EFFECTIVE USE OF LDO FIRED BOILER FLUE GAS IN CARBONATION OF PROCESS LIQUOR AT ALKALI LEACHING BASED TUMMALAPALLE MILL
Carbon dioxide (CO2 ) is a major greenhouse gas (GHG), into the atmosphere, and there is strong
evidence that the buildup of GHGs is the primary cause of the global warming that has occurred in recent decades. CO2
concentration in the atmosphere has risen about 43% since the beginning of the industrial revolution in the mid-eighteenth century—half of that since 1980.
Tummalapalle project is alkali leaching based low grade uranium ore mining and milling project of India. Leaching of ground ore (80% passing, 74 micron particles -200 mesh) takes place by addition of sodium carbonate and oxygen. Boiler House is one of the important sections of Tummalapalle Mill for generation of steam which is used in various sections ex; SDU precipitation, counter wash and sustaining cake wash temperature in leach fi lters, redissolution of SDU cake in CPL and for initialization of chemical reactions inside autoclave. Total three boilers of capacity 15MT each are present at Tummalapalle Mill out of which two are in line and one is in standby condition. Boilers are oil fi red and maximum 863 kg/hr light diesel oil can be fueled for each burner. Air pre heater is installed for each boiler in which boiler exhaust fl ue gases are used for preheating of air for burning of boiler fuel. After preheating, fl ue gas is released out to atmosphere via stack. Several experiments were conducted for carbonation of liquor by fl ue gas and satisfactory results achieved on initial stage.
KEYWORDS: Flue Gas, Carbon dioxide, Boiler House, Tummalapalle Mill, Uranium Corporation of India Limited, Caustic Lye, Sodium Carbonate, Sodium Bicarbonate.
NOMENCLATURE: SSR – Sodium Sulfate Recovery Unit, CPCB – Central Pollution Control Board, MoEF – Ministry of Environment, Forest and Climate Change, LDO – Light Diesel Oil, DG – Diesel Generator, SDU – Sodium Diuranate, ID – Induced Draught, TSS - Total Suspended Solid, TDS - Total Dissolved Solid, CPL – Clarifi ed Pregnant Liquor, OSHAS - Occupational Health & Safety Assessment Series, ISO - International Organization for Standardization, BDL – Below Detection Limit, DL – Detection Limit, APH – Air Pre Heater, IBR – Indian Boiler Regulation.
1. Alkali leaching process & chemical reactions: Sodium bicarbonate generation takes place as per exothermic reactions (1,2,3,4) inside pressurized autoclave which helps in completion of chemical reaction in presence of oxygen. For autoclave operations, low concentration NaHCO3
is suffi cient and it benefi ts in product
precipitation. Addition of NaOH is done before leaching in autoclave for maintaining suffi cient value of TDS in circuit.
2UO2 UO3
+ O2 +3Na2 CO3 2UO3 +H2 NaOH+NaHCO3
O Na4 Na2
CO3
UO2 +H2
(CO3 (1)
)3+2NaOH (2) O (3)
Figure 1. Liquor Carbonation Flowsheet of Tummalapalle Mill. 2. Objectives of liquor carbonation: a).Conversion of caustic lye liquor to desired leaching reagents, b).Neutralization of caustic lye before discharge to tailings pond, c).Minimizing procurement of reagents from other sources,
Figure 2. Four Carbonation tanks at Tummalapalle Mill.
4. Stack monitoring reports of Tummalapalle Mill: Stack monitoring of exhaust gases is a regular procedure to be followed by Tummalapalle Mill as per recommendation of CPCB/MoEF. It is done by Lab, approved by MoEF, OHSAS 18001 and ISO 9001. All values of carbon dioxide were under prescribed limit (table 1).
2FeS2 4Na2
+ 7O2 SO4 2NaHCO3 + 8Na2 CO3
+ 8NaHCO3 +Δ Na2
+ 6H2 CO3 +H2 O 2Fe(OH)2 O+CO2 +
(4) (5)
Filtration of autoclave discharge takes place after alkali leaching for solid liquid separation. Filtrate goes to clarifi cation unit for removal of remaining TSS. Precipitation of clarifi ed liquor takes place for precipitating Sodium diuranate in presence of 48% conc. NaOH (reaction 6). SDU redissolution scheme is used for increasing U3
O8
concentration in feed liquor to precipitation. 2Na4
UO2 (CO3 )3+6NaOH Na2 U2 O7 +6Na2 CO3 +3H2 O (6)
Precipitate goes to SDU thickener for solid liquid separation. Causticization of SDU thickener overfl ow takes place after addition of lime inside slaker. After passing through press fi lter, fi ltrate goes to SSR unit for recovery of byproduct sodium sulphate. Filtrate after treatment in SSR unit contains sodium hydroxide which needs to be converted into desired leaching reagent (reaction 7). Sodium carbonate is generated by addition of CO2
in carbonation tanks.
2NaOH+CO2 Na2
CO3 CO3 +H2 +CO2 +H2 O 2NaHCO3 (8)
d).Control over economical expenditures, e).Maximizing recovery of end product (SDU), f).Control over usage of process water in system.
3. Carbonation Process at Tummalapalle Mill: SDU thickener overfl ow goes to carbonation circuit after processing at Causticization & SSR. Total four tanks of capacity 150 m3
each
are available at Tummalapalle Mill for completion of carbonation (fi gure 2). pH of liquor is approximately 12.5-13.5 after receiving from SSR due to presence of caustic lye. Carbon dioxide addition takes place approximately 15-18 minutes into carbonation tanks after fi lling level approx. 75-80%. pH is monitored by online pH indicator situated inside every tanks (fi gure 1). CO2
when pH reaches at 10.5-11.5. Caustic lye converts into sodium carbonate completely. Generation of sodium bicarbonate takes place in case of additional CO2 Maximum requirement of CO2
Na2 O (7) Tummalapalle Mill. Presently, procurement of CO2 purchase from other sources.
feeding stops
addition inside carbonation tanks. may reach upto 27 MT/day in
is done through
www.envirotech-online.com AET October / November 2019
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