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
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

Sunday, March 11, 2018

Basic Technical Data Often Used in Design Calculations of Alumina Refinery

Hi Friends,

In this paper, we will present a few important data which are essentially required for carrying out process design calculations including material balance, thermal energy balance and sizing calculations for various equipment including pipelines of Alumina refinery as outlined here under:

·         Alumina means Al2O3 having molar mass = 2x27+3x16=102 g per mole.
·         Hydrated alumina means Al2O3.3H2O having molar mass= 102+3x18=156 g per mole.
·       Thus 156 tonnes of Hydrate alumina = 102 tonnes of Alumina, this implies 1 tonne of Hydrate is equal to 102/156 = 0.65385 tonne of alumina.
·         Specific gravity of dry hydrate = 2.42
·         Specific gravity of dry Bauxite = 2.55 to 2.80
·         Bulk density of alumina = 0.95 to 1.05 tonne per m3.
·         Bulk Density of Bauxite = 1.20 - 1.30 t/m3.
·         Molar mass of Na2CO3 =106 gm/mole;
·         Static Angle of Repose of Bauxite = 30 to 32 degree
·         Dynamic angle of repose of Bauxite = 19 to 20 degree
·         Molar mass of Na2O = 62 gm/mole
·         Molar mass of NaOH = 40 gm/mole
·         Hardness of Bauxite = 2.5 to 3.0 Moh
·         Bond Index = 10.5 t0 18 kW/st
·         Specific Gravity of dry bauxite residue (redmud) = 2.30
·         Velocity of slurry in pipe line = 1.5 to 2.44 m/sec.
·         Design velocity of steam in pipeline = 20 to 40 metres per sec.
·         Domestic Water requirement in township = 135 litres /
·         Potable Water requirement in Industries = 65 litres /
·         Sewage generation rate = 96 litres/
·         Solid waste generation = 0.90 kg/
·         Stack Height (in metres) = 14 x (Q)0.3;  where Q = Emission rate of SO2 in kg/hr.
·         Total water for Alumina Plant = 6.5 to 10 m3/tonne alumina
·         Norms for Green belt = 25% of total plant area (min.)
·     Residue Factor is the ratio of sum total oxides in Bauxite to sum total oxides in residue (red mud) which gives tonnes of residue generated per tonne of Bauxite processed. Residue generation rate with respect to alumina production varies from 1.10 - 1.35 tonnes residue per tonne of alumina. Higher residue factor signifies either poor quality of bauxite or lower alumina recovery or both.

Hope, all major input data have been covered here. We will appreciate your valuable remarks / comments, if any.
Best regards.

Rajendra Kunwar

Tuesday, February 6, 2018

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 270oC 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.

Monday, February 5, 2018

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.