Sewage Sludge Ash Recovery Through HCl Leaching and Solvent Extraction
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Solution Overview
Problem
Existing methods for recovering phosphorus and other valuable constituents from sewage sludge ash are inefficient, leading to low purity and yield, high energy and chemical consumption, and generation of significant waste, with many processes failing to separate individual constituents effectively.
Innovation Solution
A method involving the direct leaching of sewage sludge ash with hydrochloric acid followed by solvent extraction using TBP, with controlled management of colloidal silica and recirculation of chloride-rich streams to optimize ionic strength, allowing for the selective recovery of phosphorus, iron, aluminium, and other metals as high-value products.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If sulfuric acid is used for ash dissolution, then phosphorus can be leached from sludge ash, but gypsum precipitates increasing the amount of residue that need to be disposed
Solution Approach 1:
The patent changes the chemical parameter by using hydrochloric acid instead of sulfuric acid for leaching. This parameter change prevents gypsum precipitation because hydrochloric acid does not form insoluble calcium salts under the process conditions, thereby reducing the amount of residue requiring disposal while maintaining phosphorus leaching efficiency
Solution Approach 2:
The patent converts the previously harmful effect of calcium sulfate precipitation into a benefit by using hydrochloric acid, which allows calcium to remain in solution as calcium chloride. This enables both phosphorus recovery and calcium utilization, turning what was previously a disposal problem into a potential resource
2Quantity of substance
If sulfuric acid and hydrochloric acid are used for leaching and iron extraction, then phosphorus and iron can be recovered, but the process requires multiple acids increasing chemical consumption and cost
Solution Approach 1:
The patent makes hydrochloric acid a multi-functional reagent that performs both phosphorus leaching and iron extraction functions. By adjusting process parameters such as acid concentration, temperature, and contact time, the same hydrochloric acid solution can selectively dissolve different components, eliminating the need for multiple different acids and reducing chemical consumption
Solution Approach 2:
The patent performs preliminary oxidation of ferrous iron to ferric iron before leaching, which enhances the subsequent extraction efficiency. This preliminary action modifies the chemical state of iron to make it more readily extractable by hydrochloric acid, improving overall recovery while reducing the need for additional chemical treatments
3Quantity of substance
If evaporation is used to concentrate phosphoric acid and iron chloride products, then product concentration is achieved, but high energy input is required
Solution Approach 1:
The patent utilizes phase transition by allowing controlled precipitation of iron chloride hydrates and phosphoric acid salts from the leachate through temperature reduction and concentration adjustments. This phase transition approach enables product separation and concentration without requiring high-energy evaporation, as the products crystallize or precipitate naturally under optimized conditions
Solution Approach 2:
The patent employs self-service concentration mechanisms where the leaching process itself generates supersaturated solutions that spontaneously crystallize or precipitate products as the solution cools or stands. This self-concentration reduces or eliminates the need for external energy input in evaporation equipment, as the system uses its own thermal and chemical gradients to achieve product separation
4Quantity of substance
If wet-chemical processing with leaching and precipitation is used, then phosphorus and iron can be recovered from sludge ash, but the process is complex and individual separation of major constituents is not achieved
Solution Approach 1:
The patent segments the recovery process into distinct selective extraction stages. First, phosphorus is selectively leached under controlled pH conditions, then iron is extracted in a separate stage using different chemical parameters. This segmentation allows individual separation of major constituents while maintaining process simplicity, as each stage is optimized for a specific target element
Solution Approach 2:
The patent uses parameter changes to simplify the process by adjusting pH, temperature, and acid concentration to selectively control which elements dissolve or precipitate at different stages. These parameter adjustments enable straightforward separation of phosphorus, iron, and other constituents without requiring complex multi-step precipitation and re-dissolution cycles
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
Achieves high purity and yield recovery of phosphorus and other constituents with reduced chemical and energy consumption, minimizing waste generation and enabling the production of marketable compounds suitable for fertilizer and concrete industries.
Implementation Method 1
The LEACHPHOS process relies on leaching sludge ash with dilute sulfuric acid
Implementation Method 2
at least one of iron and phosphorus is extracted from the leachate by liquid-liquid extraction with an organic solvent
Implementation Method 3
The step of controlling is performed by promoting coagulation to grow particles by adding a silica coagulant to at least one of said start material, dissolved start material emanating from sewage sludge ash and the leachate
Data Source
Figure 1
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Figure 5~7
AI summary
A method for chemical processing of sewage sludge ash comprises dissolving (S10) a start material, emanating from sewage sludge ash, in an acid comprising hydrochloric acid. The start material comprising at least silicon and iron compounds. Undissolved residues are separated (S12), whereby a leachate remains. The amount of colloidal silica in the dissolved sewage sludge ash is controlled (S15). At least one of iron and phosphorus is extracted (20) from the leachate by liquid-liquid extraction with an organic solvent. At least a part of a raffinate at least partly depleted in at least one of iron and phosphorus originating from the step of extracting at least one of iron and phosphorus is recirculated (S90) for dissolving the start material, emanating from sewage sludge ash. The recirculated part of the raffinate at least partly depleted in at least one of iron and phosphorus comprises chloride ions. An arrangement for chemical processing of sewage sludge ash is also disclosed.