EP3966162A1

EP3966162A1 - A process for recovering elemental phosphorus

Info

Publication number: EP3966162A1
Application number: EP20724224.9A
Authority: EP
Inventors: Franciscus Henricus HORSTINK
Original assignee: Axpip BV
Current assignee: Axpip BV
Priority date: 2019-05-07
Filing date: 2020-05-07
Publication date: 2022-03-16
Also published as: WO2020225760A1; NL2023083B1

Abstract

The invention provides a process for the recovery process of elemental white phosphorus from in particularly secondary phosphate sources, such as incineration ashes of sewage sludge and bone meal, by reduction with aluminium metal in an alumino-thermic reaction at elevated temperature. Said aluminium is in an at least substantially molten state to reduce phosphates in said reaction mixture to elemental phosphorus. Phosphorus escapes from said reaction mixture in vapour form and elemental phosphorus is recovered by precipitation.

Description

A process for recovering elemental phosphorus

The present invention relates to a process for recovery of elemental phosphorus from a phosphate-containing compound.

Phosphorus is an exhaustible resource that is irreplaceable as a nutrient in agriculture. Many fertilizers contain phosphorus compounds to provide auxiliary nutrition for crop that is being cultivated. Many other applications require phosphorus in its elemental form. Elemental phosphorus has several allotropes that exhibit strikingly diverse properties. The two most common allotropes are white phosphorus and red phosphorus. Due to their reactivity with air (oxygen) these allotropes do not occur in nature in an unbound state. The most common prevalence of phosphorus is in the form of phosphorus minerals. These minerals are extracted in large quantities in countries such as Russia, Morocco and the United States. Pure phosphorus is recovered from phosphorus containing minerals by heating in the presence of carbon and silica.

Russian patent application RU 2.329.316 describes a process for the recovery of phosphorus. In this known method apatite or phosphorite is taken as source of phosphorus. These minerals are milled to a fine powder of 315 micron particle size and mixed with copper-aluminum alloy powder of similar particle size. This mixture is then heated to between 1580 and 1620 °C to obtain phosphorus-copper alloy with of the order of 14 wt.% phosphorus content. The mining of such phosphorus minerals together with the energy burden of extraction of elemental phosphorus by these known methods, render elemental phosphorus a relatively expensive raw material.

Contrary to many phosphorus containing compounds, elemental phosphorus has multiple applications in existing markets and, hence, has considerable value as a raw material.

Phosphorus is listed as a critical material by the European Union. The copper-phosphorus alloy that is obtained with this Russian process requires further processing to extract pure phosphorus.

It is an object of the present invention to provide a process for harvesting elemental phosphorus that is economically more viable and that indeed may be performed on an industrial scale that meets this demand at least to a significant extent. An ideal recovery process for obtaining phosphorus should meet the following requirements:

a high yield of phosphorus ;

a phosphorus product with multiple applications in existing markets;

flexibility in feedstock and feedstock composition;

immunity to changing feedstock prices;

a minimum waste production.

So far, all known processes for the recovery of phosphorus on an industrial scale lack one or more of these requirements.

To achieve said object, a process of the type as described in the opening paragraph, according to the invention is characterized in that a reaction mixture is formed from a

phosphate-containing waste or residual material, particularly incineration ash, and a reducing agent that is capable and adapted to reduce phosphate to elemental phosphorus, in that said reducing agent comprises metallic aluminum, in that said reaction mixture is subjected to an alumino-thermic reaction at elevated temperature, wherein said aluminium is in an at least substantially molten state to reduce phosphates in said reaction mixture to elemental phosphorus , wherein phosphorus escapes from said reaction mixture in vapour form, and wherein elemental phosphorus is recovered by precipitation.

As a component of waste or residual material, phosphorus is accumulated as phosphates for instance in sewage sludge (containing in order of the up to 30% P205) and bone meal (containing in the order of about 40% P205). These waste streams contain the most of phosphorus of all European consumption. The direct application of these waste streams as a fertilizer or animal feed supplements is controversial, because of a risk of disease or the spreading of contaminants, such as heavy metals, in the soil and water systems. Furthermore, the phosphate in sewage sludge or in a incineration residue thereof is partly present as iron or aluminum phosphate. This has an extremely low plant availability.

Recycled (recovered) phosphorus fits very well in a sustainable profile of manufactures in the phosphorus industry. The present invention makes it possible to recover such phosphorus from waste material on an economic viable scale. In this respect a preferred embodiment of the process according to the invention is characterized in that said incineration ash was obtained by burning a residual material from a waste stream, said waste stream comprising bones, sludge, in particular sewage sludge, urine and manure. If elemental phosphorus is recovered from such a secondary source, it can be purified of contaminants and will become indistinguishable from phosphorus from primary sources. Therefore, it is expected to be marketable with less effort, as it is a valuable and flexible chemical.

Such waste streams, particularly that of sewage sludge, may contain a significant amount of iron compounds (mainly oxides and phosphates). Instead of regarding the iron as a problem, the present invention makes use of this iron content in sewage sludge. The main constituents in sewage sludge, the oxidised iron and all phosphates are reduced by aluminum. Together with limited preheating, the exothermic alumino-thermic reaction provides for the energy needed to obtain and maintain sufficiently high temperatures to sustain the reaction. The non-phosphorus reaction products are in the liquid state and the formation of iron phosphides is unfavourable at the temperature reached.

The reduction of phosphate to elemental phosphorus involves a vast quantity of energy. The aluminum used in the process according to the invention is being used both as an energy carrier and as a reduction medium. After ignition, the aluminum reduces the oxides of phosphorus and any iron or sulfur, if present. To provide for a sufficient energy budget (reaction enthalpy) to auto-sustain and maintain this reaction, preferably an aluminum content of at least 90% is being used for the reducing agent. Preheating in addition to the heat of reaction, provides for a sustained and controllable alumino-thermic reaction in which elemental phosphorus is released, vented from the reaction vessel and collected, particularly in a wet condenser. The condensed product is white phosphorus. The other reaction products from the process are elemental iron and slag, which finds its way as a constructing material. This is another major advantage of the process.

To further optimize the economic efficiency of the process, a further preferred embodiment of the process according to the invention is characterized in that aluminum waste scrap is used for said reducing agent. This way, the main components of the reaction mixture, i.e. both the phosphate compound and the reducing agent, may be retrieved from residual, waste streams which increases the profitability of the process considerably. The invention will now be described in further detail with reference to a specific embodiment and a drawing. In the drawing:

Figure 1 shows a basic flowchart of an explanatory example of the process according to the invention; and

Figure 2 shows a schematic setup of a device for carrying out the process according to the invention.

It should be noticed that the figures are purely schematic and not drawn to scale. Particularly, certain dimensions may have been exaggerated to a higher or lesser extent for sake of clarity in order to highlight specific features. Same parts are generally allotted a same reference numeral throughout the drawing.

Figure 1 gives a schematic overview of a specific example of carrying out the process according to the invention. The process may for instance be carried out using the equipment that is depicted in figure 2. The installation comprises a reaction vessel comprising a graphite crucible 1 that is lined at the outside with an alumina liner 15. The crucible 1 is covered with a gas sealed stainless steel lid 2 having cooling fins (not shown). The lid 2 has a number of connections: an argon gas purge 3 of sufficient capacity to flush the inner system; a phosphorus gas vent 4 leading to a direct contact phosphorus condenser operating at for instance the order of 50°C; a central ceramic dosing pipe 5 which extends to the top of the crucible 1; and a full bore ball valve 6. The valve 6 is used to add a grain of an easily ignitable mixture, for instance potassium nitrate/aluminium in a binder, in case the reaction mixture does not ignite spontaneously in the crucible 1.

A stainless steel dosing vessel 7 is fitted on the dosing pipe 5 with a rotating sluice valve 8 in between. The vessel is equipped with a rotating spiral wall scraper 10 to keep the content free-flowing and is situated in a tube furnace 9 which has a regulated temperature. The dosing vessel 7 may be purged with argon via an inlet 11 that is provided at the top of the vessel 7.

The lower third of the crucible 1 is thermally shielded 12 and provided with an induction heater 13. The lid 2 is kept at max 650°C by regulated air draught.

Metallic aluminum is used as a reducing agent for the recovery of phosphorus from a residual (waste) material. In this example sewage sludge is taken as a source of phosphorus. The reaction is an exothermic alumino-thermic process: the reducing agent (mainly aluminum metal) is mixed with a phosphate containing compound. After ignition, the aluminum reduces the oxides of phosphorus , iron and sulfur. Preheating in addition to the heat of reaction, provides for a sustained and controllable reaction in which elemental phosphorus is released, vented from the reaction vessel and collected in a wet condenser. The condensed product is white phosphorus , which still has to be purified. Other products are slag and iron. These can be separated by their differences in density as they are in their liquid state.

In figure 1 and throughout this application the following chemical formulas will represent a group of species, unless explicitly stated otherwise:

P205 : any phosphate containing species, e.g. calcium phosphate Ca₃(P0₄)₂ = 3CaO.

P205, magnesium pyrophosphate (calcinated struvite) Mg₂P₂0₇= 2Mg0.P₂05; and/or iron phosphate FeP0₄ = 0.5(Fe2O3. P205) ;

Fe203 : any oxidised form of iron, including lower oxidised iron such as Fe₃0₄ and FeO;

S03 : any sulfate containing species, e.g. calcium sulfate (gypsum) CaS0₄ = CaO.S03

The primary phosphorus source for the process is a phosphate containing residual material, i.e. a waste stream, like meat and bone meal, sewage sludge, thermally dried urine and/or human or animal manure. This primary source material is subjected to a combustion treatment to obtain an incineration ash as that is still high in phosphate. Other or further similar secondary sources of phosphorus may be:

calcinated salts from sewage de-phosphorisation, such as struvite (magnesium and potassium types), iron and aluminum phosphates and calcium phosphate;

pyrolyzed and carbonised substances from dried or composted sewage sludge or meat and bone meal; and/or

calcinated phosphates containing waste, such as precipitated contaminated phosphoric acid and organic or mineral phosphate wastes from industry .

Since sewage sludge ash and bone meal ash will be used in this example as the prime phosphorus sources for the process, the other ashes are regarded as additives. The reaction mixture may contain further additives, to render the process economically more beneficial or to obtain a physically and a chemically optimal mixture and a controlled reaction. Such further additives may comprise booster ashes that are high in Fe203 or high in S03 (e.g., iron oxide, heavy metal waste ash and/or calcinated gypsum) and/or fluxing agents (e.g. asbestos and fly-ashes). It is favourable to use waste streams as additives. In the process, these will be up- cycled as well.

The main reducing agent for the process is aluminum metal, pure or alloyed. This aluminum is used as a chemical, rather than as a metal. Other reducing agents might be magnesium, silicium, titanium, and calcium. These are restricted to small amounts compared to aluminum, since aluminum has the most convenient fusion and boiling temperatures and is widely and sufficiently available. In practice the reducing agent will consist of at least 90% of aluminum.

The aluminum metal and its alloys are preferably scrap or aluminum waste. The aluminum scrap is either withdrawn from the recycling route or, preferably, withdrawn from incineration of municipal waste. The quality of the aluminum is not as important as recycling to aluminum metal requires. Thus, some purification steps can be omitted and even contaminated scrap can be used. Waste magnesium, silicium, calcium and titanium can partly substitute the amount of aluminum required for reduction. Silicium and titanium are applied in fine dust form. Newly produced aluminum could be a possible replacement for scrap, but this would be more expensive while scrap is readily available.

The reducing agent should be free of foreign objects that may be separated by magnetism and/or gravity after shredding and melting respectively. Also fouling, such as sand or organics, is preferably removed, for instance by washing with water and drying. Surface related substances such as paint, glue, plastic layers, oil may be removed by pyrolysis or heating.

Carbon from pyrolyzed substances and dross may be retained within the liquified metal.

A mixture of different reducing agents can be composed and melted into a liquid alloy, but the aluminum content is preferably more than 90%.

The application of pure bone meal ash may not reach the necessary reaction temperature for a sustained reaction. This implicates that iron oxides and/or sulfates have to be present in ash or mixture. Hence, sewage sludge ash, calcinated gypsum or mixtures thereof will normally be part of the reaction mixture. Because carbon reduction of phosphates is endothermic, carbon is only used as a reducing agent if there is an excess of heat of reaction in order to temper the reaction process. In the reaction mixture, carbon may originate from pyrolyzed paint on the aluminum surface of scrap aluminum or from intentionally produced phosphate containing biochar.

The ashes may undergo a treatment to increase phosphate content, making it possible to mix ashes with low phosphorus content, to decreases volatile matter (halogenides, zinc, alkali metal), to minimize fouling of the reactor equipment; and/or to decrease the sulfate content to make the ash less reactive. This pre-treatment will decrease the thermal burden of the reaction mixture by eliminating some soluble non-reactive compounds. Through this effect, the specific enthalpy of the mixture is increased. After this pre-treatment, the ash is dried and calcinated prior to further processing.

Any struvite is calcinated to pyrophosphate. Other ashes containing water, ammonia, or organics, are calcinated as well. Calcination temperature for all ashes averages 700°C. Char containing ashes can provide calcination energy by feeding a limited quantity of air into the hot biochar. This achieves temperatures considerably higher than 700°C. Before composing the reaction mixture, all ashes are milled to the a proper particle size and sieved.

The mixture should possess the following characteristics for an optimising the reaction:

physical suitability of the reactants: a particle size small enough to react; a

homogeneous mixture;

chemical suitability of the reaction: a stoichiometric amount of reducing agent with respect to Fe203, P205 and S03, with an excess of a few percent;

a total enthalpy of 2000-2200 kJ/kg mixture (ref 25°C), consisting of chemical reaction enthalpy and sensible heat, such that the reaction will be self-sustaining at a

temperature at a temperature of between 1800°C and 2300°C, in particular at a temperature around 2000°C;

fluidity of the fused slag: an adiabatic reaction temperature significantly higher than the liquidus (fusion) temperature of the slag formed;

minimal aluminum use with respect to phosphorus yield as the economical justification of the process.

The required enthalpy of the mixture depends on the heat loss during reaction. The enthalpy stated above applies to an adiabatic reaction. Generally, there may be more components in the ashes that will be reduced by aluminum. As aluminum is likely to be the most expensive agent in the reaction mixture, it should not reduce silicium oxide and oxides of metals less noble than iron, since the heat of reaction is inferior and the use of aluminum is less efficient. Different ashes can be mixed to obtain a suitable slag composition. This composition determines the fusion (liquidus) temperature of the slag, which is preferably a couple hundred degrees centigrade below the adiabatic reaction temperature.

An optimal mixture contains a minimum amount of aluminum per unit of phosphorus . The phosphorus yield should justify the aluminum input. Therefore, ashes with low phosphorus content will not be suitable for recovery unless compensated by mixing these with ashes with a high phosphorus content.

For use with the installation of figure 2, a reaction mixture is prepared from sewage sludge incineration fly ash and aluminium powder. An XRF analysis of the ash is given in table 1.

Table 1

The stoichiometric aluminium demand to reduce P205, Fe203 and S03 is 240 grams/kg ash. The enthalpy of reaction is calculated at 1860 kJ/kg mixture. This relatively high value allows a moderate preheating temperature. The ash is preheated in a furnace at 450°C. From the stoichiometric reaction mixture and the ash preheating temperature, the adiabatic reaction temperature is calculated at 1950°C and the calculated slag fusion temperature, based on CaO, Si0₂, Al₂0₃ and MgO, is calculated at 1660°C.

The ash is milled until all ash, aside from some little stones, can be screened at 0.5 mm. The ashes and reducing agent are then mixed, e.g. by a ball mill, in a low oxygen atmosphere or vacuum, at a temperature where the reducing agent is softened or just melted. At this temperature, the mixing of aluminum with the other reaction components can be done without risk because the temperature margin to auto-ignition is minimally 400°C, which can be tested before each batch is made, and the balls in the mill act as a thermal inertia. This prevents an unintentionally initiated reaction because the specific enthalpy of the total mass is lowered due to the presence of the balls. Alternatively, separately produced aluminum powder can be mixed with the ashes. In both cases the reducing agent is then dispersed into the ash as fine particles.

The alumino-thermic mixture is ready for reaction when it is homogeneous. Prior to the preparation of the reaction mixture, the bottom of the crucible is heated to 1700°C and the tube furnace is at set temperature of 600°C. Approximately 240 grams of aluminium powder is added per 1000 grams of preheated ash and is mixed by stirring and by rotating the ash container. The obtained reaction mixture is stored into the dosing vessel 7. The rotating dosing sluice 8 is switched on to introduce the mixture in the thermally insulated reaction vessel 1. As soon as the reaction mixture makes contact with the hot bottom of the crucible 1, the reaction starts. The induction heating 13 is switched off. The reaction mixture charge is dosed steadily while the reaction is maintained and the slag and iron levels in the crucible rise. The phosphorus gas stream, entraining some dust, is liquified in a condenser (not shown) connected to the outlet 4. The process terminates when the dosing vessel 7 is empty and the reaction has ended. The crucible is allowed to cool down naturally and the solidified slag is analysed. The iron phase (ferrophosphorus) appears to contain 16% phosphorus and the calculated phosphorus recovery efficiency is 74%.

The total enthalpy prior to the reaction consisting of the heat of reaction and the preheating energy should be within a range of roughly 2000-2200 kJ/kg. Generally, the reaction mixture has an enthalpy of approximately 2100 kJ/kg. The heat of reaction is determined by the amount of P205, Fe203 and S03. This is adjusted to approximately 1500 kJ/kg by composing a proper reaction mixture. The remainder of the heat required, is supplied as sensible heat. This enthalpy is already present during the dispersion of the reducing agent. Thus, preheating lowers the consumption of aluminum, which is a prerequisite for a profitable process. The heat of reaction is exothermic and is calculated from the standard heat of formation of the appropriate species.

Also, the reaction can be assisted by a heat supply in the reaction zone through electric arc or by induction. Both the reactants and the products contain elemental metals, so induction is possible without adaptations. A third heat assistance is the minimizing of heat loss in the reaction zone by a heated jacket. The latter two thermodynamic measures shift the reaction to a higher phosphorus yield and lower viscosity of the slag.

The main reactions and their reaction enthalpy are as follows:

Al + ³/io P205 ®> 7₂ AI₂0₃ + 3/10 P₂ -127kJ/mole

Al + V₂ Fe203 ®> V₂ Al₂0₃ + Fe -416 kJ/mole

AI + 7₈ S03 ®> 7₈ AI₂0₃ + 7₈ AI₂S₃ -397 kJ/mole

The reaction is carried out in the reaction vessel 1 (crucible) at an inert argon atmosphere and comprises the following three stages:

initiation : the reaction mixture is ignited by an electric arc or by addition of a small quantity of ignition mixture to the preheated reaction mixture, such as potassium nitrate/aluminum;

propagation : the heat of reaction is sufficiently high to create a propagating front passing through the reaction mixture. The velocity of this front is low enough to control this phase; and

termination : when the amount of reaction mixture is exhausted the reaction is terminated.

These three stages are prominent in the processing of a batch. The process according to the invention may also be in the form of a continuous process. In that event the reaction mixture is fed to the reaction zone in accordance with the reaction rate. Any liquid reaction products are withdrawn from the reactor periodically or continuously. The propagation phase is then extended to the desired time.

The process provides for a sustained and controllable alumino-thermic reaction in which elemental phosphorus is released. The form of phosphorus in the reaction zone can be mono phosphorus (P) and di-phosphorus (P₂). These recombine at lower temperatures to normal phosphorus gas (P₄). This is vented from the reaction vessel to a condenser where it is condensed and extracted for further processing. The condensed product is white phosphorus . The other reaction products from the process are ferro-phosphorus and slag. A significant efficiency of phosphorus recovery is reached by the above process without real difficulty and without special provisions for ashes that are detailed in table 1. The phosphorus yield can be optimised by increasing the reaction temperature. Iron and most heavy metals present in the ashes are reduced to their elemental state and are separated from the remaining slag. Heavy metals and arsenic oxides may be reduced to their elemental state or remain present as oxides. Then the species may be evaporated, dissolved in the liquid iron, or remain in the slag. Evaporated metals (zinc, cadmium etc.) can subsequently be separated as dust from the phosphorus gas by fractionated cooling.

In short the process described hereinbefore and with reference to figure 1 encompasses the following unique and distinguishing features:

producing a phosphorus gas alumino-thermally;

selective formulation and preheating of the reaction mixture;

positive use of iron and sulfate in the phosphorus source ash; and

dispersion of the liquified reducing agent into the oxide matrix.

Although the invention has been described with reference to merely a single explanatory embodiment it will be appreciated that the invention is by no means limited to such example. On the contrary many variations and alternatives are feasible for a skilled person without requiring him to exercise any inventive skill and to depart from the true spirit and scope of the present invention as further elaborated on in the claims to this application. In general, the present invention provides a solution for phosphorus recovery from contaminated (e.g. heavy metals) and problematic (low plant availability of phosphate) waste streams.

Claims

1. A process for recovery of elemental phosphorus from a phosphate-containing compound, characterized in that a reaction mixture is formed from a

phosphate-containing waste or residual material, particularly incineration ash, and a reducing agent that is capable and adapted to reduce phosphate to elemental phosphorus , in that said reducing agent comprises metallic aluminum, in that said reaction mixture is subjected to an alumino-thermic reaction at elevated temperature, wherein said aluminium is in an at least substantially molten state to reduce phosphates in said reaction mixture to elemental phosphorus , wherein phosphorus escapes from said reaction mixture in vapour form, and wherein elemental phosphorus is recovered by precipitation.

2. Process according to claim 1, characterized in that said reducing agent comprises an aluminum content of at least 90%.

3. Process according to claim 1 or 2, characterized in that aluminum waste scrap is used for said reducing agent.

4. Process according to claim 1, 2 or 3, characterized in that said aluminum is subjected to a pre-treatment, comprising at least one or more of purifying, cleaning, shredding, and heating, in particular heating to a temperature of at least 400 °C.

5. Process according to one or more of said preceding claims, characterized in that said incineration ash was obtained by burning a residual material from a waste stream, said waste stream comprising bones, sludge, in particular sewage sludge, urine and manure.

6. Process according to one or more of said preceding claims, characterized in that said ash is calcinated and/or enriched before introducing said ash into said reaction mixture.

7. Process according to one or more of said preceding claims, characterized in that said ash is milled and sieved to form a powder with a defined grain size.

8. Process according to one or more of said preceding claims, characterized in that a residue of said reaction mixture is withdrawn from said reactor as low-phosphate slag.

9. Process according to one or more of the preceding claims, characterized in that metallic iron is withdrawn from the reaction mixture in liquid form.

10. Process according to one or more of said preceding claims, characterized in that said reaction mixture comprises one or more of said following additives:

booster ashes, particularly rich in iron oxide and / or sulfur oxide, such as rust, heavy metal slag and calcinated gypsum;

fluxing agents, such as asbestos and fly ash; and

carbon.

11. Process according to one or more of said preceding claims, characterized in that said reducer is present in said reaction mixture in substantially a stoichiometric ratio with respect to said components to be reduced therein, preferably with a slight excess.

12. Process according to one or more of said preceding claims, characterized in that said reaction mixture has a total enthalpy of said order of 2000-2200 kJ/kg.

13. Process according to one or more of said preceding claims, characterized in that said reaction maintains itself at a temperature of between 1800 °C and 2300 °C, in particular at a temperature around 2000 °C.