Mechanical Operations - AMIChE Exam Questions

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1. A ball mill 1.3 in dia is being run at 60 rpm. It is found that the mill is not working satisfactorily. Analyze the operation and suggest the remedy

2. A ball mill 1.2 m in dia is being run at 0.85 Hz. It is found that the mill is not working satisfactorily. Suggest any modification in the conditions of operation

3. 89.5 KW power is required to crush certain material from 50.8 mm to 6.35 mm. What will be the power required using the same feed to crush to 3.18 mm?

4. Calculate the average recirculation load of a mill which is closed circuit with a classifier and grinds 300 Ton of dry ore per day. The screen analysis of the mill discharge, classifier overflow, and classifier underflow are as follows:

   Mesh	Mill Discharge %	Classifier
   		Overflow %	Underflow %
   -65+150	15.2	19.6	13.8
   -150+200	6.1	12.4	4.1

5. Anthracite coal from a pulverization unit has been found to contain an excess of fine material (75% by weight). In order to remove these fines, it is screened using a 1.5 mm screen. Estimate the effectiveness of the screen from the following data:

   Particle size mm	Oversize from screen	Undersize from screen
   -3.33+2.36	0.143	0.00
   -2.36+1.65	0.211	0.098
   -1.65+1.17	0.230	0.234
   -1.17+0.83	0.186	0.277
   -0.83+0.59	0.196	0.149
   -0.59+0.42	0.034	0.101
   -0.42+0.29	0.00	0.141

6. The size analysis of a powder is carried out by sedimentation in a vessel having sampling point 180 mm below the liquid surface. If the viscosity of liquid is 2 mNs/m² and the density of the powder and liquid are 2500 and 1100 kg/m³ respectively. Determine the time which must elapse before any sample will exclude particles larger than 20 mm size.

7. It is desired to separate into two pure fractions a mixture of quartz and galena of a size range from 0.015 mm to 0.065 mm by the use of hindered settling process. What is the minimum apparent density of the fluid that will give this separation? How will the viscosity of fluid affect the minimum density. Density of galena = 7500 kg/m³
   Density of quartz = 2690 kg/m³

8. A mixture of silica and galena particles having a size range of 5.21 mm to 2.50 mm is to be separated by hydraulic classifier. The specific gravity of silica is 2.65 and that of galena is 7.5. Calculate the size range of the various fractions obtained in settling.

9. Quartz having a specific gravity of 2.65 and hematite having a specific gravity of 5.1 are present in the mixture of particles. It is desired to separate them by sink and float method using a suspension of fine particles of ferrosilicon having a specific gravity of 6.7 in water. At what consistency in Vol% ferrosilicon solids in water should the medium be maintained for the separation?

10. A slurry containing 5 kg of water per kg of solids is to be thickened to a sludge containing 1.5 kg of water per kg of solids in a continuous operation. Laboratory tests using five different concentrations of the slurry yielded the following results:

    Concentration kg water/ kg solids	5.0	4.2	3.7	3.1	2.5
    Rate of sedimentation mm/sec	0.20	0.12	0.094	0.074	0.05

    Calculate the minimum area of a thickener to effect the separation of 1.33 kg of solids per second.

11. An experimental filter press having an area of 0.041 m² is used to filter an aqueous BaCO₃ slurry at a constant pressure of 267 kPa. The filtration equation was obtained as:
    t/V = 10.25 x 10⁶ V + 3.4 x 10⁶ where t is in sec and V is in m³. If the same slurry and conditions are used in a leaf press having an area of 6.97 m², how long will it take to obtain 1.00 m³ of filtrate?

12. In a plate and frame filter (no cloth resistance) it takes 2 hour 15 minutes to filter 18 m³ of suspension. Filter cake is washed with 2 m³ of water having viscosity of filtrate. If washing velocity is 1/4th of the filtration velocity at the final moment, calculate washing time.

Last Modified on: 04-Feb-2022

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