Phosphorus Recovery from Sludge Ash via Acid Leaching
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Solution Overview
Problem
Current methods for recovering phosphorus from sewage sludge ash are inefficient due to high energy requirements, low fertilizer value of recovered products, and high costs, particularly when dealing with co-incinerated sludge ash with low phosphorus content, and fail to effectively separate and recycle other elements like iron, aluminium, and heavy metals.
Innovation Solution
A method involving the dissolution of sludge ash in hydrochloric acid, followed by precipitation with lime, and subsequent recycling of the leach solution to produce high-quality, water-soluble phosphorus compounds, along with the recovery of calcium, iron, and heavy metals, using a system that includes ash leaching, residue separation, and alkaline dissolution to enhance phosphorus recovery and reduce costs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If heating sludge ash to 1,400°C to vaporize elemental phosphorus is used, then phosphorus recovery is achieved, but large amounts of energy are required and efficiency is reduced due to iron phosphate slag formation
Solution Approach 1:
The invention changes the chemical parameters of the system by using hydrochloric acid to dissolve phosphorus from sludge ash at relatively low temperatures, avoiding the need for high-temperature heating to 1,400°C. This chemical dissolution approach transforms the energy-intensive thermal process into a chemical reaction process, dramatically reducing energy consumption while maintaining effective phosphorus recovery
Solution Approach 2:
The invention introduces hydrochloric acid as an intermediary substance that mediates the extraction of phosphorus from sludge ash. The acid acts as a chemical mediator that dissolves phosphorus compounds at low temperatures, forming soluble phosphorus-chloride complexes that can be easily separated, thereby avoiding direct thermal vaporization and the associated high energy requirements
2Object-affected harmful factors
If earth metal chlorides are added and heating to above 900°C is used to evaporate heavy metal chlorides, then heavy metals are removed, but phosphorus remains water-insoluble and phosphorus concentration is reduced due to dilution
Solution Approach 1:
The invention extracts phosphorus from sludge ash using hydrochloric acid in a selective dissolution process. By controlling the acid concentration and reaction conditions, phosphorus is extracted into the solution phase while heavy metals remain in the solid residue, achieving separation without the need for high-temperature evaporation that would dilute the phosphorus concentration
Solution Approach 2:
Instead of removing heavy metals first and then recovering phosphorus, the invention inverts the sequence by selectively dissolving phosphorus in hydrochloric acid while leaving heavy metals behind in the insoluble residue. This inverted approach allows phosphorus to be recovered in concentrated form without dilution, while heavy metals are automatically separated in the solid phase
3Quantity of substance
If ion exchange is used to recover iron, aluminium and phosphorus from ash leach solution, then element recovery is achieved, but high costs result due to large excess of regeneration chemicals and low concentration recovery
Solution Approach 1:
The invention replaces expensive ion exchange resins that require costly regeneration chemicals with a simpler, disposable hydrochloric acid dissolution approach. The acid can be easily neutralized and disposed of, eliminating the need for complex regeneration systems and expensive chemical regenerants, thereby significantly reducing process costs while maintaining effective element recovery
Solution Approach 2:
The invention substitutes the complex mechanical and chemical ion exchange system with a simpler chemical dissolution system using hydrochloric acid. Instead of using ion exchange resins that require mechanical filtration and chemical regeneration, the invention uses acid dissolution followed by simple precipitation, replacing a complex multi-step system with a simpler, more cost-effective process
4Adaptability or versatility
If co-incineration with biomass is used, then dedicated incineration plants are not required, but phosphorus concentration in ash is reduced to below 5 percent
Solution Approach 1:
The invention changes the chemical parameters of phosphorus extraction by using concentrated hydrochloric acid to dissolve phosphorus from low-concentration co-incineration ash. Even though the starting phosphorus concentration is low (below 5%), the strong acid dissolution efficiently extracts phosphorus, and subsequent evaporation and precipitation steps concentrate it into high-purity products, transforming low-concentration input into high-concentration output
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enables the extraction of phosphorus and other elements in valuable forms, such as phosphoric acid and calcium phosphate, with high plant availability and minimal heavy metal contamination, while reducing the weight of residual ash and lowering transportation costs, thus providing a cost-effective and environmentally friendly process.
Implementation Method 1
dissolving of sludge ash in hydrochloric acid
Implementation Method 2
precipitation with lime
Implementation Method 3
insoluble residues from the dissolving step are separated
Data Source
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AI summary
A method for production of phosphate compounds comprises dissolving (405) of a raw material comprising phosphorus, aluminium and iron, in a mineral acid. Insoluble residues from the dissolving step are separated (410). Iron hydroxide is added (420) causing precipitation of phosphate compounds. The precipitated phosphate compounds are removed (425). The phosphate compounds are dissolved (302) by an alkaline solution. Iron hydroxide is filtered out (304). Lime is added (306), causing precipitation of calcium phosphate. The precipitated calcium phosphate is separated (308). The leach solution after the separating of precipitated calcium phosphate is recycled (310) to be used for dissolving phosphate compounds by an alkaline solution.