WO2020111955A1

WO2020111955A1 - Chamber for combustion of white phosphorus and method for combustion of white phosphorus therein

Info

Publication number: WO2020111955A1
Application number: PCT/PL2018/050067
Authority: WO
Inventors: Damian MILDE; Leszek Urbańczyk; Michał OCHMAN; Marcin Figura
Original assignee: „Alventa” Spółka Akcyjna
Priority date: 2018-11-29
Filing date: 2018-12-14
Publication date: 2020-06-04

Abstract

The invention relates to the chamber for combustion of white phosphorus comprising a tank, particularly having a cylindrical cross-section, with a lid transforming into a process gases collector, wherein the chamber is equipped with a furnace and phosphorus feeding system. The chamber in the embodiment is characterized by that the furnace (7) is rotatable, and above the furnace (7) near its upper edge, an outlet of the melted phosphorus delivery system (4) is placed, wherein air lances (2) suitable for supplying compressed air are placed in the tank (9) circumference. The essence of the invention relates also to a method for combustion of white phosphorus in this chamber, serving the purpose of obtaining phosphorus pentoxide with a reduced level of arsenic, wherein: liquid phosphorus is dosed onto a rotating furnace (7) in presence of rotational movement of air in the combustion chamber (9), the furnace rotation rate being adjusted so that the phosphorus is combusted superficially on its entire surface; then, the liquid phosphorus influx to the combustion chamber is increased concurrently with the increase in the amount of the supplied compressed air, in order to maintain 1.5 times stoichiometric excess of oxygen in relation to phosphorus, the phosphorus combustion temperature in the process gases collector being maintained in the range of 500°C÷600°C; then, the gases that are formed as a result of the phosphorus combustion are cooled down, wherein at the first stage of condensation, the average temperature of the gases at the inlet to the condensation chamber is 420°C, while at the outlet of the chamber it is 270°C, on average; then, the product is collected at the bottom of the chamber in an indirect or direct way.

Description

Chamber for combustion of white phosphorus and method for combustion

of white phosphorus therein

The solution according to the invention relates to a white phosphorus combustion chamber and a method for combustion of white phosphorus in this chamber, as a result of which phosphorus pentoxide with a reduced level of arsenic is obtained. The chamber is equipped with a rotatable furnace, air nozzles and a gland system, and the phosphorus combustion occurs in this chamber at a lower temperature of exhaust gases. The invention is associated with the industrial chemistry field.

In prior art, construction solutions used for phosphorus combustion in air are known. For example, US Patent Application No. US3598525A discloses a solution including elements used for pressurized phosphorus outflow from diffusion flame burners. In this solution, dispersed phosphorus-air mixture flowing out of the burner undergoes a spontaneous ignition. In turn, US Patent Application No. US4219533A discloses that process gases reach a very high temperature in the combustion chamber, ranging from 1200 to 1600°C, and must be cooled down before leaving the combustion chamber to a temperature close to the temperature of phosphorus pentoxide condensation. The cooling efficiency depends on the presence of humidity in the air that directed to combustion. In the known solutions, disclosed, among others in US Application No. US4603039A, it is recommended that a highly dried air with a dew-point at about -70°C is used. Humidity brought in combustion together with the air results in formation of hot polyphosphoric acids on the cooling surfaces, hindering the heat exchange and causing severe corrosion of steel.

In turn, a diffusion flame burner placed in a combustion chamber and used for phosphorus combustion is known from the description of Chinese utility model No. CN206799171U. The burner comprises an external tube, an internal tube with phosphorus, a water jacket for molten phosphorus heating, and a diffusion nozzle, in which the molten phosphorus is mixed with air. The mixture obtained in the burner is atomized in the combustion chamber.

Depending on the source, white phosphorus available on the market contains a number of impurities in its composition. The main impurities are substances such as arsenic, iron or unidentified organic compounds originating from the raw material, e.g. phosphorites or apatites, or a reducer used in the phosphorus obtaining process, e.g. carbon. A common practice in obtaining a high quality P2O5 consists in a pre-purification of phosphorus by its distillation in an oxygen-free atmosphere. The distillation process requires a significant amount of energy and a complex heating and cooling apparatus, and it should be carried out under reduced pressure. An example of a process of phosphorus distillation was disclosed in US Patent Application No.

US4575403A. A necessity to use an additional stage requiring reduced pressure and elevated temperature increases the process costs and thereby, a higher market price of the manufactured P2O5 in relation to direct production from commercial phosphorus.

US Patent Application No. US2016185599A also teaches a method for purification of yellow phosphorus as a raw material for phosphorus acid. In this method, purification of yellow phosphorus consists in the removal of impurities from yellow phosphorus by mixing with an addition of an oxidizing agent, and in a subsequent stage, removal of impurities by mixing addition of a solution containing an additive with a functional group exhibiting an affinity to phosphorus. This method enables purification of yellow phosphorus with a reduced

consumption of water. A disadvantage of this solution comparing with the discussed application based on the combustion chamber with a rotatable furnace, consists in a necessity to use at least two additional stages of phosphorus purification, i.e. oxidation of impurities in a hot solution and isolation of phosphorus from the mixture system.

In turn, European Application No. EP3281913 A1 teaches a method for purification of contaminated phosphorus including contacting the contaminated liquid phosphorus with active carbon in a continuous way, with a constant intake of the purified phosphorus from a stationary bed or active carbon bed. The disclosed method increases the cost-effectiveness of the process and limits the loss of phosphorus connected with the disposal of the used up active carbon. One should note that this method produces an additional waste, namely the active carbon saturated with impurities and residues of phosphorus. The process requires continuous mixing of the hot liquid phosphorus with a prepared active carbon suspension and selective separation of this mixture. The discussed method based on a direct phosphorus combustion in a rotatable furnace does not include the above inconveniences.

Despite phosphorus combustion chambers equipped with systems for its combustion being known in the prior art, construction solutions that would provide optimization of the combustion process, including reduction of the combustion temperature, increase in the process efficiency and quality of the obtained product are still being sought. Methods to obtain phosphorus pentoxide that would provide optimization of the combustion process, including reduction of the combustion temperature, increase in the process efficiency and improvement of quality of the obtained product, especially with a reduced level of arsenic, are also looked for. The goal of the invention was to develop a phosphorus combustion chamber, in which phosphorus would be combusted entirely at a lower temperature, i.e. at the process gases temperature below 600°C, which facilitates condensation of phosphorus pentoxide while maintaining the process efficiency. Another goal purpose of solution according to the invention was to develop a method for phosphorus combustion where its full combustion would occur at a lower temperature, i.e. at the process gases temperature below 600°C. The obtained lower temperature of the process gases enables a more effective condensation of phosphorus pentoxide without a necessity to use process additives, while maintaining the process efficiency.

The essence of the solution according to the invention is a chamber for combustion of white phosphorus, comprising a tank, particularly a tank with a cylindrical cross-section, having a lid transforming into the process gases collector, wherein the chamber is equipped with a furnace and a phosphorus feed system, characterised in that the furnace is rotatable, and above the furnace, at its upper edge, an outlet of the molten phosphorus feed system is placed, and wherein in the tank circumference, air lances for compressed air supply are located. Melted phosphorus is poured onto the surface of the furnace with a specific rate, to create a flat pane of liquid phosphorus enabling its uniform combustion. Walls of the chamber according to the invention may be covered with thermal insulation.

Preferably, there are 3 to 7 air lances located in the circumference of the tank of the chamber for white phosphorus combustion, and the lances are installed on the circumference of the tank at a height between the chamber base and the upper edge of the furnace.

Particularly preferably, the lances are installed at the angle of 20÷45°, preferably 40°, in relation to the diameter of the circle constituting the A-A cross-section of the combustion chamber, wherein the deviation angle is consistent with the furnace rotation direction. During the research carried out, it was found that it is important for the angle of the lances to be consistent with the furnace rotation direction. Such an orientation of the lances causes a rotatable movement of the air between the chamber walls and the furnace. As a result of the observed phenomenon, the combustion chamber walls are cooled down and they are physically isolated from the contact with P2O5, which prevents formation of corrosive polyphosphoric acids on them.

Preferably, the furnace in the solution according to the invention has a shape of an inverted cone with its top mounted on a motoreducer shaft. Due to this shape of the furnace, a uniform surface of phosphorus combustion is obtained, resulting in obtaining a better quality phosphorus pentoxide.

Equally preferably, at the chamber according to the invention has a motoreducer installed, and the motoreducer drive shaft is sealed with a gland system in the place where it passes through the combustion chamber base. The gland system consists of a small chamber localized centrically around the drive shaft. Particularly preferably, a gap between the gland and the drive shaft at the external side of the combustion chamber is smaller by 1 to 2 mm than the gap at the internal side side of the combustion chamber. The gland system ensures that an overpressure is formed in the gland chamber, generated by a continuous supply of compressed air. Due to the difference between the dimension of the gaps around the shaft, the excess of air flows to the interior of the combustion chamber, which guarantees its air-tightness and cools down the drive shaft surface and the bottom part of the rotatable furnace. Air used in the gland system performs a sealing function for the combustion chamber, cools the furnace drive shaft and it is used in the phosphorus combustion directly.

The invention also relates to a method for combustion of white phosphorus resulting in obtaining phosphorus pentoxide with a reduced level of arsenic, wherein:

a) liquid phosphorus is dosed to the combustion chamber onto the rotating furnace together with the rotational movement of air, the furnace rotation speed being regulated so that phosphorus is combusted superficially on its entire surface;

b) then, the influx of liquid phosphorus to the combustion chamber is increased

concurrently with the increase in the amount of the compressed air supplied, so as to maintain stoichiometric excess of oxygen in relation to phosphorus equal to 1.5, the phosphorus combustion temperature in the process gases collector being maintained in the range of 500°C÷600°C;

c) then, the gases formed as a result of the phosphorus combustion are cooled down, wherein at the first stage of condensation, the average temperature of the gases at the inlet to the condensation chamber is 420°C, while at the outlet, it amounts to 270°C on average;

d) then, the product is collected at the bottom of the chamber in an indirect or direct way.

Preferably, at stage a) of the method according to the invention, the rotational movement of air is achieved by supplying compressed air with an operating pressure in the range of 3.5÷4.5 bar and flow in the amount of 45÷50 m³/h.

Preferably, at stage a) of the method according to the invention, liquid phosphorus having a temperature in the range of 75÷95°C is dosed in the amount of 4 kg/h with the chamber capacity in the range of 1 ,5÷2 m³.

Preferably, at stage b) of the method according to the invention, the liquid phosphorus dosing speed is increased until it reaches 8 kg/h with the chamber capacity in the range of 1 ,5÷2 m³. Equally preferably, at stage b) of the method according to the invention, the process gases temperature in the collector amounts to 520÷560°C.

Preferably, the stage c) of the method according to the invention, is followed by an additional stage c’) which constitutes a second stage of condensation, wherein the process gases are cooled down to a temperature of about 200°C while measured at the chamber outlet. Particularly preferably, the stage c’) is followed by a stage c”) which constitutes a third stage of condensation, wherein the process gases are cooled down to the temperature of about 120°C while measured at the chamber outlet.

The advantage of using the chamber according to the invention in the process of obtaining high quality phosphorus pentoxide consists in a possibility to carry out the process at a relatively low temperature ranging from 450 to 550°C, while compressed air with a dew point ranging from -5 to 5°C is supplied. As a result, a high concentration of phosphorus pentoxide in the process gases is obtained. An additional advantage of the solution according to the invention consists in a reduction of liberation of polyphosphoric acids which corrode the chamber. Such a solution does not require cooling of the furnace walls and minimises the formation of polyphosphoric acids simultaneously, which protects the furnace and the combustion chamber from corrosion. An additional advantage of application of the method according to the invention consists in tha fact that due to the superficial combustion of phosphorus as a result of its free outflow onto the rotating furnace, a high purity phosphorus pentoxide is obtained directly from non -purified phosphorus. It was found during experimental tests that this method for obtaining phosphorus pentoxide allows for separating the impurities from the product stream at several stages.

While carrying out the combustion process in furnace systems known from the prior art, the whole material, including impurities, undergoes dispersion and enters the product directly. During a free combustion of phosphorus on a rotatable furnace, non-volatile impurities of phosphorus, such as iron and other metals and their oxides, do not pass into gaseous phase, and thus, they do not contaminate the product. Incombustible impurities accumulate at the bottom of the rotatable furnace in the form of sludge which must be removed from the combustion chamber periodically and disposed. This is the first of the process stages for obtaining pure P2O5 directly from contaminated phosphorus.

The second stage of the purification occurs in the entire volume of the combustion chamber with a rotatable furnace. The lances supplying compressed air to the combustion chamber are installed at the angle of 45° in relation to the chamber cross-section diameter, in the direction identical to the direction of rotation of the phosphorus furnace. An appropriate positioning of the lances causes formation of a rotational movement of air in the combustion chamber. It was found in the experiments that due to such a movement, an additional phenomenon of gas purification occurs in the combustion chamber. During this stage of the process, phosphorus pentoxide obtained during the process of combustion has mostly the form of a gas or -to a slight degree— a volatile fog which is easily raised by the rotational movement of air and directed to the exhaust collector. In case of other, heavier substances, e.g. fine fractions of sludge raised from the furnace and solid impurities of phosphorus, the rotational movement of air causes a centrifugal force and the solid particles are directed towards the chamber walls to a laminar zone. The impurities moved away to the laminar zone flow slowly to the bottom of the combustion chamber, while not mixing with the process gases.

The third stage of purification starts in the process gases collector. Due to the application of a chamber with a rotatable furnace, it is possible to obtain the process gases at a temperature of about 500°C. It allows to utilise the difference between the condensation temperatures of phosphorus pentoxide and arsenic oxides. In the case of classic furnace systems, the process gases reach temperature exceeding 1000°C, precluding physical separation of P2O5 from arsenic oxides. Under the process conditions, arsenic being a basic non-metal impurity of phosphorus, combusts in an analogues way as phosphorus, forming arsenic oxides, mainly AS2O3. The condensation temperature of gaseous AS2O3 is about 465°C, whereas in case of P2O5, a liquid form of this substance practically does not occur. It is only at a temperature below 423 °C that the phosphorus pentoxide resublimation process directly from the gaseous to the solid phase begins, proceeding with a fastest rate at a temperature higher than 360°C. The gases which leave the combustion chamber with a rotatable furnace are characterized by a temperature close to the arsenic oxide condensation temperature. Due to this fact, AS2O3 undergoes a condensation already at the stage of the process gases collector, where it is blown away from the system in a form of a fine fog, whereas phosphorus oxide requiring a lower temperature for resublimation, undergoes condensation not earlier than in the further part of the system. At the stage of the first condensation chamber, strong cooling of the process gases occurs, leading to precipitation (resublimation) of solid P2O5 in the form of flakes and dust. As a result of a small amount of humidity present in the air used for phosphorus combustion (dew point of about -5°C), the dust and flakes of P2O5 conglutinate to bigger agglomerates and fall out of the stream of the process gases. Arsenic oxide does not have any strong hygroscopic characteristics like those of P2O5, and, consequently, it does not undergo further agglomeration and is blown away from the system in the form of a fog, together with residual gases.

In the solution being the subject of the invention, it is possible to use several condensation chambers connected in series. Due to such a solution, another grade of P2O5 is obtained in each chamber. The pentoxide obtained in the first chamber is characterized by the lowest arsenic content and the highest bulk density, because only a coarse-grained P2O5 can precipitate in the first chamber, whereas fine fractions and AS2O3 fog are blown away with the air to further chambers. The products obtained in every subsequent condensation chamber are characterised by higher arsenic contents, lower bulk densities and smaller grain sizes. This relationship is shown in the plot constituting Fig. 1.

In order to utilise the separation of P₂O₅ into fractions, a specific construction of the condensation node is used. The first condensation chamber has the smallest operating capacity. Particularly preferably, the chamber has a conical shape with a water-cooled jacket. To achieve the phosphorus combustion output up to 10 kg/h in a chamber with a rotatable furnace, the condensation chamber should have a height of 2000 mm and a top diameter of 1200 to

1500 mm, wherein process gases are supplied from the top by a centric pipeline with a diameter of 200 to 250 mm. The bottom diameter of the condensation chamber should be equal to the process gases pipeline, i.e. 200 to 250 mm, application of an air-tight system for receiving P₂O₅ being necessary. Every consecutive condensation chamber (V_n+i) should be characterised by an operating capacity larger by 15-35% while compared to the preceding one (V„), preferably by 25%. This dependence is described by the formula:

V_n+1 = V_n * (1.25 ± 0.1)

As contrasted with the first chamber, the subsequent chambers should not be equipped with a cooling water jacket, because at this stage, slow cooling of the process gases was found to be most preferable.

The subject of the invention is shown in embodiments that do not limit the scope of the invention, and in the drawing, where:

Fig. 1 is a longitudinal section of the combustion chamber,

Fig. 2 is cross-section of the combustion chamber in the plane that includes the air lances,

Fig. 3 shows a distribution of the phosphorus pentoxide bulk density in the individual condensation chambers;

Fig. 4 shows the change in the quality of the obtained P2O5, concerning the arsenic content in the individual condensation chambers.

Referring to Fig. 1 and Fig. 2, the phosphorus combustion chamber in one embodiment is constituted by a cylindrical tank 9 with a conical lid transforming centrally into a collector of process gases 10 which are directed to the phosphorus pentoxide condensation system. The chamber is made from acid resistant steel. The external walls of the chamber are covered with thermal insulation 1. Inside the chamber, on its bottom, a rotatable furnace 7 is located centrally. The furnace rotation movement is circular. In the base of the chamber, a system having a form of a stub pipe 4 feeding molten phosphorus to the movable furnace 7 is located.

The chamber has a compressed air collector 3 connected, equipped with supply lances 2. In the cylindrical part of the tank 9, at the height of the upper edge of the furnace 7, seven air lances 2 are built in. In another version of the solution according to the invention, three lances 2 were also used. The lances 2 are used to distribute compressed air inside the chamber in a way that guarantees a rotational movement of the process gases. Referring to Fig. 2, the lances are installed at an angle of 40°, but they can be mounted at any angle in the range of 20÷45° in relation to the diameter of the circle constituting the A-A cross-section of the combustion chamber. The configuration of the lances 2 ensures that compressed air is distributed inside the chamber in a way that guarantees a rotational movement of the process gases.

In the cylindrical part of the tank 9, there is a single air-tight inspection opening 8 for the purpose of observation of the chamber interior during the process. In the top lid, a temperature sensor 11 is located for the purpose of control and adjustment of the process gases temperature. The furnace 7 has a shape of an inverted cone with its vertex mounted on the motoreducer shaft 5. The furnace rotation is regulated by a frequency converter, so that a uniform outflow of the molten phosphorus onto the interior surface of the furnace 7 is provided.

The motoreducer 5 is installed outside of the combustion chamber. The motoreducer 5 drive shaft is sealed with a gland system 6 in the place where it passes through the base of the combustion chamber. The gland system 6 consists of a small chamber located centrically around the drive shaft. The gland system 6 chamber is supplied with compressed air. The gap between the gland 6 and the drive shaft at the external side of the combustion chamber is narrower by 1 to 2 mm than the gap at the internal side of the combustion chamber. This solution ensures that an overpressure generated by a constant inflow of the compressed air occurs in the gland chamber 6. Due to the difference in the dimension of the gaps around the shaft, the excess of air flows to the interior of the combustion chamber, guaranteeing its air-tightness and cooling the surface of the drive shaft and the bottom part of the rotatable furnace 7.

Through the supply system 4, molten phosphorus flows freely and without any pressure onto the surface of the furnace rotating with a rate of 4÷8 1/h, and automatically combusts in the atmosphere of air supplied to the combustion chamber through the system of nozzles from the compressed air collector with an operating pressure of 3.5÷4.5 bar and a dew point from -5°C to 5°C. In one of the experiments, liquid phosphorus was dosed onto a movable furnace in the combustion chamber at a temperature of 85°C and in an amount of 2 1/h. The furnace movement rate was set to 20 revolutions per minute. The lances supplying the air, in the number of 6, were deviated in the direction consistent with the furnace rotation to the angle of 40° in relation to the diameter of circle constituting the A-A cross-section through the combustion chamber. The amount of the dosed phosphorus was gradually increased while maintaining the air excess at a level of 1.5 times in relation to the stoichiometric oxygen demand. Due to application of a movable furnace and nozzles set at an angle, the maximum dosing speed of phosphorus for combustion could be increased to 8 1/h without causing overheating of the chamber above 600°C.

A comparative test for a combustion chamber with a stationary furnace was also carried out. In this case, liquid phosphorus at a temperature of 85°C and in the amount of 2 1/h was dosed onto a stationary furnace of the combustion chamber. Air for combustion was supplied using 6 lances positioned perpendicularly to the centre of the chamber. The amount of the dosed phosphorus was gradually increased, while maintaining the air excess at a level of 1.5 times in relation to the stoichiometric oxygen demand. Approaching the phosphorus dosing speed of 5 1/h, overheating of the combustion chamber to a temperature exceeding 600°C was observed, which constitutes an adverse phenomenon due to mechanical and corrosive resistance of steel.

In the embodiment, phosphorus combustion chamber 9 with a rotatable famace 7 was supplied with compressed air with operating pressure of 3.5÷4.5 bar and, the initial rotational flow of air was set to 45÷50 m³/h. In order to obtain an appropriate rotational flow of air, the compressed air lances 2 were positioned at the angle of 45° in relation to the cross-sectional chamber diameter.

Afterwards, when the air flow got stabilized, an engine 5 driving the rotatable furnace was turned on, and liquid phosphorus having a temperature of 75÷95°C and in the amount of 4 kg/h was started to be dosed. Additionally, the course of the process was observed via inspection openings 8 in the combustion chamber 9, adjusting the furnace rotation rate so that the combustion of phosphorus was superficial on the entire surface of the furnace. The rotation of the furnace enabled uniform superficial combustion of phosphorus, which has a significant impact on the purity of the obtained product, ensuring minimal raising of solid impurities which remain in the layer of sludge under the liquid phosphorus. In the case of a non-uniform combustion of phosphorus, raising of solid impurities is greater, deteriorating the quality of the product.

After adjustment of the furnace revolutions, the inflow of liquid phosphorus to the combustion chamber was gradually increased from 4 kg/h to 8 kg/h together with a simultaneous increase in the amount of compressed air flowing to chamber 9 through the lances 2 collector, so that an excess of oxygen at the level of 1 ,5 times of that resulting from stoichiometric amounts of this reaction in relation to phosphorus was maintained. A correspondingly high excess of air results from the necessity of a complete combustion of the phosphorus in order to avoid formation of intermediate phosphorus oxides. Additionally, a rapid flow of air through the lances set at an angle ensures that a rotational movement of gases occurs and centrifugal force arises, directing the heavier fractions towards the combustion chamber walls and removing them from the stream of the process gases.

After reaching the phosphorus inflow value level of 8 kg P/h, the subsequent process was carried out within a temperature range from 500°C to 600°C. If the temperature was lower than 500°C, the amount of the dosed phosphorus was increased, whereas if the temperature increased and approached 600°C, then the amount of the phosphorus being dosed was reduced. If 600°C is exceeded, an increased corrosion of the system occurs, deteriorating the quality of the obtained P2O5, mainly by an increase in the iron content in the product. The phosphorus combustion process was especially effective at the temperature of 520÷560°C in the process gases collector.

At the first stage of the condensation, the gases forming due to the phosphorus combustion in chamber 9 are cooled down, wherein the average temperature of the gases at the inlet to the condensation chamber is approx. 420°C with the phosphorus amount combusted of 8 kg/h, while at the outlet of the chamber, it is approx. 270°C. At this stage, condensation takes place in the first condensation chamber cooled with water. The condensation process is particularly effective when the gases temperature at the inlet is below 450°C, which enables initial condensation of arsenic oxides in the form of a fog which is subsequently moved away by the stream of gases in the centric part of the condensation chamber and does not mix with P₂O₅ which desublimates near the walls of the condensation chamber. Due to a low temperature of the process gases in such a solution, phosphorus pentoxide of a high purity and a low arsenic content is obtained in the first condensation chamber.

Subsequent stages of condensation may be carried out to increase the yield of P2O5 and precipitation of finer, dusty fractions of the product. In order to ensure deposition of finer, dusty fractions, the operating capacity of the subsequent chambers must be greater and greater (described by the formula of Fig; 2)

At the second stage of the condensation process, the gases are being cooled down from the temperature measured at the inlet to the second condensation chamber, being approx. 260°C with phosphorus burnt in the amount of 8 kg/h, whereas at the outlet, it is approx. 200°C.

At the third stage of the condensation, the process gases are being cooled down from the temperature measured at the inlet to the third condensation chamber, being approx. 190°C with phosphorus burnt in the amount of 8 kg/h, whereas at the outlet, it is approx. 120°C. The third condensation chamber has the largest operating capacity and enables precipitation of a fine light fraction of P₂O₅ with the bulk density below 250 kg/m³ (Fig. 3). Together with the increased precipitation efficiency of the phosphorus pentoxide fine fraction, the coefficient of precipitation of the arsenic oxide fog increases as well. As a result, the product obtained from the last chamber contains the highest As content comparing to the previous chambers (Fig. 4). After leaving the last condensation chamber, the process gases are characterised by a low temperature that enables directing the residual gases to the phosphorus acid production scrubber or absorber.

In the plot constituting Fig. 3, the characteristic of the qualitative change of the obtained P2O5 in terms of the arsenic content is shown. A linear increase in the arsenic content was observed for the three-stage condensation process in the system of three condensation chambers. The results obtained during the tests were compared with various P2O5 grades available on the market and with the results for P2O5 obtained while carrying out the combustion in the system with furnaces. Based on information included in the specification sheet, a typical phosphorus pentoxide on the market contains a declared arsenic amount < 100 ppm. During chemical analysis, it was shown that P2O5 obtained in the system of furnaces contains 70÷80 ppm of As on average, and this amount can be greater if the pentoxide is obtained from phosphorus of a worse quality. It may even exceed 100 ppm of As. In the discussed method, when a low purity phosphorus (Kazakh) was used, very pure P2O5 was obtained in the first condensation chamber, with arsenic content < 40 ppm. The comparison was presented in Table 1.

Table 1

Claims

1. A chamber for combustion of white phosphorus, comprising a tank, particularly having a cylindrical cross-section, comprising a lid transforming into a process gases collector, the chamber being equipped with a furnace and phosphorus delivery system, wherein the furnace (7) is rotatable, and an outlet of the melted phosphorus delivery system (4) is placed above the furnace (7) at its upper edge, with air lances (2) for supplying compressed air being placed in the tank (9) circuit.

2. The chamber for combustion of white phosphorus according to claim 1, wherein 3 to 7 air lances (2) are located in the tank circuit (9).

3. The chamber for combustion of white phosphorus according to claim 1 or 2, wherein the air lances (2) are placed at the circumference of the tank (9) at a height between the chamber base and the furnace (7) upper edge.

4. The chamber for combustion of white phosphorus according to any claim 1 -3, wherein lances (2) are positioned at the angle of 20÷45°, preferably 40°, in relation to the circle diameter that constitutes an A-A cross-section of the combustion chamber, the deviation angle being consistent with the furnace rotation direction.

5. The chamber for combustion of white phosphorus according to claim 1, wherein the furnace (7) has a shape of an inverted cone with its vertex mounted on the motoreducer (5) shaft, the furnace angle measured between the horizontal axis of the combustion chamber and the furnace cone edge (7) being at least 20°, preferably 25÷45°.

6. The chamber for combustion of white phosphorus according to claim 1 or 6, wherein a motoreducer (5) is mounted to the chamber, and the motoreducer (5) shaft is sealed with a gland system (6) in the place where it passes through the combustion chamber base, the gland system (6) comprising a small chamber placed centrically around the drive shaft, and a gap between the gland and the drive shaft at the external side of the combustion chamber being smaller by 1 to 2 mm than the gap at the internal side of the combustion chamber.

7. A method for combustion of white phosphorus in the chamber according to claims 1-6, the method intended to obtain phosphorus pentoxide with a decreases arsenic level, wherein:

a. liquid phosphorus is dosed into the combustion chamber (9) onto a rotating furnace (7) with rotational movement of air, the furnace rotation rate being adjusted so that it enables phosphorus to be combusted superficially on its entire surface;

b. then, the inflow of liquid phosphorus to the combustion chamber is increased together with a simultaneous increase in the amount of compressed air supplied, so that at least 1.5 times stoichiometric excess of oxygen in relation to phosphorus is maintained, the phosphorus combustion temperature inside the process gases collector being maintained within the range of 500°C÷600°C; c. then, the gases which are formed when phosphorus is combusted, are cooled down, with the average temperature of gases at the inlet to the condensation chamber at the first stage of condensation being 420°C, while at the outlet from the chamber being 270°C on average;

d. then, the product is collected directly or indirectly at the bottom of the chamber.

8. The method according to claim 7, wherein at the stage a), the rotational movement of air is obtained by supplying compressed air with an operating pressure in the range of 3.5÷4.5 bar and a flow rate in the range of 45÷50 m³/h.

9. The method according to claim 7, wherein at the stage a), liquid phosphorus with a

temperature in the range of 75÷95°C is dosed in the amount of 4 kg/h with the chamber capacity between 1.5÷2 m³.

10. The method according to claim 7, wherein at the stage b), liquid phosphorus dosing rate is increased until it reaches 8 kg/h with the chamber capacity between 1.5÷2 m³.

11. The method according to claim 7, wherein at the stage b), the process gases temperature in the collector is 520÷560°C.

12. The method according to claim 7, wherein additionally, after stage c), stage c’) occurs constituting a second stage of condensation, wherein the process gases are cooled down to a temperature of approximately 200°C measured at the outlet of the chamber.

13. The method according to claim 7, wherein additionally, after stage c’), stage c”) occurs constituting a third stage of condensation, wherein the process gases are cooled down to a temperature of approximately 120°C measured at the outlet of the chamber.