Issue 22/2012 - Yellow phosphorus tail gas purification and utilization technology

Yellow phosphorus tail gas is rich in more than 85% CO, and contains impurities such as sulfur, phosphorus, arsenic, fluorine, etc.
, sulfur exists in the form of hydrogen sulfide and a small amount of organic sulfur, the content is 2000~3000mg/m3; Phosphorus exists in the form of phosphine and elemental phosphorus, with a content of 800~1500mg/m3; Arsenic and fluorine are low in content and both exist as hydrides, most of
which can be removed by washing with water or alkali.
The main task of yellow phosphorus tail gas purification is to remove sulfur and phosphorus
from the gas.
From the perspective of CO content, this gas has high industrial application value, but if it is directly used, there are not many
industrial applications suitable for it.
At present, most manufacturers will yellow phosphorus tail gas through extensive purification (water washing or alkali washing), only as fuel instead of bituminous coal to dry raw material ore, coke and peat phosphorus, etc.
, this use of the use is limited, less than 20% of the total exhaust gas emissions in the production process, the rest is still combustion and emptying, which not only wastes precious CO resources, but also causes serious pollution
to the environment.

There are two main ways to use the CO rich in yellow phosphorus tail gas: (1) use its combustion heat; (2) Use CO as the synthetic raw material
of carbon-one chemical industry.
Different ways of use put forward different requirements
for the degree of purification.

In recent years, some universities, research institutes and some yellow phosphorus plants have proposed a variety of yellow phosphorus tail gas purification methods, mainly including water washing + alkali washing method, sodium hypochlorite oxidation method, variable temperature and pressure adsorption method, catalytic oxidation method, etc
.
These purification methods have their own limitations, such as substandard purification depth, cumbersome purification cost, and unstable process
.
Finding an economically feasible and technically reliable purification technology is a prerequisite
for the full utilization of yellow phosphorus tail gas.

1.
Traditional yellow phosphorus tail gas purification technology

1.
Water washing or alkali washing can
achieve the purpose of cooling and dust removal, but the impurities in the yellow phosphorus tail gas are not very soluble except HF, so the efficiency of simple water washing to remove impurities in the gas is relatively low; When using alkaline solutions, the amount of acid dissolved in the solution, such as H2S in the exhaust gas, can be increased, but a large amount of CO2 in the exhaust gas relative to PH3 and H2S will consume a considerable amount of lye
.
The efficiency of this method to purify phosphorus and sulfur impurities is also not high, and the purified gas is mainly used to dry raw material ore, coke or peat and phosphorus
.
This method is an extensive type of purification and is often used as a pretreatment process
for other purification methods.
When the gas purified by this process is burned as fuel, the sulfur, phosphorus, etc.
in the gas will still cause pollution
to the environment.

2.
The method of variable temperature and pressure swing adsorption method
requires the gas to be washed with water, alkali and desulfurized before pressure swing adsorption and temperature adsorption, the whole process is long, the investment is large, the cost is high, and the composition of yellow phosphorus tail gas will fluctuate with the fluctuation of production conditions, which will also lead to the instability
of the process operation.
The impurity gas after variable temperature and pressure desorption needs to be further treated in another process, otherwise it will still lead to secondary pollution
.

3.
Sodium hypochlorite oxidation method
is a representative of chemical oxidation absorption method, other oxidants include hydrogen peroxide, sulfuric acid, potassium permanganate, etc
.
The main purpose is to remove PH3 in yellow phosphorus tail gas, and the chemical oxidation absorption method has defects
such as large oxidant consumption, difficult operation, and purification efficiency decreases with the decrease of oxidant concentration.
This approach
is rarely used in industry at present.

4.
The catalytic oxidation method
is mainly used to remove PH3 and elemental P4
from gas.
PH3 and elemental P4 in yellow phosphorus tail gas are oxidized by oxygen in the gas on the catalyst to generate P2O3 or P2O5 adsorbed in the
catalyst bed.
The adsorption capacity of these two on the catalyst bed is much greater than PH3 and elemental P4
.
Using this adsorption property, the total phosphorus in the yellow phosphorus
tail gas is purified.
When the catalyst bed adsorbs a certain amount of phosphorus oxides, the catalyst bed is penetrated and needs to be regenerated to continue dephosphorization
.
The disadvantage of this type of catalyst is that it needs to be regenerated, and the catalyst will increase with the increase of the number of regeneration, the efficiency of catalytic oxidation of total phosphorus will decrease significantly, at this time the catalyst is consumed and a new catalyst
needs to be replaced.
In this method, a purification device was built in Yunnan Xuanwei Phosphorus Power Co.
, Ltd.
, and the purified gas was used to co-burn with coal for power generation
.

Second, yellow phosphorus tail gas continuous phosphorus purification technology

1.
The content of hydrogen fluoride and hydrogen arsenide in the yellow phosphorus tail gas of the DePOx purification process
is low, and most of them can be purified by water washing and alkali washing, and the problem of industrial desulfurization, whether it is crude desulfurization or fine desulfurization, is also applied
in many fields.
Therefore, the key problem of yellow phosphorus tail gas purification is the problem of total phosphorus removal
.
Beijing Peking University Pioneer Technology Co.
, Ltd.
has developed a continuous phosphorus removal catalyst, which can continuously remove total phosphorus from yellow phosphorus
tail gas.
The process was tested in the lateral line of a phosphorus power company in Yunnan Province, and the experimental gas speed was 5333ml/gh and the catalyst loading capacity was 600g
.
The concentration of PH3 at the inlet is 500mg/m3, and the concentration of PH3 at the outlet has been lower than 10mg/m3; The total phosphorus concentration at the gas inlet was 770mg/m3, and the total phosphorus concentration at the outlet was always lower than 20mg/m3.
The inlet and outlet concentrations of hydrogen sulfide and other sulfides remained largely unchanged
.
The experiment was carried out continuously, and the total amount of purified gas exceeded 1200m3
.
This sideline experiment verified the adaptability of the catalyst to the real yellow phosphorus tail gas atmosphere, determined the practicability of the purification process, and solved the problem
of total phosphorus removal in the yellow phosphorus tail gas.

The characteristics of the catalyst are as follows:
(1) Continuity - the dephosphorization reaction is a continuous process, and there is no need to regenerate during the operation of the catalyst, which greatly simplifies the process flow and reduces the operating cost
.

(2) Stability - the catalyst has stable catalytic performance during the operation of continuous removal of total phosphorus, can run for a long time, and has good
stability.

(3) Sulfur resistance - the catalyst has good sulfur resistance, and the total sulfur content in the yellow phosphorus tail gas does not change before and after removal, and does not affect the continuous phosphorus removal performance
of the catalyst.

In this process, the total phosphorus can be removed in the yellow phosphorus tail gas atmosphere, and then the desulfurization method and desulfurization depth
are selected according to the utilization mode of yellow phosphorus tail gas.

2.
Utilization of
gas after purification in DePOx process (1) Combustion power generation yellow phosphorus production power consumption is large, yellow phosphorus tail gas is used for combustion power generation
, can reduce yellow phosphorus production costs
.
The DePOx process can purify the total phosphorus in the yellow phosphorus tail gas to less than 20mg/m3, which can meet the requirements of
combustion power generation.
The purified yellow phosphorus tail gas can be mixed with coal to burn boilers for power generation, or it can be directly burned to steam turbines for power generation
.

(2) The total phosphorus content of yellow phosphorus tail gas after DePOx purification of carbon-one chemical synthetic raw materials is less than 20mg/m3, and after further deep dephosphorization, the total phosphorus in yellow phosphorus tail gas is less than 1mg/m3, at this time, it is a gas containing H2S in CO atmosphere, and the gas after crude desulfurization and fine desulfurization can be directly used as a synthetic raw
material
for carbon-one chemical industry 。 At present, we have completed the experiments of deep desulfurization and phosphorus of yellow phosphorus tail gas in the laboratory, and the total sulfur and total phosphorus in yellow phosphorus tail gas can be removed to less than 1mg/m3, laying a good foundation
for yellow phosphorus tail gas to be used as a raw material for carbon-one chemical synthesis.