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# Lecture 2

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### Lecture 2

1. 1. Prof.Dr.Salah Elmofty Ph.D.professor Material Technology Department of Mining Petroleum &Metallurgical, Faculty Of Engineering E-mail : [email_address]
2. 3. The main purposes of fine grinding during dressing of minerals: <ul><li>1-Liberation of valuable minerals from waste minerals of the ore. </li></ul><ul><li>2-Grinding to size meeting specific process requirements (for flotation). </li></ul><ul><li>3-Grinding to meet commercial product requirements. </li></ul>
3. 4. Principal draw backs of the grinding mills: <ul><li>1-Fine grinding is a relatively expensive operation. </li></ul><ul><li>2-Fine grinding machines are inherently inefficient. </li></ul><ul><li>3-The production of excessive quantity of material is a waste both of </li></ul><ul><li>energy &of ore values </li></ul>
4. 5. Theory of mill operation <ul><li>. </li></ul><ul><li>π </li></ul><ul><li>π 2 </li></ul>Mv r = Mg cos N 2 N Mg M = Mass of ball V = linear velocity r = radius of the mill n = number of revolutions per min . v= 2 π rn COS N = 4 2 g n r 2 mv r 2
5. 6. Grinding action <ul><li>- The grinding in ball mill is therefore caused due to: </li></ul><ul><li>1-Cascading action of grinding media. </li></ul><ul><li>2-By impact due to fall of grinding media by gravity. </li></ul><ul><li>3-Crushing by heavier and harder lamp. </li></ul><ul><li>4-Kinetic energy of falling and rolling grinding media. </li></ul>
6. 7. -The grinding in ball mill based on: <ul><li>1-Nature of the ore. 2-Feed size. </li></ul><ul><li>3-Pulp density. 4-Balls size </li></ul><ul><li>5-Liners design. 6-State of dispersion. </li></ul>
7. 14. Critical speed of the mill <ul><li>-The faster the mill is rotated,the higher the balls will be lifted. </li></ul><ul><li>-The minimum speed at which a mill charge will centrifuge is known as the critical speed of the mill and is given by formula: </li></ul>√ N = 54.19 R N = Critical speed . R=Radius of the mill .
8. 15. Power consumption <ul><li>-The energy that mill consumes is a function not </li></ul><ul><li>only of the speed but also of the ball load,the </li></ul><ul><li>specific gravity of the ore and the dilution of the </li></ul><ul><li>pulp. </li></ul>
9. 16. The advantages of ball mill <ul><li>(i) Low power consumption for a given reduction . </li></ul><ul><li>(ii) Simple operation . </li></ul><ul><li>(iii) If over loaded ,it can be brought back to </li></ul><ul><li>normal in a very short time owing to its free discharge and rapid forward propelling action . </li></ul>
10. 17. Liners <ul><li>-Various materials such as cast-iron , carbon and alloy steels are used for liners , manganese steel liners are most common in </li></ul><ul><li>ore milling plants . </li></ul><ul><li>-Recently soft rubber is gaining important in the </li></ul><ul><li>line </li></ul>
11. 18. Mill shell liners
12. 19. Tube mill Vs ball mill <ul><li>1- Tube mills are cylindrical mills of relatively great length. </li></ul><ul><li>2- Ball mills in general include all mills in which </li></ul><ul><li>steel balls are used as grinding media . </li></ul><ul><li>3- Tube mill 5’ 6” in diameter and 22’ long are standard. </li></ul>
13. 21. Rod mill <ul><li>-Rod mill is similar to ball mill in appearance and general principals. </li></ul><ul><li>-Rod mill differ from ball mill in its grinding action , in that the rods are kept a part by the coarsest particles . </li></ul><ul><li>-Consumption of rod and liners of energy is almost the same as in ball mill employed for a similar duty using steel balls. </li></ul>
14. 25. Autogenous mill <ul><li>-Grinding takes place only between ore and ore. </li></ul><ul><li>-Autogenous mills are tube mills in which bigger ore lump ( 4” to 6” size ) are used instead of flint pebbles. </li></ul><ul><li>. </li></ul>
15. 30. Closed circuit and open circuit <ul><li>(a) In closed circuit operations there is closer approach to a uniform ratio of ball size to average particle size throughout the length. </li></ul><ul><li>(b) Over grinding can be avoided in closed circuit as there is continuous reduction in proportional of finished size in the mill . </li></ul><ul><li>(c) Power consumption is lower in open circuit than closed circuit. </li></ul><ul><li>(d) Time of grinding is lower in open circuit than closed circuit. </li></ul><ul><li>(e) There is higher tonnage in open circuit than closed circuit. </li></ul>
16. 31. <ul><li>Moussa El Mewafy </li></ul><ul><li>Hany Salah Ahmed </li></ul><ul><li>Emad El Dein </li></ul><ul><li>Ahmed lotfy </li></ul><ul><li>Assem Abd El naby </li></ul>
17. 32. Thank You