Fuming Furnace

The principle of fuming is a reduction and volatilization process, which is to
blow air and pulverized coal into the slag in the fuming furnace, generate heat
and carbon monoxide after combustion, keep the slag at high temperature, and
make the compounds and free ZnO and PbO in the slag reduced to Zn and Pb
vapors, the vapor will rise to the upper space of the furnace, meet CO2 or
inhaled air and oxidize to zinc oxide and lead oxide again, and are collected
in the state of soot.
The four walls of the fuming furnace are all composed of water jackets. The
lowermost water jacket on the side is provided with a tuyere. The inner wall of
the water jacket is welded with Φ16mm and 50mm long pins, and the distance
between the nails is 50mm, so that the condensed slag adheres to it and forms
slag layer protects the water jacket and reduces heat loss.
There is a slag outlet on the water jacket at the front end of the furnace.
The lower layer of the water jacket and the inclined flue are water-cooled and
circulated, and the upper layer of the water jacket is cooled by vaporization.
The tuyere structure adopts a tuyere with a ball valve, and there are two
branch pipes. The one near the water jacket is called the primary air coal
pipe, and the one outside is the secondary air pipe.
In order to prevent pulverized coal from escaping out of the furnace, the
secondary air pressure is slightly higher than the primary air pressure (about
100mmHg).
There are three air outlets on the upper part of the furnace, which is
convenient for operation and observation, and can inhale air oxidized metal
vapor.
 
Fuming Furnace Operational
In order to achieve good results in the fuming process, the following
conditions must be well controlled:temperature, fuel and air supply volume,
slag composition, molten pool depth.
 
1. Temperature
If the temperature is too high (such as exceeding 1350°C), iron deposits or
Zn-Fe, Sn-Fe alloys may be formed, which is harmful to the fuming operation. If
the temperature is too low, the reduction rate of metal oxides will slow down,
the volatilization rate will decrease, the fluidity of the slag will
deteriorate, and there may even be a danger of furnace formation, so the
temperature in the fuming practice is kept in the range of 1150-1300 °C.
 
2. Fuel and air supply volume
The most active factor affecting the volatilization rate in the fuming process
is the air supply volume. Because the temperature in the furnace, the ratio of
CO/CO2, the amount of gas, and the vapor pressure of the metal are all related
to the air supply volume. The volume of the air supply depends on the
pulverized coal consumption and the air utilization coefficient (α). The
larger the air utilization coefficient, the higher the thermal effect of the
fuel, the greater the partial pressure of CO2, and the higher the temperature
in the furnace; the smaller the air utilization coefficient , the thermal
effect is reduced, the partial pressure of CO becomes larger, and the reducing
ability is enhanced.
Pulverized coal is both a reducing agent and an exothermic agent in the fuming
process, so the CO/CO2 should have an appropriate ratio during combustion. The
air utilization factor actually adopted by the factory is variable. Usually, at
the beginning of fuming, an air utilization factor close to 1.0 causes almost
all carbon to be burned into CO2 to increase the temperature of the slag. In
the reduction stage, adjust the air utilization factor to make carbon into CO
as much as possible to improve the reduction ability.

3. Slag composition
In practical the actual zinc content of the slag feed into the fuming furnace
should be >6%. Below 4%, fuming treatment is uneconomical.
Regarding the effect of slag composition on the volatilization rate of zinc, it
can be summarized as follows:
The increase of CaO content can increase the volatilization rate of zinc;
The increase of FeO content in slag has little effect on ZnO activity;
The content of SiO2 increases, and the volatilization rate of zinc decreases.
Therefore, the silicic acidity of the slag is preferably in the range of 1.1 to
1.2.
 
4. Molten pool depth
The thickness of the slag layer in production is generally in the range of 700-
1000mm above the tuyere area.

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