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Flow Processing of Liquor in a Mineral Refining Plant

The aim of this project is to design a flow processing system of liquor (slurry) in a mineral (aluminum) refining plant.

Aluminum is manufactured in two phases - the Bayer process of refining the bauxite ore to obtain alumina oxide, and the Hall-Heroult process of smelting the alumina oxide to produce pure aluminum. These two phases require two separate plants - a refinery to produce alumina oxide and a smelter to obtain pure aluminium from the alumina oxide powder. These two plants are usually located at a distance from each other and therefore a transportation system of the finished alumina oxide powder from the refinery to the smelter is installed.

In alumina refineries, the Bayer process employed to obtain alumina oxide from bauxite has three major steps-Digestion, Clarification, Precipitation and Calcination. It takes about 2 kg of bauxite to produce approximately 0.5 kg of aluminum metal.

For the digestion process, bauxite which contains alumina is first ground into small particles and then dissolved in hot solution of sodium hydroxide (NaOH) at 175° C where steam and pressure are applied during this process.

Waste solids are separated from the liquor stream in the clarification process.

In the precipitation process, the alkaline liquor is cooled in a precipitating tank where alumina hydroxide precipitates out of the solution which is heated to give off moisture to produce alumina oxide (powder) during the calcination process. The finished alumina oxide powder is then transported to a smelter and/or to other destinations.

Alkaline liquor after the digestion process is transported to a thickening tank where waste solids are separated from the liquor stream by settling. The clarified liquor then is directed to open-top precipitating tank where it is agitated, cooled and seeded and the liquor then is transported to a tertiary tank. All tanks are large and the horizontal distance between the first two tanks is about 200 m. The precipitating tank has a liquor height of

27 m while the cylindrical thickening tank has a liquor height of 18 m and both the tanks are open to the atmospheric pressure. The tertiary tank which is vented to atmospheric pressure and 14 m in height is sitting on an elevation of your choosing and it is at a distance of 800 m from the precipitating tank. The flow rate of the liquor to be maintained between the tanks is in the range of 0.06m3/s to 0.1m3/s (you may choose a different flow rate for valid technical reasons).

For our analysis, we consider these liquors as Bayer liquors. The liquor (R.D = 1.16, μ = 0.024 Pa.s (assumed constant for simplicity), Pvap = 13.4 kPa) is first pumped from the thickening tank to the precipitating tank at 60° C and then is led out of the precipitating tank into the tertiary tank where the temperature of the liquor is 32° C. The density and viscosity of the liquor in the precipitating tank are increased and are R.D = 1.33, μ = 0.086 Pa.s (assumed constant) respectively. It is important that the viscosity of the liquor flowing out of the precipitation tank be measured in real time and on-line. This can be done using an on-line rheometer or other methods.

It is to be emphasized that the Bayer liquors are super-saturated with alumina and impurities such as silica and sodium oxalate derived from digestion of the bauxite ore at various stages of the process. These liquors are very prone to scaling (deposition of materials on surface). Scaling leads to serious on-going technical problems and is a major cause of production loss due to equipment downtime required for descaling and cleaning operations.

For this project you are required to undertake a limited investigation into scale mitigation/suppression in a precipitation tank. You can get the required information on this topic from the literature and some technical articles that have been published in various journals. One such article is by J. Wu et al (Swirl flow agitation for scale suppression, International Journal of Mineral Processing, 112-113 (2012) 19-29).

Research indicates that scale in a precipitating tank can be significantly reduced if the wall velocity of the liquor is in the range of 0.5 m/s to 2 m/s. It is desired that such a velocity be produced using the blade agitator in the precipitating tank. You are directed to Fig.1 of the above article for your reference. You are expected to analyze the range of wall velocity for suppression of scale in the precipitation tank as discussed in the published articles. Your task is to design an agitator system using appropriate blades with the required speed for scale suppression in the precipitation tank.

1. For the liquor slurry transportation system.
Your submission must have a schematic diagram of the layout of the system featuring

(a). The proposed piping system for the liquor with appropriate pumps and fittings (b). An agitation system for suppression of scale formation in the precipitation tank (b). An on-line capillary rheometer or any other method for viscosity measurement

and showing the locations and elevations of all elements you would include in the systems. The actual position/location of the pumps, tanks, fittings etc are your choice.

You need not provide costing of the materials in your design. However, it is expected that technical skills and professional judgment are used in selecting the number and type of materials in your design.

You should include common practical features that go with this design, namely

(i) Pipes and pumps: their types, material and size appropriately selected from technical analysis
(ii) The valves (eg, globe/gate valves), bends, expansion, reducer, tees in the system (suction and delivery).
(iii) Your design must also consider minimum "Health and Safety" practices required for a process plant where the liquor is found corrosive.

Your report and design should have a through technical analysis and, in particular, should include
(i). System equation(s)
(ii). Selection of pipe material and size
(iii). Selection of type and size of pump(s) (pump characteristics) (iv). Duty points
(v). Efficiency value & safe operation range without cavitation (vi). Electricity cost per day for your chosen design.

(i). Brief research on scaling in precipitation tank and possible methods of scale mitigation
(ii). Design of a simplified flow/agitator system in the precipitation tank
(ii). An analysis of velocity and power required for your design for scale suppression.

(i). Brief research on composition and rheological properties on the behavior of bauxite slurry (liquor)
(ii). Theory and schematic design of rheometer (description of theory, its location and about how the rheometer would work in your design).

2. For the transportation method of alumina oxide powder to the smelter

You need to consider the following:

Alumina oxide is a ceramic with properties suitable for a coating material and electrical insulator. It has a density of 3.95 kg/m3 and its other mechanical and electrical properties are readily available from the public domain.

Currently, most of the refineries in Queensland transport alumina oxide to smelter via conveyer belts. It is expected that a limited research is conducted for an alternative method which could include mechanical/electrical or pneumatic (vacuum/pressure) method. The transportation system need not run continuously, however, the system should be capable of running as many hours as needed to provide the required supply.

This design should include as a minimum a schematic diagram and technical description of the operating principles of the proposed system and a brief technical justification (may include basic calculation if needed) for a comparative assessment of the merit of the proposed system.

Note that any information NOT given is at DESIGNER'S CHOICE. You are strongly recommended to make use of reference materials from the library, relevant websites and any other sources, for example product suppliers.

PART A: General
A.1 Introduction, aim and objectives of the project
A.2 Brief literature review relevant to this project
A.3 Brief description of related systems
A.4 Assumptions and data presentation
A.5 Referencing and indicated team members' % contribution

PART B: Pump system design and calculation
B.1 Clarity and schematic drawing showing relevant components
B.2 System equation (static and dynamic head, head loss, etc.)
B.3 Fittings-K values Table
B.4 Pipe consideration (material, diameter, length, f, losses etc.)
B.5 Duty point
B.6 Pump characteristics
B.7 Pump selection (Specific Speed)
B.8 Pump power, efficiency, electricity cost, etc.
B.9 Cavitation check (NPSHA)
B.10 Correct method/accuracy

PART C: Scaling and agitation system in precipitation tank
C.1 Brief literature on scaling and scale mitigation
C.2 Design of a simplified agitator system in precipitation tank
C.3 Analysis of velocity and power required for scale suppression

PART D: Rheometer design
D.1 General description and assumption
D.2 Design/schematics and theory
D.3 Location of installation and soundness of operation

PART E: Alternate transportation system design
E.1 General discussion on type of transportation
E.2 Schematics and operating principles
E.3 Justification/comparative assessment with existing method

PART F: Health and safety issue
F.1 Safety procedures, pipe, pump safety and discussion
F.2 Storage and handling (obstacles and dangerous location)
F.3 Applicable control measures, storage and safety colours

PART G: Others
G.1 Conclusion and recommendations
G.2 Oral presentation and overall report presentation

*Note: Only part D from the criteria is required with designing of rheometer.

Mechanical Engineering, Engineering

  • Category:- Mechanical Engineering
  • Reference No.:- M93113272

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