Process Overview


The Ash2Phos process can be divided into two concepts (as can be seen in the picture above) depending on the size of the plant and its location: the City concept and the Industrial concept.

City concept

The city concept is an option for smaller ash volumes and stops at the intermediate products, which will have to be upgraded to final products at a different industrial site. To lower operation costs, waste acid (low-quality hydrochloric acid, HCl) from the gas cleaning of a waste incineration plant can be used to dissolve the ash in the Ash2Phos process. Since the City concept handles lower amounts of ash, locally produced waste acid is often enough to match the City concept's acid demand. In addition to a lower operational cost for the Ash2Phos process, the incineration plant will also lower it's operational costs since it no longer has to neutralize and treat the waste acid.

Industrial concept

The Industrial concept includes the steps necessary to produce commercially valuable products, which are already traded on the market.

The Ash2Phos process consists of 3 steps: a first acidic step, a second alkaline step where intermediate products are produced, and finally a conversion step where the intermediates are processed into final products.

The main inputs of the process are

  • Ash from incinerated sewage sludge
  • Acid (hydrochloric acid, HCl)
  • Lime

The outputs of the process are

  • Phosphorus, preferably as ammonium phosphate (fertilizer, requires addition of ammonia to the process) or calcium phosphate (feed additive). It is also possible to produce phosphoric acid or super phosphate fertilizers.
  • Iron as iron chloride or iron sulfate
  • Aluminum as aluminum sulfate
  • Residual sand (undissolved ash consisting mainly of silicates)
  • Heavy metal precipitate (for safe disposal)
The process itself

The process consists of several successive chemical reactions undertaken at room temperature (though one process step may benefit from a temperature of 40 degrees C). There is no need for pressurized vessels or for exceptional materials to be used for the equipment. The mass balance of the process is favorable, since all input chemicals become part of the products.

Possible Phosphorus Products


Monoammonium phosphate, MAP

  • Uses: Fertilizer in agriculture or horticulture. Animal feed ingredient.
  • Quality: Technical grade, 26,5% P (61% P2O5) and 12% N
  • Cadmium content: < 1 mg Cd/kg P (< 0.5 mg Cd/kg P2O5)
  • Efficiency in application: Very high, the product is fully water soluble and plant available
  • Corresponds to CAS number: 7722-76-1

Ammonium phosphates are commercially produced in two quality levels: fertilizer grade and technical grade. Fertilizer grade is the large volume phosphorus commodity and contains 21-22% P (48% P2O5). Fertilizer grade products are not fully water soluble and can only be distributed to the crop in solid form. It also contains impurities from the production process, such as gypsum, and metals originating from the minerals used as raw material. The Ash2Phos process always produces the highest quality of ammonium phosphate, i.e. technical grade, which is fully water soluble and can be used for fertilization in solid or liquid form.

Dicalcium phosphate, DCP

  • Uses: Fertilizer in agriculture or horticulture. Animal feed ingredient.
  • Quality: Equal to currently available products with a phosphorus content of 18% P (41% P2O5)
  • Cadmium content: < 1 mg Cd/kg P (< 0.5 mg Cd/kg P2O5)
  • Efficiency in application: Equal to currently available products
  • Corresponds to CAS number: 7783-28-0

Phosphoric acid

It is also possible to produce phosphoric acid with the same low cadmium content of < 1 mg Cd/kg P (< 0.5 mg Cd/kg P2O5).

  • Corresponds to CAS number: 7664-38-2

Superphosphates

A less likely alternative is to produce single or triple super phosphate. These products are lower grade phosphorus fertilizers which were common in the past, but have been mostly replaced by ammonium phosphates.

Iron and Aluminum Products


Iron

Two different iron products can be produced in the industrial concept from the intermediate product ferric hydroxide.

Iron chloride, FeCl3

  • Application: Coagulant used for precipitation of phosphorus and organic materials in waste water treatment plants
  • Quality: The product equals the current commercial product
  • Correspond to CAS number: 7705-08-0

Iron sulfate, Fe2(SO4)3

  • Application: Coagulant used for precipitation of phosphorus and organic materials in waste water treatment plants
  • Quality: The product equals the current commercial product
  • Correspond to CAS number: 7782-36-0
Aluminum

Aluminum sulfate, Al2(SO4)3

  • Application: Coagulant for precipitation of phosphorus and organic materials in waste water treatment plants
  • Quality: The product equals the current commercial product
  • Correspond to CAS number: 10043-01-3

Residual Sand & Heavy Metal Precipitate


Residual sand

The remaining ash residue after dissolution

  • The washed sand consists mainly of silicates
  • The sand has several possible applications such as cement or filler in concrete
Heavy metal precipitate

The heavy metals are separated as a sulfide precipitate for disposal. Metal sulfides are chemically stable and sulfide precipitation is a state-of-the-art technology which is widely in within industrial water treatment.

Summary of the Ash2Phos Process


A chemical process with the following characteristics

  • Phosphorus from sludge ash is recovered as high value products (ammonium or calcium phosphates)
  • High phosphorus recovery rate (> 90%)
  • Clean phosphorus products with a very low cadmium content of < 1 mg Cd/kg P (< 0.5 mg Cd/kg P2O5)
  • Recovery of iron & aluminum as precipitation chemicals
  • Separation of heavy metals as chemically stable sulfides for safe disposal
  • Favorable mass balance
  • Low energy consumption since the process operates at ambient temperature and atmospheric pressure
  • Can use sludge ashes with lower phosphorus content than ash from mono-incineration
  • Can use waste acid from incineration plants

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