Tertiary crushing

Before magnesite is calcined, it undergoes a third crushing step (tertiary), where it is generally reduced in size to less than 1 in. using short head cone crushers. 

---

If the process has undergone primary, secondary, and tertiary crushing, a process of screening is undertaken to separate the very coarse from finer particles. The particles are then further reduced in size by milling in rod nulls with the addition of water. The still dilute slurry is transported to a ball mill where steel balls achieve further reduction in size. The mixture of slurry and steel balls undergoes a separation at the exit from the ball mill (by using the difference in momentum between the two materials). Slurry is then diverted into a pump box. which is connected to slurry pumps. The slurry is pumped to cyclones, where the coarse particles are further separated from the fine particles. 

Crushing the rocks is often achieved in two or three stages. The secondary and tertiary crushing machines resemble the machines used during primary crushing. They consist of vertical cone crushers or horizontal cylinder crushers. The former type is the most widespread. 

A novel comminution device, the roll press, has achieved commercial success [Schoenert, International J. Mineral Processing, 22, 401-412 (1988)]. It is used for fine crushing, replacing the function of a coarse-baU mill or of tertiary crushers. Unlike ordinary roll crushers which crush individual particles, the roll press is choke fed and acts on a thick stream or ribbon of feed. Particles are crushed mostly against other particles, so wear is veiy low. Energy efficiency is also greater than in ball mills. 

By showing numerous impellers, motors and effieieneies for one pump, the family eurve has a lot of information crushed onto one-graph. So to simplify the eurve, the effieieneies are. sometimes shown as eoneentrie eireles or ellipses. The eoneentrie ellipses demonstrate the primary, seeondary and tertiary effleieney zones. They are most useful for eomparing the pump eurve with the system eurve. (The system eurve is presented in Chapter 8.)... 

Primary gyratory Secondary c ne crushing Tertiary short-head crushing One- or two- age grinding... 

These tertiary crushers employ smooth or toothed heavy-duty impact and abrasion-resistant steel-rimmed rolls. The rolls are mounted inline in a horizontal manner and turn toward each other at equal speeds to create a nip into which a friable feed material is introduced (Fig. 4). Heavy-duty compression springs with automatic reset are used to dampen crushing shock and to protect the crusher from tramp iron and oversize material. An adjustable screw that adjusts spring tension changes the crusher opening. A flywheel is used to even out pulses and economize on power consumption. These crushers have a theoretical maximum reduction ratio of 4 1 and will only crush materials to about 10 mesh. Roll crushers produce a controlled product size distribution without a lot of fines. The narrow particle size distribution is achieved by controlling a combination of variables including roll speed, gap measure, differential speed, feed rate, and roll surface. 

Tertiary crushers are used to help balance the demand between sizes. By selecting primary and secondary crushers that produce more coarse material than is required on average, the amount of fines produced is kept to a minimum. Tertiary crushers are generally used to crush surplus quantities of larger products into the smaller sizes. 

Primary crushing may reduce as mined ore to less than 200 mm and then, in the conventional approach, it is further reduced in secondary and tertiary crushers followed by rod and ball mill grinding to the required particle size. Final sizing classification is usually achieved by operating the ball mill in closed circuit with hydrocyclones. 

Lorry or van bombs Lorge devices (>40 kg) Lorge secondary missi es ore formed Fragmentation injuries, tertiary injuries and crush from building collapse... 

---