PRESENTATION ON FUME TREATMENT PLANT AT NALCO

About NALCO

Incorporated in 1981, as a PSU, National Aluminium Company Limited (Nalco) is Asia's largest integrated aluminium complex encompassing bauxite mining, alumina refining, aluminium smelting and casting, power generation, rail and port operations.

Based on energy efficient state-of-the-art technology of smelting and pollution control, the Smelter Plant is in operation since early 1987.

Initially 3,45,000 tpa capacity Aluminium Smelter, located at Angul in Orissa, is expanded to 4,60,000 tpa.

FTP & its NEED

The FTP (Fume Treatment Plant) is designed to treat gas from hooded aluminium reduction pots where the purposes of the plants are to recover particulate & gaseous fluorides & collect & remove dust of alumina and carbon from pot gas.

The generation of HF is an unavoidable process accompanying the Hall-Heroult aluminum production.

Any hydrogen source entering the electrolyte will potentially create hydrogen fluoride.

Three main sources of HF are the water and hydroxyls associated with the fed alumina, hydrogen content in the anode and moisture in the air.

2AlF3(diss) + H2O(g) = 1/3Al2O3(diss) + 2HF(g)

3NaAlF4(g) + 3H2O(g) = Na3AlF6 + Al2O3(s) + 6HF(g)

Constituents Of Cell Fumes:

Gases

HF 

H2S 

CF4 

CS2 

C2F6 

SO2

SiF6 

CO2 

H2O 

CO 

C Carbon soot AlF3 Aluminium Fluoride

AL2O3 Alumina CaF2 Calcium Fluoride

NA3AlF6 Cryolite Na5AlF14

Chiolite

Particulates

Process Flow diagram

The proposed mechanism of HF adsorption on alumina

Physical adsorption of HF on the alumina surface (hydroxyl groups bonded to surface and/or physisorbed by water) to form alternating layers of HF and H2O. Basically, it is a surface process where humidity is present.

Direct chemisorption of HF at reactive sites on the alumina surface with formation of Na-F and/or Al-F species and with additional monolayer being physically adsorbed on top of chemisorbed layer is the Lewis acid-base complex. Al3+ is by definition Lewis hard base and F- is Lewis hard acid. Due to their chemical properties they form together most stable complex.

Due to their chemical properties they form together most stable complex.

Al2O3 + 6 HF 2 AlF3 + 3 H2O

Factors affecting Dry scrubbing

It is important to get a good, equal mixing in the injection zone where the relative velocity between gas and alumina particles is high. As soon as particles and gas are moving with the same velocity, the HF scrubbing efficiency drops down because of an HF concentration gradient around the alumina grains.

BAG FILTER SYSTEM

The main purpose of the bag filter is to collect the particles in the gas stream.

The alumina serves two purposes:

1. Adsorbent for the hydro fluorine gases.

2. To create a dust layer on the filter bags, this will collect particles even smaller than filter fabric mesh.

As the mixture of gas & alumina passes through the fabric, the dust accumulates on the outside of the bags. Each bag is supported on the inside by a metal cage to prevent the bag from collapsing. The clean gas flows up & out of the top of the compartment.

The proposed mechanism of HF adsorption on alumina

The cleaning of the bag is accomplished by a pulse jet technique which consists of a low pressure reverse pulse of air introduced into the open top of the filter bag.

The bag filter resistance or pressure drop depends on

Dust and alumina concentration and composition

Gas flow per unit time and filter area

Cleaned frequency and time

Cleaned energy(pressure)

Gas temperature

Recent Up gradation

Particulate aluminium oxide is mixed with the gases• contact reactor

The mixture is separated with bag

filters

sulphur dioxide is to a certain extent adsorbed in these processes

sulphur dioxide thus accompanies the aluminium oxide back to the aluminium-production process

The modified process

-• In the adsorption process the gas is treated in a

first dry adsorption stage with at least partly spent particulate aluminium oxide,

-• The aluminium oxide with adsorbed fluorine-

containing substances is separated from the gas downstream from the first adsorption stage

-

• A part of the separated aluminium oxide with adsorbed fluorine-containing compounds is removed from the adsorption process, and the remainder of the aluminium oxide is recirculated in the first adsorption stage

-• At the same time the gas is transferred to a

second dry adsorption stage arranged downstream from the first adsorption stage

The sulphur dioxide thus is not removed from the gases, but is instead in undesirable fashion recycled and concentrated in a system including the aluminium-production furnace and the gas-cleaning equipment, and one further obtains an increase of the sulphur-dioxide content in the air on the premises.

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Aluminium oxide has a much higher affinity with hydrogen fluoride than with such gases as sulphur dioxide.

By partly recirculating the adsorbent in at least the first adsorption stage, the sulphur dioxide already adsorbed will be desorbed, while hydrogen fluoride takes the place of the sulphur dioxide on the active surface

As a result, the adsorption of undesirable substances can be monitored and minimized, such that it is only a minimum of these substances that is recycled to the electrolysis furnace together with the adsorbent.

This recirculation is motivated by two reasons. 1. One wishes to control and optimize the adsorption of gaseous fluoride from the

process gas in the first adsorption stage. 2. One wishes to obtain the aimed- at desorption of such substances as sulphur

dioxide, which have been adsorbed on the aluminium oxide in the second adsorption stage