Bottom Ash Carbonation Process for Slag Treatment
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current methods for treating bottom ash or slag from municipal waste incineration are inefficient, requiring complex plants and large surfaces for water washing and carbonation processes, which are time-consuming and non-uniform due to natural carbonation occurring in a non-homogeneous manner.
Innovation Solution
A compact process that combines washing and carbonation of bottom ash or slag in a single stage within a drum mixer, using a controlled flow of CO2 to rapidly lower the pH and promote carbonation, while also removing heavy metals and soluble salts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If water washing plants are used to lower chloride and TOC content, then the purity of slag is improved, but the device complexity and surface area required increase
Solution Approach 1:
The patent combines washing and carbonation operations into a single integrated process unit, eliminating the need for separate washing plants and carbonation facilities. This merging of functions directly reduces device complexity while maintaining the purity improvement benefits of both processes
Solution Approach 2:
The carbonation reactor serves multiple functions simultaneously: it acts as a washing unit where water removes soluble contaminants, a carbonation unit where CO2 converts calcium oxide to calcium carbonate, and a pH adjustment unit. This multi-functionality eliminates the need for separate dedicated equipment for each function
2Object-affected harmful factors
If natural carbonation is used to lower pH, then the harmful factors are reduced, but the duration of action and surface area required increase
Solution Approach 1:
The patent applies preliminary action by pre-moistening the slag with water before introducing CO2, which prepares the material for faster carbonation. Additionally, the continuous mixing action is established beforehand to ensure uniform CO2 distribution, enabling the rapid pH reduction to occur within hours rather than months
Solution Approach 2:
The patent transforms the static natural carbonation process into a dynamic controlled process by implementing continuous mixing and controlled CO2 injection. This dynamic approach ensures uniform contact between CO2 and slag particles throughout the mass, dramatically accelerating the carbonation rate and reducing the time required to achieve the desired pH level
3Object-affected harmful factors
If natural carbonation is used, then the harmful factors are reduced, but the homogeneity of treatment deteriorates
Solution Approach 1:
The continuous mixing operation dynamically redistributes the slag particles throughout the reactor, ensuring that all portions of the slag mass receive equivalent exposure to CO2 and water. This dynamic mixing prevents the formation of carbonated surface layers that would otherwise block CO2 penetration, achieving uniform pH reduction and homogeneous carbonation throughout the entire slag mass
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 process achieves significant reduction in chloride and organic content, lowers the pH to non-corrosive levels, and effectively removes hazardous heavy metals and anions, making the treated slag suitable for reuse as artificial aggregates within a remarkably short time frame of less than 1.5 hours.
Implementation Method 1
the mixing, inside a rotating body, of the bottom ash and/or slag and a quantity of water in a weight ratio between 0.8 and 2, while simultaneously a flow of gaseous CO2 in a concentration between 90 and 100% v/v passes through the rotating body
Implementation Method 2
CO2 in water forms Carbonic Acid (H2CO3) which can react with the basic compounds present in the slag. For example, the calcium oxide, CaO, present in the slag, in water is in the form of calcium hydrate, Ca(OH)2 which reacts with carbonic acid, H2CO3, according to the acid-base reaction: Ca(OH)2 + H2CO3 → CaCO3 + 2H2O
Implementation Method 3
water washing plants are increasingly being used to lower the chloride and TOC content below the acceptable limits for reuse
Data Source
Figure 1
AI summary
Treatment process for bottom ash or slag in general characterised by the fact that it includes the following phases: - removal of ferrous material content between 7% and 10% and non-ferrous content between 2% and 3% of the slag, - removal by sieving of the fraction > 12-15 mm, - sending the fraction < 12-15 mm with water in a slag/water ratio between 1:1 and 1:3, inside a drum mixer (2) where washing/carbonation takes place, - injection of CO2 into the drum (in a concentration close to 100% vol/vol) at a flow rate of approximately 0.2-0.5 litres/minute per kilo of slag, - introduction of FeSO4* 7H2O (ferrous sulphate heptahydrate) into the drum, - mixing for about 10-15 minutes under continuous pH control to promote the completion of the reactions involved: reduction of Hexavalent Chromium, precipitation of Heavy Metals and solubilization of Salts, - sending the washed and carbonated slag with the washing water to a rotating sieve (8) with holes of 2-4 mm diameter, - sending the fraction above 2-4 mm to a further washing apparatus (10) and separation of the liquid from the washed fraction > 2-4 mm, - sending the solid phase > 2-4 mm to a collection container (12) and the liquid to a water reuse circuit (14), - sending the under-sieve (< 2-4 mm) exiting the sieve (8) to a second separating sieve (16) with 0.3-0.5 mm holes, - sending of the solid fraction > 0.3-0.5 mm and of the soaking water to a washing and separation group (18) with collection of the solid with granulometry > 0.3-0.5 mm in a collection container (20) and sending the liquid to the water reuse circuit (14), - sending the fraction <0.3-0.5 mm exiting the sieve (16) to a vacuum filtration system (22) to obtain a fraction < 0.3-0.5 mm which is sent to a washing unit (24) from which the solid fraction - sent to the collection container (26) - is separated from the liquid sent to the water reuse circuit (14), - sending the filtration liquid exiting the filter (22) to a chemical-physical purifier (28), the chemical-physical purifier having two possible outputs: water to the drain and purification sludge.