Tuesday, June 19, 2018

Grand Success of My Book on " A Complete Guide to Alumina Technology"

Hi Friends,

I am highly obliged to my seniors for their guidance in completing the publication of my book on "A Complete Guide to Alumina Technology" as planned.

I am sincerely thankful to all my friends for their positive feed back about the book. The book has earned good reputation among the friends of bauxite-alumina fraternity within such a short span of time. Your encouraging words will always remain memorable for me.

Now, I realize that I have published this book well on time and request my friend to share such technical data / information with others which adds value to the group and individual as well.
Truly, I can not express my feelings in words. I will always remember you.
Best regards.

Rajendra Kunwar

Friday, June 15, 2018

Tube Digestion Technology for Processing of Boehmitic Bauxite

Hi Friends,

In previous papers, we have covered variety of technological, design, engineering and operational issues of Alumina refinery. It’s true that there is no end to advancements and thus tremendous scope for enriching our knowledge base. In continuation to our approach and efforts, we will present here in brief about Tube digestion technology as an alternative to Double digestion process for extraction of alumina. 

The digestion of predominantly Boehmitic Bauxite is carried out at an elevated temperature ranging from 220 deg C to 280 deg C, thus total thermal energy requirement for processing Boehmitic Bauxite is very high. As such Double digestion technology coupled with Pressure decantation system is a proven and established technology and many Alumina refineries are operative with this technology. The estimated overall energy consumption in efficient Alumina refinery with high temperature digestion technology is about 15 GJ per tonne of calcined alumina.

Another efficient digestion technology which was developed sometime in 1960s is popularly known as Tube Digestion Technology. In this process, wet ground Bauxite in caustic liquor followed by desilication is passed through long jacketed tubes / pipes at high velocity. The particle size of the Bauxite is controlled at around 100% minus 1000 microns. The tube jackets contain high pressure saturated steam for raising the temperature of reaction mixture indirectly to ensure dissolution of alumina. The higher velocity of slurry in the pipelines (tubes) enhances the heat transfer coefficient and thereby heat transfer rate resulting in lower digestion time requirement. The retention time during digestion is generally not dictated by high dissolution rate of alumina rather dependent on chemical kinetics of quartz with caustic liquor. It has been practically observed in laboratory as well as plants that higher retention time across tubes during digestion at higher temperature above 250 deg C causes Quartz attack due to dissolution of Quartz with caustic soda which increases the formation of DSP and Cancrinite and thus additional consumption of caustic soda for alumina production. As per my personal experience with Boehmitic Bauxite, 15 minutes of digestion time in tube digesters will be adequate. The outcome of thermal energy calculations reveal that energy requirement with Tube digestion technology is comparatively less than that for other high temperature digestion technology. 

Advantages of Tube digestion technology: 
There is a growing trend in adoption of Tube digestion technology in a few Alumina refineries in planning and execution stages in the World. An Alumina refinery of 1.8 Mtpa production capacity has been newly constructed and commissioned a few years back in Saudi Arabia adopting tube digestion technology. The plant has adopted this technology with Basic engineering developed jointly by two leading engineering consultancy companies. This will be the biggest Alumina refinery in the World adopting Tube digestion technology.

Tube digestion technology has following major advantages over other high temperature digestion technologies available in the World-
·         Lower thermal energy consumption for digestion,
·         Easy maintenance or replacement of equipment,
·         Lower retention time for digestion,
·         Flexibility in processing wide range of Bauxite,
·         Lower evaporation requirement,
·         Lower capital investment and
·         Marginally lower operating cost. 

Total thermal energy consumption for alumina production adopting Tube Digestion Technology works out to about 12 GJ/t alumina which accounts for 9.0 GJ/t alumina for Bayer process (Hydrate) area and 3.0 GJ/t for Calcination area of Alumina refinery.

Disadvantages of Tube digestion technology: 

The scaling rate in inner surface of tubes will be high because of Boehmite reversion in tube digestion stage because of high alumina loading in liquor in presence of solid Boehmite. As such, the hardness of scale will be very high due to presence of DSP and Cancrinite. Also, caustic soda consumption will be higher due to dissolution of quartz and formation of Cancrinite as well. Thus following are the disadvantages of Tube digestion technology-
·         High scaling rate in tube digesters and
·         High caustic soda consumption.
At times, certain chemicals, like MgO, are added to bauxite slurry before entry to tube digesters for making porous scales for easy dislodging.

Since high temperature digestion beyond 250oC causes conversion of quartz to reactive silica, thus it is always recommended to maintain the digestion temperature at around 240-245oC and should avoid crossing 250oC to combat high caustic soda consumption for alumina production.

The statements presented here are purely based on personal experience and technical studies of the author on the subject. Hence, it is advised to carry out detailed evaluation of process by thorough scrutiny of process control data and related efficiency figures before implementation in any plant for commercial production.

We solicit your valued comments / remarks for further improvement from our end.
Rajendra Kunwar

Wednesday, May 30, 2018

The Book on "A Complete Guide to Alumina Technology"

Hi Friends,

As informed through previous article, the book on "A Complete Guide to Alumina Technology" is ready now in final stage of publication. Given below, please find the actual physical appearance of the book.

Assimilation of technical data, information and calculation related to design & engineering took long time as utmost care was taken on accuracy aspects of the book.

As communicated, the book will be available on www.amazon.in sometime in the first week of June 2018 onward.

Best regards.

Rajendra Kunwar

Saturday, May 26, 2018

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.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 and entrepreneurs as it elaborates techniques for pre-investment studies 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 optimising the capex and opex of Alumina refinery. 
Best regards.

Rajendra Kunwar

Sunday, April 22, 2018

Features of Draft Tube and Open Impeller Agitators for Precipitators in Alumina Refinery

Hi Friends,

Precipitators are most critical equipment considered in Alumina refinery because of its effect on process efficiency and product quality as well. Relevant process efficiency refers to liquor productivity during crystallization process in precipitators which has direct bearing on total thermal and electrical energy consumption for alumina production in Bayer process. On product quality front, product coarseness along with granulometry of hydrated alumina is controlled to the desired level in precipitators.

As discussed in earlier posts, in precipitators, freshly prepared seed hydrate is added in the saturated and filtered aluminate liquor at controlled temperature and the whole content is kept under agitation / circulation resulting in crystallization of hydrated alumina. The crystallization process requires about 40 hours of slurry holding time. There are three types of agitation / slurry circulation systems prevalent in Alumina refineries-
  • Circulation  by air lift,
  • Draft tube agitators and
  • Open impeller agitators.
Slurry circulation by air lift requires huge flow of compressed air, thus it has lower slurry agitation efficiency. Till 1960s, many Alumina plants had compressed air based slurry agitation system in precipitators but now most of them have modified the system adopting either draft tube agitators or open impeller agitators. In addition to inefficiency, air lift system has the capacity limitation for bigger volume precipitators. Earlier days, the crystallization circuit had smaller capacity precipitation tanks with volume of each precipitation tank ranging from 1000 m3 to 3000 m3. Now a days, all modern Alumina refineries installed in 1980s and afterwards has higher volumetric capacity varying from 4000 m3 to 4500 m3 per precipitation tank. Installation of higher capacity precipitation tanks has resulted in lower space requirement for installation, lower man power requirement for operation and substantial reduction in operating and maintenance expenses. Agitators of precipitation tanks requires following key features-

1. Higher pumping capacity for keeping solids in suspension and homogeneity of solids,
2. Low shearing rate of impeller to minimize particle attrition and
3. Lower electrical energy requirement.

As per my personal experience and interactions with professionals of Alumina refineries in India and abroad, both draft tube agitators as well as open impeller agitators are equally efficient for solid consistency in precipitation tanks ranging from 400 to 500 gpl hydrate as alumina. This level of solids in precipitation tanks is adequate for achieving the liquor productivity upto 75 gpl Al2O3 and most of the plant in the World has liquor productivity in the same level. But, all new Alumina refineries under execution and planning stage have precipitation tanks of further higher capacity of 5500 m3 to 6000 m3 with solids in slurry over 600 gpl Al2O3 with the prime objective to have liquor productivity to the level of plus 92 gpl Al2O3.

For higher solid consistency, open impeller agitators will not have any problem, however, certain design modifications may be required in draft tube agitators for meeting the process requirement for its successful uninterrupted operation with high solids in precipitation tanks of Alumina refinery.

The views expressed here are purely my personal experience without any bias / prejudice for any particular individual / organization involved in designing of such a complex equipment. I have freely shared my views and request you to put your views / remarks / comments, if any, if you have different experience on the subject.

Rajendra Kunwar

Friday, March 30, 2018

Key Process Control Techniques to Produce Improved Quality of Alumina

Hi Friends,

It’s proven fact that improved quality of products / services have the edge over normal quality. Quality always plays vital role in value addition and there by attributes in sustained business attracting confidence of customer. This paper describes key operational and process control measures to produce consistent quality of hydrate as well as calcined alumina in any Alumina plant. Major reasons affecting the quality of product are highlighted along with key control measures for improvement.
There are two main factors which badly affects the quality of product as listed below-
1.       High soda content in product hydrate and alumina
2.       High suspended solids in Thickener overflow pregnant liquor.

Step-I: In different Alumina refineries across the globe, we have experienced high soda content in product hydrate as one of the basic reasons for deterioration of product quality. The identification of this issue is confirmed through simple analytical technique by analyzing the Na2O content in product hydrate ready for dispatch to respective customers. If total Na2O content in Hydrate is found more than 0.30% then it confirms that Hydrate generated in decomposers has not been washed properly on product filter wherein the maximum component of leachable soda is removed. After observing this high soda content, vigorous washing of hydrate using hot water at temperature around 95-98oC helps in achieving total Na2O content below 0.30%. For further improvement on washing efficiency of hydrate, steam-hood at the discharge side of product filter has been found to be more advantageous.

Step-II: High Fe2O3 content in product hydrate above 0.010% is the indication of high suspended solids in filtered aluminate liquor feeding to Decomposers (Crystallizers / Precipitators). The suspended solids must be controlled below 10 mgpl in pregnant liquor in feed to decomposers. Higher solids in pregnant liquor happens because of inadequate settling of suspended red mud particles in Liquor thickener for the major reasons listed below-
(a)    High generation of fines during grinding of Bauxite:  
Control fineness of Bauxite at optimum level to ensure higher settling rate of red mud particles in thickener. Generally, minus 60 mesh fraction in ground bauxite slurry is maintained more than 85%. Check and control it accordingly.

(b)    Higher viscosity of thickener overflow liquor because of its lower temperature: 
(c)     Temperature of Thickener overflow liquor must be controlled at around 100-102oC if the concentration of liquor in thickener overflow is at more than 145 gpl Na2O. High concentration profile is the need of modern Alumina refinery in order to have improved productivity so as to minimize thermal and electrical energy consumption.
(d)    Selection of right settling agent: 
Settling agent used for faster settling of residue particles play vital role in achieving desired quality of product. There are different types of natural and synthetic high density polymers are available for different applications. It has been observed that mixed natural starch with particular type of synthetic flocculent are cost effective for Alumina refinery. 
(e)    Selection of proper filter cloth for polishing filtration equipment: 
Selection of proper filter cloth is equally essential for controlling suspended solids in filtered aluminate liquor fed to crystallizers. Polypropylene filter cloth of more than 30 EPI and 32 PPI have been found to be the most suitable cloth for security filtration.
(f)     Controlling P2Oconcentration in aluminate liquor: 
At times, high P2O5 content in Bauxite affects the settling rate of bauxite residue (Red mud) in decanters. To eliminate this issue, small quantity of Burnt lime (CaO) may be added with bauxite being fed to Grinding mills. This results in reducing the Phosphorus content in aluminate liquor forming Calcium Phosphate.
(g)    Control Filter feed pressure below 2.0 kg/cm2.g: 
Lower filtration pressure results in better filtrate quality. Pressure around 2.0 kg/cm2.g at inlet of security filtration system has been found to be optimum for security filtration.
(h)    Control solids in Thickener overflow Liquor well within 100 mgpl: 
Suspended solids in feed liquor to polishing filter should be controlled well within 100 mgpl so as to avoid extra load of residue on polishing filtration unit thereby improving the filtrate quality to decomposers,
(i)      Optimize dosing of TCA: 
TCA (Tri calcium aluminate) addition to filter feed liquor helps in getting improved filtrate quality. It has been established in various plants across the globe that mass flow of TCA equal to 10 times of suspended solids in decanter overflow liquor is optimum. This is considered as main controlling parameters for reducing the impurities in filtered aluminate liquor and subsequently the quality of product hydrate and calcined alumina. TCA dosing has been found to be the most effective step in improving the filtrate clarity.

Step-III: Replacement of Filtration Equipment:
Even after checking and monitoring all the above mentioned parameters stated under Step-II, if there is no appreciable improvement in filtrate clarity, then it concludes that something wrong with filtration equipment itself. In such situation, replacement of existing Polishing filters with suitably sized and designed security filters is the ultimate solution to this problem. Since this step requires additional capital expenditure for the plant, thus decision needs to be taken only after thorough investigations of results obtained under the guidance of process expert.

We would welcome your comments / remarks for further improvement.

Rajendra Kunwar

Sunday, March 18, 2018

Design of Air Coolers and Water Coolers for Alumina Refinery

Hi Friends,

Generally, fluidized bed coolers are preferred for lowering down the temperature of calcined alumina in medium to big size Alumina refineries. But for small capacity Alumina plant is not economical as sophisticated fully mechanized  alumina cooling facilities may not be cost effective, thus installation of semi-mechanized equipment like Air cooler and Water cooler are considered for the purpose.

In previous post, we have covered Bauxite digestion technology for gibbsite-boehmite mixed bauxite. 

Also, we have published a very interesting article on "Impact of design defects in performance of Alumina refinery."

In present paper, we would broadly discuss the methodology adopted for sizing of air cooler and water cooler including the calculations for working out the rating of their drive motors required for their successful operation. Here, production rate of 1.00 tph Refractory grade alumina (RGA) has been considered in the sample calculations for sizing the Air cooler and Water cooler used as Primary cooler and Secondary cooler for lowering down the temperature of alumina to desired level.

Sizing of Air Cooler:

In first step, hot calcined alumina coming out from Rotary Kiln at around 1400oC is cooled with air in Air cooler. The generated hot air is passed in counter-current direction of alumina flow through cooler. Let the temperature of alumina drops to about 300oCand temperature of air rises from 30oC to 95oC. The hot air generated is fed to the Kiln for combustion of HFO.
Heat given by alumina = 1000*0.27*(1400-300) = 297000 k.cals/hr.
Heat lost by radiation = 25% of total heat = 0.25*297000 = 74250 k.cals/hr.
If  w kg of air is passing through the Air cooler for absorbing heat,
Then heat taken by air = w*0.24*(95-30) = w*15.6 k.cals/hr.
Since Heat In = Heat Out.
Thus w*15.6 + 74250 = 297000.
Therefore, w = 14279 kg/hr.
Hence, air flow through Air cooler = 14279 kg/hr.
For design of Air cooler, mass velocity of air through Air cooler is taken as 45,00 kg/hr.m2 .
Thus, the cross sectional area of Air cooler = 14279 / 4500 m2 = 3.17 m2.
  (π/4)*D= 3.17                                
 D = 2.0 meters.
For Air coolers, L/D =4; therefore, L = 8.0 meters.
Drive motor rating for Air cooler = 4.D= 4* (2)= 16.0 kW.
Sizing of Water Cooler:

Calcined alumina coming out from Air cooler is further cooled in Water cooler. Here, cooling water is sprayed over rotating shell, but the contact of water with alumina is completely avoided. In this case, rise in temperature of water is less but desired cooling of product alumina is easily achieved. It is assumed that final temperature of product alumina is controlled at 90 deg.C and temperature of water rises from 30 to 40 deg.C.
Hence heat given by alumina = 1000*0.27*(300-90) = 56700 k.cals/hr.
Heat taken by water = w*1.0*(40-30) = 10*w k.cal/hr.
Since heat loss = heat gain,
Thus 10*w = 56700
Therefore, w = 5.67 m3/hr.
Thus water flow to Water cooler ~ 6 m3/hr.
Overall heat transfer co-efficient for water cooler = 50 k.cals/hr.m2.deg.C.
So, 56700 = 50*A*(40-30).
Therefore, A = 113.4 sq.m.
For Water cooler, L/D = 7.
Hence, π*D*L = 113.4,
Or,  π*D*7D = 113.4
Thus, D =2.27 m and L = 15.9 m.
Drive motor rating for Water Cooler = 4.(D)2 = 4*(2.27)2 =20.6 kW.
Trust, design calculations for sizing of Air cooler and Water cooler required for Alumina refinery / any other industries have been described systematically. Please put your remarks / comments.
Rajendra Kunwar