Bauxite Digestion Technology for Processing of Gibbsite-Boehmite Mixed Bauxite

Hi Friends,

We appreciate your remarks on "Typical schedule for commissioning of Alumina Refinery" as realistic and systematic approach. All our papers are based on our experience in operative Alumina plants in various parts of the globe.

In this technical paper, we would like to present different techniques for processing of Gibbsite-Boehmite mixed Bauxite prevalent in various existing Alumina refineries in the World and placing our views taking into account all techno-financial aspects. 

Presence of boehmite in the range of 4 to 6% of boehmite mixed with gibbsite in Bauxite is considered negligible as it is not considered economical to recover such small component of boehmite. Thus minimum boehmite content of 8% with remaining gibbsite compensating for module more than 8 is considered viable to process using Modified Bayer process. Various technological options for digestion of boehmite-gibbsite mixed bauxite in Alumina refinery are briefly described below-

1.       High temperature digestion technology: 

In this process, mixed bauxite is digested at temperature between 240 to 250 degree centigrade at about 35 kg/cm2 pressure with 5 to 7 stages of flashing for heat recovery system. Total thermal energy requirement in this process is very high to the tune of 18 to 21 GJ per tonne of alumina. Though many existing plants with this technology are operative even today but not being adopted in new Alumina refineries now because of high operating cost.

2.       Double digestion technology:

This technology has advantages over high temperature digestion technology as energy consumption is comparatively lower. In this process, the Bauxite is first digested at low pressure at digestion temperature varying from 140 to 145 degree centigrade and the settler underflow residue is again digested at high temperature at process conditions same as high temperature digestion. Thus overall energy consumption is substantially lower compared to high temperature digestion. This technology reduces the overall energy consumption of about 1.5 GJ per tonne of alumina compared to high temperature digestion technology.

3.       Two step digestion technology:

This digestion is almost same as Double digestion technology wherein Bauxite at 1st stage is digested at atmospheric pressure at around 105 to 110 degree centigrade keeping 2nd stage exactly the same as high temperature digestion. In this technique, the bauxite is digested at atmospheric pressure and settler underflow residue is again digested in second stage of digestion circuit. This technology consumes less thermal energy and lower caustic soda consumption due to partial formation of sodalite complex. Alumina extraction is comparable other available digestion technology discussed above. The total thermal energy requirement adopting two step digestion is estimated at around 14.0 GJ per tonne of Alumina.

4.       Tube Digestion technology:

Tube digestion technology has been developed on pilot plant scale and in the process of commercialization. The details of Tube digestion technique will be covered in forthcoming posts. In this technique, bauxite slurry with desired amount of caustic soda is passed at high velocity through bundle of tubes. The residence time requirement for digestion is very low. Material and energy balance calculations with optimized process parameters and heat recovery system reveal that total thermal energy requirement adopting tube digestion technology should be around 12.0 GJ/t alumina which accounts for about 9 GJ/t for Bayer process (Hydrate) area and 3.0 GJ/t for calcination area.

Out of above four techniques, it is evident that Tube digestion technology will require less capex and opex. Pilot plant scale trials with this technology have shown encouraging results in all respect. A newly constructed Alumina refinery has adopted Tube digestion technology for their 1.8 Mtpa Alumina refinery in Saudi Arabia. We strongly feel that the presence of lime at high digestion temperature of 260 to 270^oC will cause high scaling rate on inner surface of tubes. Hence it may be essential to take care of the tube cleaning while adopting this technology in any Alumina refinery.

Our technical paper on "International benchmark of energy consumption in Alumina refinery" may of interest to you.

It is advisable to carry out careful evaluation of all pros and cons of considered digestion technology before taking final decision in implementing any of the above-mentioned digestion technology in the plant. The content of this paper is purely our own experience and views in the interest of global bauxite-alumina-aluminium fraternity.

Your comments / remarks on above will be highly appreciated.

Best regards.

Rajendra Kunwar

CETI Enterprises.

www.ceti.co.in

Impact of Design Defect on Steady Operation of Alumina Refinery

         Hi Friends,

We are well aware that alumina production is a continuous hydro-metallurgical process. The steady operation of plant and equipment results in remarkable plant performance. At times, minor design defects causes severe operational problems in Alumina refinery. Unsteady operation of Alumina refinery directly impacts the efficiency of the plant. Thus, all out efforts are made to minimize the design defects during implementation / modification stages of plant / equipment / facilities. 

In earlier posts, we have highlighted a few design defects and the methodology to overcome those limitations as well. In present post, we will discuss in brief a few additional design defects which have been experienced by the author in the operative plants in India and abroad. A few design defects are briefly outlined in subsequent paragraphs-

1.       Bauxite slurry heating system: 

Generally, heating of ground bauxite slurry at around 100 degree centigrade is carried out either during pre-desilication or before feeding to pre-desilication tanks. The heating of slurry is carried out to attain the desilication temperature by following four methods-

·       Direct injection of steam to pre-desilication tanks,

·       Indirect heating using Double pipe heat exchangers,

·       Indirect heating using spiral heat exchangers and

·       Indirect heating using Shell & tube heat exchangers.

Direct injection to bauxite slurry increases the dilution to process and hence never considered. Double pipe heat exchanger for bauxite slurry heating is not recommended because of acute scaling problems for the reasons already explained in earlier post. Last two options stated above are recommended among which the last one i.e. Shell & tube heat exchanger is the preferred choice because of lower capital investment as well as ease in maintenance.

2.       Schedule of pipes for slurry handling: 

Slurry generated in red area of Alumina refinery has high abrasiveness because of presence of silica which causes erosion of pipelines. Thus, pipelines of adequate thickness and schedule 80 should be installed.  Schedule-40 pipes should always be avoided as it will require frequent replacement.

3.       Selection of MoC for pumps, valves and fittings: 

As such carbon steel is the best suited material of construction (MoC) for Alumina refinery having alkaline environment. But presence of silica causes fast erosion in stationary as well as rotating equipment. Thus, for handling abrasive slurry in red area, equipment / components made of carbon steel as well as Nicast are never used. For such critical service conditions, Nihard / high nickel steel / high chrome steel is the right choice. Never compromise on MoC of impeller and casing of slurry pumps, connected valves and inlet Tees of slurry flash tanks particularly for handling Bauxite slurry. Ensure the right MoC for these critical components for steady operation of Alumina refinery.

4.       Minimum shell thickness of Digesters:

Digesters are the pressure vessels installed for dissolution of alumina present in bauxite in caustic liquor. The digestion vessels are designed on the basis of its recommended operating conditions. The shell thickness is always based on the pressure but shell thickness is always kept more than 20 mm to take care of any eventualities because of instrument failure and operational lapses as well.

5.       Provision of non-return valve in steam lines to process vessels: 

Alkalinity in boiler feed water is controlled below 0.01 gpl Na2CO3. Though direct injection of steam to process are always avoided but a few designers keep provisions. In such situation, non-return valve in steam line near process area is a must as sudden drop in boiler pressure for any reason may case backing of alkaline liquid to steam lines which may develop stress corrosion ultimately causing sudden failure of steam line within short duration. It is essential for safeguarding the equipment as well as safety of operating personnel in the plant.

6.       Provision of flanges in pipelines: 

Sometimes, flanges are not provided in slurry pipelines to minimize capital investment in project stage. But, it cause severe operational problems because of difficulties in cleaning of welded lines. Thus flanges must be provided at the interval of 6m to 10m depending of practical experience operating personnel.

7.       Adequate capacity of Evaporator Hot well: 

Tail pipe of Barometric condenser is kept submerged to a depth of about 0.50 meter in water of Hot well of evaporation unit. The Hot well is designed to maintain adequate seal water for circuit. The volume of Hot well is always kept more than five times the submerged volume of leg of Barometric condenser. If water volume in Hot well is inadequate, the vacuum seal will be broken and air drawn through tail pipe will drastically affect the performance of evaporation unit.

8.       Minimum plate thickness of tail pipe of Barometric condenser: 

Generally, the vacuum requirement for efficient evaporation system is 685 mm of mercury. Thus, the minimum acceptable plate thickness for the tail pipe is 6 mm. Otherwise, sudden increase in vacuum during start up with negligible load of non-condensable may cause rupture of tail pipe of Barometric condenser. This problem was faced in a newly constructed Alumina refinery which could be rectified by carrying out modification for re-startup.

9.       RPM of impellers of slurry pumps: 

RPM of impeller is the prime criteria for selecting the pumps for Alumina refinery. In Alumina refinery, impeller rpm for handling bauxite residue as well as hydrate is relatively kept at lowest possible level so as to avoid breaking of the flocs and particles. This issue has already been discussed at length in earlier posts.

These are just a few examples of design defects of Alumina refinery. There are many such design defects observed in Alumina refinery which badly affects the continuous operation of Alumina refinery and thereby by huge monetary losses for the company. Hence, role of Owner’s consultant becomes very important to avoid such design defects during engineering state of execution of the plant. We would like to have your comments / remarks for further improvement.

You may like the article on "Systematic approach for design of Slurry mixing agitator" published earlier.

Best regards.

Rajendra Kunwar

Principal Consultant-Engineering

CETI Gurugram, Haryana, India.

www.ceti.co.in

MAJOR ENGINEERING & EXECUTION ACTIVITIES FOR SETTING UP NEW CHEMICAL INDUSTRIES

Hi Friends,

Before going through the current article, you will like to have a look at "Caustic embrittlement in Alumina Refinery". This will help you to know the impact of caustic at high temperature.

As such it is difficult to list out major engineering and execution activities, however all out efforts have been made to assimilate the information in systematic manner for guiding the young professionals engaged in engineering of Mineral processing / Specialized Chemical Industries as outlined here under-

A.      Pre-Project Activities

1.       Finalization of Product mix

2.       Quality specifications of products

3.       Preparation of Conceptual Report

4.       Technological testing of  input materials

5.       Site selection and land acquisition

6.       Identification of available infrastructure like roads, rails, rivers and port facilities

7.       Estimation of Raw material consumption factors

8.       Estimation of utilities requirement like steam, power, chilled water & compressed air

9.       List of major equipment with broad specifications

10.   Estimation of Capex and Opex

11.   Development of Plot plan

12.   Preparation of Feasibility Report

13.   EIA / EMP Report

14.   Statutory clearances for site

15.   Receipt of Consent to Establish (CTE)

B.      Basic Engineering

1.       Process Design Basis

2.       Fixing the process & operating parameters

3.       Material flow balance

4.       Thermal energy balance

5.       Water Balance

6.       Caustic Balance

7.       Alumina Balance

8.       Condensate balance

9.       Development of  Overall Process Flow Diagram

10.   PFDs for All Units

11.   P&IDs for All Units

12.   Broad specifications of major technological equipment

                                                 i.       Primary & secondary crusher

                                                ii.      Conveyors

                                              iii.      Vibro-pulse screens

                                              iv.      Ball mills

                                                v.       Slurry pumps

                                              vi.      Heat Exchangers

                                            vii.       Autoclaves / Digesters

                                           viii.      Flash Tanks

                                              ix.       Evaporators

                                                x.      PHEs

                                              xi.       Boilers

                                             xii.      Compressors

                                           xiii.       Cooling Towers

                                           xiv.      Rotary Disc Filters

                                             xv.      Rotary Drum Filters

                                           xvi.      Calciners

13.   List of major equipment with motor ratings

14.   Specifications of other process equipment

15.   Instrumentation and control philosophy

16.   List of cranes and hoists

17.   Sizing of heat exchangers

18.   Sizing of Desilicators

19.   Sizing of Liquor decanters & washers

20.   Sizing of Precipitators

21.   Sizing of condensate & other liquid storages tanks

22.   Specifications for liquor and water pumps

23.   Sizing of pipelines

24.   Sizing of valves

25.    Acid cleaning system for heat exchangers

26.   Chemical cleaning system for precipitators

C.      Detailed engineering

1.       Preparation of tender documents

2.       Development of General layout of the plant

3.       Preparation of shop layout

4.       Assistance in evaluation of offers

5.       Assistance in inspection of equipment

6.       Assistance in procurement of plant & equipment

7.       SLD for Power Control Centre (PCC)

8.       SLDs for Motor Control Centre (MCC)

9.       Design of Digesters

10.   Design of Flash tanks

11.   Design drawings for tanks and vessels

12.   Inspection and expediting activities

13.   Drawings for process & utility piping network

14.   Design drawings for Civil & Structural Buildings

15.   Foundation drawings for all equipment & facilities

16.   Certification of bills payable to contractors and suppliers

D.      Construction activities

1.       Preparation of schedule of activities

2.       Planning for materials, machines and manpower

3.       Planning of construction activities

4.       Appointment of Competent Contractors

5.       Supervision of construction at site

6.       Cost & quality control during erection

E.       Assistance in Testing & Commissioning

1.       Checking Mechanical completion

2.       Cold trial run

3.       Hot trial run

4.       Commissioning of equipment

5.       Plant commissioning procedure

6.       Performance guarantee test

F.       Stabilization of plant

1.       Finalization of  process and operating parameters

2.       Guiding the team for steady  plant operation

G.      Quality control methodology

H.      Safety and Environmental Management & Control Techniques

I.        Preparation of As-Built Drawings.

The above list clearly indicates the volume of engineering and execution activities involved in putting up any new chemical industries in any geographical location across the globe. 

Also, you may be interested to read our paper on "Systematic approach on testing and commissioning of Newly Constructed Alumina Refinery" .

We  will welcome your feedback and suggestions for further improvement. 

For any assignment related to design and engineering activities, we may be contacted at rajendra@ceti.co.in. Details of services are available at our website.
Best regards.

Rajendra Kunwar
India
www.ceti.co.in

Systematic Approach for Design of Slurry Mixing Agitators

         Hi Friends,

In various technical forums, process experts as well as equipment manufacturers have opined that the design of agitators for mixing process slurry handled in Chemical industries is complicated and tricky issue. In this paper, we would like to discuss the subject in very simplified manner with systematic approach starting with brief description of involved terminology, associated design parameters and methodology with sample motor rating calculations for the slurry mixing agitator.

Basic Formula Drive Rating Calculations for Agitator:

Power Required for Agitator Drive Motor = N_p X ρ x N³ x D⁵ watt for 1 Impeller.

Where Np = Power number (Dimensionless number dependent on type of impeller),

ρ = Density of Fluid in kg/m3,

N = Revolution per second and

D = Diameter of Impeller in m.

Indicative Power Numbers for Various Types of Impellers:

S.No.	Type of Agitator Impeller	Power Number (Np)
1.	Four bladed 45 degree Pitch Turbine	2.1
2.	Rushtom Turbine	5.5
3.	Propeller Type Impeller	0.70
4.	Hydro-Foil Type Impeller	0.30 to 0.51

Sample Calculations to Arrive at Drive Motor Rating for Agitator: 

Simplified sample calculations have been presented to arrive at the drive motor rating for the agitator of a Process tank having around 3000 m³ gross capacity with key input figures as well as realistic assumptions-

Fluid height in the tank, H = 16 m and Diameter of tank, D_t = 14 m

Slurry volume in tank   = π *D_t²*H/4 =  π * (14)²*16 /4 = 2463 m³   

Solid consistency in Slurry = 50 % (w/w),   Density of slurry, ρ = 1602 kg/m³,

Viscosity of slurry, μ = 550 cp

Agitator Impeller Diameter, D_i= 33 % of tank diameter = 14 * 33% m = 4.62 m

Tip speed of Impeller = π *D_i *N = 3.1416 * 4.62 *20 = 290 m/minute,      

 Drive motor RPM = 1500 rpm, Gear Box Reduction Ratio = 75

∴ Agitator RPM = Drive Motor RPM/Gear Box Reduction Ratio = 1500/75 = 20 rpm,

Thus revolution per second of impeller, N = 20/60 = 0.333 Revolution per second.

Flow Number N_q   = 0.56 and Power Number,     N_p  = 0.51 (assumed figures for Impeller)

∴ Pumping Capacity  =  N_q * N * D_i³  m³/ second

= 0.56 * 0.333 * (4.62)³ = 18.41 m³/sec.

Area of Tank               = π  * D_t²   = π  *(14)² / 4 = 153.94 m²

Bulk fluid Velocity      = pumping capacity/area of tank

= 1104.44 / 153.94= 7.18 m/min. = 23.55 ft./min.

Degree of Agitation   = bulk fluid velocity / 6     

(For 6 ft/min., degree of agitation =1 and Degree of agitation varies from 0 to 10)

= 23.55 / 6 = 3.93 ~ 4

Annular Area = π  * (D_t²- D_i² ) /4  

Where D_t = Diameter of tank      and        D_i = Diameter of impeller in meters.

= 3.14 * (14² – 4.62²) / 4 = 137.18 m²

Rising velocity of particles = pumping capacity / annular area

= 1104.44 / 137.18 = 8.051 m/min.  = 0.1342 m/sec. 

Tank Turnover rate   = Pumping capacity / tank capacity

= 1104.44 / 2463 = 0.45 times / min.

Power Number N_p    = 0.51

Shaft Power,      P      = N_p* ρ *(D_i)⁵ * N³ /1000 kW

Where N_p = Power number of impeller (Dimensionless number),

D_i = Diameter of impeller in meters, Shaft RPS,             N = revolutions per second,

∴ Shaft Power, P   = 0.51 * 1602 * (4.402)⁵ * (0.333)³ /1000 kW = 49.86 kW

Taking Gear Box Efficiency   = 80% and Drive Motor Efficiency = 95%,

Design margin           = 1.15

∴ Drive Motor Rating = 1.15 * 49.86/(0.80 * 0.95) =73.95 kW.

Thus the drive motor of about 75 kW shall be adequate for successful operation of agitator of 3000 m³ Process tank containing fluid of considered solid content. 

Conclusions:

The developed methodology clearly reveals that motor rating calculations for any slurry mixing agitator can be carried out easily by simply replacing the associated input process conditions, operating parameters, dimensions of tanks / vessel and appropriate power number for impeller in above simplified derivation. 

Trust, you will like the methodology adopted in this technical paper. We would like to have your comments.

In case, your company needs assistance of our team in resolving any technological, process, engineering or operational problems of your plant; we may be contacted. We are operating out of Gurugram, India. You may like to have a look at our website.

Regards.

Rajendra Kunwar

E.Mail: rajendra@ceti.co.in

Our Website: www.ceti.co.in