound practice to use a crusher setting that is near the center of the range of settings shown in the catalog for the same reasons outlined in the jaw crusher discussion.