How to enhance liquor productivity in Alumina refinery?

Hi Friends,

In earlier posts, we have already discussed the principles of crystallization in Bayer circuit. In present post, we will discuss our views to enhance the liquor productivity in existing Alumina refineries operative at lower productivity level.

Production of sandy alumina with highest possible liquor productivity across crystallization circuit is the key issue in alumina production units so as to have minimum energy consumption resulting in lower production cost of calcined alumina. In earlier posts, we have also listed in detail the  factors affecting the liquor productivity in Alumina refinery. Thus by judicious changes in associated parameters will result in liquor productivity. It is prudent to mention here that changes in any of these parameters may have adverse effect on other parameters. For example, higher concentration profile of aluminate liquor in Bayer circuit may require changes in total water management for the plant otherwise there is a fair chance losing more caustic soda in residue washing circuit.

Most of the Alumina refineries in the World are operative with liquor productivity figures at around 60 gpl Al₂O₃, few at about 75 gpl Al₂O₃ and very few refineries are continuously achieving the liquor productivity figure at plus 95 gpl Al₂O₃. Such wide gap in productivity figure gives the tremendous scope for low productivity alumina producers to critically analyze the process conditions and make suitable changes in process circuit and process parameters for improvement in liquor productivity figure. Changes in process circuit may require some additional capital expenditure but the return on investment will be very high and thus the payback period for such investments will be around two to three years with certain realistic assumptions.

Changes required in the Alumina refineries will be purely plant specific but to start with following major aspects may be scrutinized in the existing plant and corrective actions may be taken suitably –

1. Check the bauxite charging ratio with respect to digestion liquor concentration and change these parameters to reasonably higher level considering the alumina saturation at particular concentration. The indicative range of A/C charging ratio (~ Blow-off ratio) may vary from 0.630 to 0.710 with digestion liquor concentration at 260 gpl to 290 gpl as Na₂CO₃.
2. Water draw for residue washing circuit to be controlled within 2.1 m³ per tonne of residue which will attribute in dilution control within concentration drop of about 30 gpl up to feed point of precipitation circuit.
3. Minimize process liquor dilution from miscellaneous sources to the extent possible.
4. Covert single stage precipitation circuit to two stage continuous circuit (agglomeration and growth circuit) immediately by planning necessary changes in piping and pumping system.
5. Operate the agglomeration circuit at high temperature of saturated aluminate liquor at around 80 to 85^oC and crystal growth circuit at around 56 to 60^oC.
6. Fine seed hydrate to be added to agglomeration stage at around 100 to 150 gpl hydrate as Al₂O₃, however seed charge to crystal growth circuit should be around 400 to 600 gpl Al₂O₃ depending on slurry handling capacity of installed agitation system in precipitators.
7. Liquor circulation time (retention time) in agglomeration stage should be kept at around 16 to 24 hours for generation of agglomerated seed hydrate to be used in growth circuit. However, the same is maintained at around 40 hours in crystal growth circuit. Required number of precipitators may be worked out based on the plant capacity and circulation time.
8. About 30% of total aluminate liquor should be passed through agglomeration circuit and balance 70% from the crystal growth circuit.
9. Adopt efficient hydrate classification techniques to ensure product quality with required coarseness as well as seed generation rate to meet the process requirement.

The above mentioned changes / modifications will result in improving the liquor productivity of the plant to plus 80 gpl Al₂O₃ level. Request to put your views / suggestions / remarks / comments, if any. We will welcome your suggestions. 

Regards.

Kunwar Rajendra

Systematic approach for learning process design calculations

Hi Friends,

It is extremely difficult to develop simplified methodology and procedures to carry out  calculations for complex hydro-metallurgical process involved in continuous production method of calcined alumina. To honor the requests of our friends of alumina fraternity, we initiated actions and took it as a spectacular challenge to complete it progressively in phased manner. We have already published many process design as well as equipment sizing calculations and will continue our efforts to share balance topics in the form of technical papers in future.
Particularly for the benefit of our keenly interested and enthusiastic younger generation, it would be essential to focus the preliminary basic requirements for understanding the complex process, design and engineering calculations as listed below- 

• Awareness about the detailed chemical and mineralogical composition of bauxite,
• Familiarize with the involved process technology and efficiency figures,
• Fair idea about the unit operations and unit processes involved in alumina production,
• Command on various conversion factors frequently used in process, design and engineering calculations,
• Fully aware about the chemical reactions involved in unit processes,
• Assimilation of each and every operational and process control parameters,
• Conversant with process terminology of alumina production process, 
• Must have knowledge of broad specifications of all input materials, utilities and services,
• Data related to characteristics of process streams with respective values of specific gravity, viscosity, thermal conductivity, specific heat, latent heat, specific volume, % solids, hardness, abrasiveness etc. as applicable,
• Interest to learn chemical engineering calculations,
• Ability to understand insights of process particularly with respect to phase changes,
• Capability to read the invisible derivations between the visible lines,
• Seek help from friends immediately to get clarified without piling up the bunch of doubts and
• Understanding the entire content of the technical document before printing  / xeroxing the same.

The methodology elaborated above are purely based on our own experience. In addition to these points, we have also experienced that sharing the knowledge with others always sharpens the knowledge base of individual in developing improved techniques for future. Trust, you will agree with my views. Please put your suggestions / remarks / comments, if any.

Kunwar Rajendra

Factors Affecting Extraction Efficiency of Alumina

Hi Friends,

Extraction efficiency is the measure of percentage dissolution of total alumina present in Bauxite across digesters and thus calculated by using analytical results of alumina in Bauxite processed and alumina present in blow-off liquor coming out from digesters.  It is always desirable to achieve alumina extraction efficiency in digesters close to 100%. Main factors affecting the alumina extraction efficiency are briefly described in subsequent paragraphs-

1. Particle size of Bauxite: We are aware that grinding of Bauxite is done to increase the surface area of Bauxite for improving its reactivity with caustic soda. Finer Bauxite particles get dissolved very easily but may cause settling problem of finer residue particles in decanters and residue washers. The deterioration in residue settling rate poses problems in polishing filtration of decanter's overflow liquor and may limit the production rate and quality of product as well. On the other hand, coarser Bauxite particles may not dissolve completely in caustic soda and part of Bauxite may remain unreacted. Hence grinding of Bauxite is done to achieve optimum size of Bauxite particles feeding to digesters. It has been experienced in many operative Alumina plants in the World that Bauxite particle size  of over 80% of minus 147 microns is the optimum particle size, however, the exact particle size of Bauxite of particular deposit may be  worked out based on laboratory test work under similar digestion conditions. 
2. Steady control of digestion liquor concentration: Digestion liquor prepared with uniform dosing of fresh caustic to spent liquor feeding to grinding mill and digestion circuit results in steady control of digestion liquor concentration and ratio as well. The steady control of digestion liquor concentration results in improved digestion efficiency. 
3. Stability of Digestion liquor: The addition of lime slurry to the process circuit maintains the stability of NaOH in liquor to the maximum level and hence better extraction efficiency of alumina across digesters.
4. Optimum Retention time: Adequate retention time in digesters must be provided to attain complete dissolution of alumina present in Bauxite. Excessive retention time in digesters to be avoided.
5. Optimum digestion temperature: Control of digestion temperature at optimum level is the key parameter for higher extraction efficiency as lower digestion temperature may not dissolve the alumina in caustic liquor completely. At times, the higher digestion temperature causes reversion of gibbsitic alumina due to presence of boehmitic alumina which acts as seed in digesters. Thus higher temperature as well as higher residence time may affect the extraction efficiency.
6. Continuous agitation of slurry in digesters: It has been established from laboratory and pilot plant tests that continuous digestion of reacting mixture in digesters improves the dissolution rate and thereby extraction efficiency of alumina in digesters.

The above stated parameters are the key factors for maximizing the extraction efficiency of alumina in digesters.

Please process your valued remarks highlighting the missing parameters.

Regards.

Rajendra Kunwar

A Complete Guide to Alumina Technology - Process Design and Engineering

Hi Friends, 

We have pleasure to inform you that approval from concerned authority of Government of India was received yesterday for publication of my book "A Complete Guide to Alumina Technology." Soon after approval, the process of publication has been initiated by the publishing house.

Given below please find the pictorial view of the book which will be available at www.flipkart.com and www.amazon.in from 8th June 2018 onward.

This book is the exposition for engineers involved in preparing basic engineering, carrying out detailed engineering and developing various process design calculations for Alumina refinery. It provides systematic approach, required technical data and methodology for process design & engineering in execution of Alumina refinery projects. It provides systematic approach, methodology and calculations pertaining to material & energy balance, sizing of critical technological equipment & pipelines, estimation of utility requirements, arriving consumption factors of input materials, determining mining cost of bauxite, working out manufacture cost of calcined alumina and estimation of capital cost of Alumina projects. 

This book will be proved as a reference guide for strategic planners, engineering consultants and entrepreneurs as it elaborates methodology for pre-investment studies as well as project engineering techniques with focus on specific capital investment, profitability and other associated techno-economic aspects of the project. This book covers key technical data and information with regard to composition of major input materials and products including data sheets and specifications of equipment. It gives the glimpse of quality specifications, applications and production process routes for specialty grades of hydrate, alumina, by-products and value-added alumina chemicals. This book also provides, tips, tricks and techniques in carrying out engineering activities efficiently thereby optimizing the capex and opex of Alumina refinery. 

Best regards.

Rajendra Kunwar

www.ceti.co.in

Calculations for Mining and Transportation Cost of ROM Bauxite

Hi Friends,

In earlier posts, we have elaborated the Bauxite mining and modes of transportation of ROM Bauxite to Alumina Refinery. In present post, we will discuss the systematic approach to estimate the landed cost of ROM Bauxite in Alumina refinery. This derivation is purely based on certain factual information and a few major assumptions as described here under-

Though there are many methods to work out the mining and transportation cost of Bauxite, but the simplified method has been presented here for easy understanding. Please put your views / suggestions / remarks / comments, if any.

If you like this article, then please press your rating as  +1   appearing at the footer of this page.


Thanks and regards.
Kunwar Rajendra

Motor Rating Calculations for Pumps in Alumina Refinery

Hi Friends,

In earlier posts, we have already discussed about NPSHA, head loss due to friction and other useful derivations like empirical formula to estimate the specific gravity of slurry required for various process design and engineering design calculations for Alumina refinery. Now, we have to proceed further to understand the detailed methodology to arrive at motor rating calculations for centrifugal pumps. 

In present post, we will discuss the detailed sample calculations for working out the motor rating of a pump discharging raw water from source river or canal to the storage pond of Alumina refinery site. Assumed basic input data and step by step procedures have been elaborated in subsequent paragraphs-

Sample Input Data:-

Elevation of source river / canal       =    50 m above MSL

Elevation of Alumina Refinery site    =    130 m above MSL

Distance from source plant site        =   70 km = 70000 m  

Design velocity of water in pipelines =   1.5 - 2.44 m/sec.

Required water supply rate              =   1400 m³/hr

Material of pipes and bends             =   Carbon Steel.

Details of Suction line of pump:-

Length of suction pipe to pump             = 20 m

No. of standard 90^o elbow                    = 1

No. of  fully open gate valves                = 1

Details of pump delivery line up to Plant site:-

Length of delivery pipeline to plant        = 70000 m

No. of standard 90^o elbow                    = 5

No. of standard 45^o elbow                    = 20

No. of standard Tee                             = 1

No. of  fully open gate valves                = 1

Design of Delivery Line:-

We know that

Discharge      = Cross sectional area x velocity

\ Q             = A x V

Here, Q        = 0.3889 m³/sec. and V = 1.5 m/sec.

\ 0.3889     = p/4 x d² x 1.5

\ Diameter, d= 0.5746 m

                   = 5746 mm.

          Say,       600 mm NB.

Design of Suction Line:-

Generally suction line is taken one size bigger than delivery.

\Suction line diameter, D = 700 mm NB.

Calculations for Total Head of Raw Water Pump:-

Total head, H =  H_d + H_g.

Where,       H = Total head to be developed by the pump.

                 H_d = Delivery Head &

                 H_g = Gravitational or Static Head.

Suction head is negligible as considered flooded.

Discharge head is also zero, as water will be discharged at atmospheric pressure in raw water storage in plant premises.

Calculations For Static Head-

Static Head = Gravitational Head

       = 130 meters – 50 meters

                 = 80 m.

Calculations For Frictional Head Loss in Delivery Line-

           f.L_e.V²

H_fd =  -------- .

           2.g.d

Where, f = Frictional factor

L_e = Equivalent length of delivery line with fittings.

V  = Velocity of water in delivery line.

Now, d    = 600 mm = 0.6 m

        Q    = 1400 m³/hr.

        Q    = A. V

\0.3889 = p /4 x (0.60)² x V

\  V       = 1.38 m/sec.

Now, Reynold number,

Re          = r.V.d/m

Where, r = 1000 kg/m³ for water

           V = 1.38 m/sec.

m    = Dynamic viscosity of water

   = 1 centi poise = 1 x 10^-3 kg/m³

Re   = 1000 x 1.38 x0.6 / (1 x 10^-3)

      =  828000.

      = 8.28 x 10⁵

Since Re > 2200,

Hence the flow of water in delivery line is turbulent.

                                         1.325

 Friction factor, f = --------------------------------------

                            [ln{(e/3.7 d) + (5.74/Re ^0.9)}]²

 Where, e = Surface roughness   = 4.5 x 10^-5   for Steel   

d = Dia. of delivery pipe = 0.60 m

Re = Reynold Number    = 8.28 x 10⁵

                                                       1.325

\ Friction factor, f = ---------------------------------------------------------

                                [ln{(4.5x10^-5/3.7 x 0.6) + (5.74/828000^0.9)}]²

                              1.325                     1.325

f = -------------------------------------------- =    ----------

           [ln{2.03 x10^–5 + 2.71 x 10^-5}]²     99.14

 i.e.  f = 0.01336.

Calculations for Equivalent Length-

Straight length of pipe line = 7000 m

L/D ratio for Fully opened Gate valve = 17

Size of Gate valve,   D = 0.5 m

No. of Gate Valve in Delivery Line          = 1

Equivalent length for Valve, L_Valve  =17 x 0.6 x 1.

\  L_Valve = 10 m

Equivalent Length of Standard Tee-

L/D ratio for Standard Tee = 60

Size of Standard Tee , D = 0.6 m

No. of Standard Tee in Delivery Line =1

Equivalent length for Std. Tee, L_Tee = 60 x 0.6 x 1.

\       L_Tee    =  36 m

L/D ratio for 90^o standard Bend = 30

Size of 90^o standard Bend D = 0.6 m

No. of 90^o standard Bend in Delivery Line = 5

Equivalent length for 90^o standard Bend = 30 x 0.6 x 5.

\  L_bend90      =  90 m

 L/D ratio for 45^o standard Bend = 16    

Size of 45^o standard Bend, D       = 0.6 m

No. of  45^o standard Bend in Delivery Line = 20

Equivalent length for 45^o standard Bend L_bend45 = 16 x 0.6 x 20.

\ L_bend45   = 192 m

\Total equivalent Length, L_e = 70000 + 10 + 36 + 90 +192 m.

\ L_e = 70328 m

\ Frictional Head Loss,

           f.L_e.V²         0.01336 x 70328 x (1.38)²

H_fd =  --------  = ----------------------------------

           2.g.d              2 x 9.81 x 0.6

                    = 152 m.

\ Total Head = Static Head + Frictional Head.

                    = 80 m + 152 m = 232 m.

Calculations for Drive Motor Rating:-

Basic Assumptions:

Efficiency of Pump   = 75%

Efficiency of Motor  = 97%

Water Discharge Rate = 0.3889 m³/sec.

Specific gravity of Water     = 1.0

Fluid Power =r .g. Q. H

                  = 1.00 x 9.81 x 0.3889 x 232 kW.

                             = 885 kW.

                                 Fluid Power

\Connected Motor = -----------------

                                h_Pump x h_Motor

                                    885

\Connected Motor = ---------------

                                0.75 x 0.97

                             = 1216 kW.

                             ~ 1200 kW.

Thus motor rating of 1200 kW shall be required to be connected with the pump for the purpose.

Trust, the methodology outlined here is simple, systematic and easy to understand and the same can be adopted for working out motor rating for any pump for required duty conditions just by putting the required input parameters. 

Please put your views / suggestions / remarks / comments, if any.

If you like this article, then press your rating as  +1  .

Thanks and regards.

Kunwar Rajendra

Production & Uses of High Purity High Alpha Calcined Alumina

Hi Friends,
In present post, we will discuss the specifications, applications and production process route for High purity high alpha calcined alumina.

Product Specifications:
Total Al2O3 : 99.99% min.
Alpha Al2O3: 99.5% min.
SiO2: 0.010% max.
Fe2O3: 0.010% max.
Porosity : 60-75%,
BET surface area: 5 to 10 m2/g,
Water absorption: 1% max.


Input Raw Material: 
High purity hydrate.


Production Process Route:
Mainly two process steps are followed for production of high purity high alpha grade of specialty calcined alumina as described here under-


Step-I : Calcination of High purity hydrate in Rotary kiln at around 1650-1700 degree C.
Step-II: Micronisation of calcined alumina either by jet milling or grinding in ceramic lined Ball mill.


Applications:

• Abrasives for magnetic tape,
• Watch jewels,
• Ceramic balls,
• Fibre connectors,
• High strength ceramic tools,
• Translucent tubes for high pressure sodium lamps,
• Precision polishing,
• Technical ceramics and
• Catalyst support for production of Ethylene oxide.

Trust, the useful data / informations have been presented to the extent possible. However, further details with process flow sheet and list of equipment with broad specifications shall be published in future posts.

Please put your views / remarks / suggestions / remarks / comments, if any.

Regards.

Kunwar Rajendra