and G. Stockinger2
1 Sepro Mineral Systems Corporation
Langley, Canada V1M 4A3
(*Corresponding author: [email protected])
2 Brantner Oesterreich GmbH
3500 Krems an der Donau, Austria
A recent trend in wet processing of incinerator bottom ash is complemented by the development of a fine ash treatment module for size fractions – 2 mm. Options for the extraction of the fine size fractions in both wet or conventional dry process plants is presented and the economical, environmental and health & safety aspects of extracting and processing the fines are discussed. Ash samples are exceptionally heterogenous in their composition and this particularly applies for precious metals. Particle shapes also play a significant role in the selection of suitable recycling technology. Comprehensive testwork at laboratory scale has identified a compatible gravimetric separation route and it is explained why only a combination of multiple devices satisfies the diverse requirements to produce a marketable heavy non-ferrous metal product. The diverse metal composition and potential precious metal “nugget effect” poses a sampling challenge. To enhance the confidence level in sampling, pilot scale tests can be considered, for which a practical and cost effective concept is presented. Experience from laboratory and prototype operations has shaped the process and led to the successful launch of a commercial scale plant at Brantner Oesterreich GmbH‘s operation in Austria. Operating data from 2018 allows for a comparison between laboratory experiments and plant
operations, which can be applied for scale-up calculations in similar projects.
Incinerator Bottom Ash Introduction
Incinerator bottom ash (IBA) is the residue of municipal and industrial waste–to-energy (WTE) plants. It consists mainly of inorganic (mineral or glassy) matter, unburnt organic matter and metals . Differentiation is made between ash types according to their origin: The terms incinerator bottom ash (IBA), combined ash and fluidized bed ash refer to incinerator type or mode of ash collection. Wet discharged incinerator bottom ash (IBA) is the default ash type considered in this paper. Modern ash processing facilities are primarily targeting the recovery of ferrous, aluminum, copper, stainless steel and zinc. A secondary objective of ash processing is the diversion of bulk materials from landfills when those can be utilized as construction aggregates instead. Approximately 20-30% of ash is finer than 2 mm and contains ferrous metals, heavy non-ferrous (HNF) metals, mainly copper, zinc, lead and precious metals. Precious metals trace back to electronics and jewelry. Wet gravimetric separation in a fine slag treatment plant (FSTP) has been identified as the most practical and cost effective route for metal recovery.
DEFINITION OF ‘FINE’ ASH
The industry has a variable understanding of ‘fine’. For this paper, fine ash is considered – 2 mm for the
- The efficiency of wet screening drops sharply for screen apertures finer than 2 mm
- Industry established gravimetric separation devices for fines are commonly designed for particle size ranges up to 2 mm
- Pumping and slurry suspension becomes increasingly difficult with coarser particle sizes
Slimes are here defined as the particles of – 0.1 mm, which report to the overflow of dewatering hydrocyclones. Slimes are carried with the process water to the water treatment circuit and ultimately discharged in the form of a filter cake or sludge.
IBA PROCESSING ROUTES: WET AND DRY
There are two technological routes for the processing of ash from WTE plants:
Dry Processing Incinerator Bottom Ash
Conventional processing plants rely on a 4-6 week aging period of the ash. During this time the moisture content is reduced from approximately 25% to 10% and chemical – physical reactions lead to metals oxidation and carbonisation / solidification of the bottom ash. The aging period is necessary before effective metal separation can be performed by magnetic and eddie current (EC) separators.
The ash may be crushed to increase metal liberation. It is then sized, demagnetized (ferrous metal removal) and presented to eddie current separators for recovery of HNF and LNF metals. Sometimes, hand-sorting or sensor-sorting is applied for oversize streams or the recovery of stainless steel. There is no meaningful metal revenue from the fine size fractions in a dry process, most of which remains unliberated or responds poorly to EC separators due to the residual moisture and unfavourbale density / conductivity ratio inherent with heavy non-ferrous metals.
The adaptation of a fines treatment plant (FSTP) to a conventional process is possible and provides benefits:
- The above mentioned aging period can be eliminated
- Due to the timely processing of the ash, considerably less storage area is required
- Dust generation is practically eliminated because the fine size fractions are separated in a wet
- process and the coarse size fractions remain moist
- Material buildup on conveyors and chutes in conventional processing is minimized when
- processing moist + 2 mm ash
- Existing conventional process plants require no technical modifications
- It is apparent that the lack of hydraulic adhesion from fines generates higher quality metal
- Better liberation allows for higher metal recovery rates
- Processed ash + 2 mm exhibits significantly better chemical / physical properties in regards to
- environmental and aggregates applications
- A closed loop process water treatment system requires no discharge of water, but instead
- requires the additon of make-up water; no additional waste streams are generated if slimes
- (filter cake / decanter sludge) can be landfilled conventionally.
Wet Processing Incinerator Bottom Ash
The utilization of a jig is exemplary of a wet process component of a plant: The raw ash is submerged in water while exposed to repeated lifting and suction strokes resulting in particle stratification according to specific gravity. The mechanism achieves the recovery of HNF metals and the separation of floats, organics or unburnt material from the ash. The removal of water suspended slimes – 0.1 mm from the ash is a key feature of wet processing in improving the quality of bottom ash.
The jig process is followed by dewatering of the lights and heavies fractions. Conventional eddie current separators recover light non-ferrous (LNF) metals from the lights fraction. Fresh ash yields measurably higher metal recovery rates, which is due to improved metal liberation as no time is allowed for carbonisation / solidification. Dust emmissions are practically eliminated in a wet process.
The developed fines treatment module for the 0.1 – 2 mm size fraction adapts to the existing infrastructure with little or no technical modifications required to the plant or water treatment system.
Fines Extraction Options
In a wet process, – 2 mm fines can be extracted with the suction strokes of a jig and from the underflow of the subsequent dewatering screens. In the case of a conventional dry process, fines can be removed with a fines removal screen ahead of the plant. Wash water is applied, which reports with the fines to the screen undersize. The +2 mm screen oversize continues to the conventional dry processing plant (Figure 3).
PROCESS WATER TREATMENT
The slimes portion of bottom ash (IBA) is les than 0.1 mm in size and constitutes approximately 10% of the mass of raw bottom ash. The percentage diminishes as the ash solidifies during the aging process. Sizing of the sedimentation and filtering equipment must take into consideration the expected sludge extraction rates and process water circulation rates.
Water treatment systems consist mainly of the following components:
- Sedimentation equipment (Thickener)
- Solid-liquid separation equipment (Filter press or decanter centrifuge)
- Flocculant dosing system
An example process water circuit for wet processing is displayed in figure 4.
The slimes from the overflow of a hydrocyclone report to the thickener. Sedimentation allows slimes to settle to the bottom, from where they are withdrawn and pumped to a filter press. If a decanter centrifuge is used instead of the filter press the drawpoint for settled sludge may be installed halfway up the tank to allow coarser solids to collect in the conical bottom for separate collection. Solid-liquid separation in the filter press produces a
dewatered filter cake of approximately 20-30% moisture, while decanter sludge holds approx. 50% moisture.
Filtrate water is returned to the circuit. The thickener overflows into a surge tank from where process water is returned to the plant. Reagent additon is performed at the point of first contact between ash and process water (de-areation or antifoam) and prior to slimes sedimentation (flocculant).
Fresh water consumption is the difference in moisture content between all outgoing streams (including filter cake) and incoming ash, plus evaporation and spillage losses and can be assumed to be 10-50 liters of fresh make-up water per tonne of processed IBA.
The removal of salts and dissolved solids (TDS) from the ash is limited by the amount of fresh water added: Once the process water in circulation has reached saturation levels, salts and soluable solids can only be absorbed by fresh make-up water. When processed ash is to be used as construction aggregate, the allowable heavy metals and salt content is governed by regulatory thresholds (i.e. leachability or heavy metals content) and by suitability for the application (i.e. acceptable salt content in concrete).
A wet process differs from a wash process: In both cases process water is used to remove fine particulate, but in wet processing only slimes (filter cake or decanter sludge) are removed from regenerated process water, while in a washing application, the process water is actively relieved from its salt and TDS levels, in order to reduce salt and leachable metals in the washed ash. As required per “Green Deal” in the Netherlands, washing of the ashes will practically become a requirement from 2020 to produce freely applicable construction aggregates from IBA. 
As sampling is a statistical problem, there can never be complete confidence in the result of a sampling exercise, but with larger samples comes greater confidence. When sampling bottom ash for metal
content at a precision of ±10% one can derive from sampling theory the following equation (1) :
The minimum sample mass (MSM) depends very strongly on particle size. Large particles require much more MSM than small particles. Due to the “nugget effect” the MSM also increases with decreasing mass fraction of the metal in question. Considering the representativity of fine HNF (0.1 – 2 mm), a sample size of approx 1 kg would be considered representative, but a sample size of >1000 kg is required to consider a sample representative of its precious metals content. Laboratory testwork utilizing 20-50 kg of sample mass is meaningful for the evaluation of HNF deportment, but only indicative of gold and silver deportment. Pilot scale testwork with sample sizes greater than 1000 kg can increase confidence in precious metal deportment.
Eight ash samples (0.1 – 2 mm) from European WTE plants were processed at Sepro Laboratories in Langley, BC, Canada. Sepro specializes in wet process technology for fine mineral applications.
A microscopy study was undertaken to understand the size distribution and shape factors of HNF metals in fine ash. The study concluded qualitatively that HNF metals are found throughout the entire particle size range of 0.1 mm – 2 mm and that particle shapes are dominated by 1-dimensional wires and 3-dimensional quasi-spherical shapes.
Heavy non-ferrous metals exhibit a distinct difference in specific gravity from the mineral and glassy ash components as illustrated in Table 1.
Table 1 – Specific gravity of relevant minerals and metals
The gravity response of the three pay metals was investigated on four, industry standard, gravity separation devices: Holman 800 shaking table, Falcon L40 centrifugal concentrator, Multotec SC-20 heavy minerals spiral separator and a custom built slot separator, all of which differ in their working principles:
Spiral Separators are Affected by Shape Factors
Fine ash in water suspension gravitates downwards along spiral flights towards a splitter box, where heavies, middlings and lights are collected.
Particle shape effects compete with the effect of specific
gravity on the respective travel path: Spherical HNF metal beads tend to roll towards the outside of the curved motion flow and report to the lights. For this reason spiral separators are unsuitable for ash applications.
Slot Separators Overcome the Shape Factor
Fine ash in water suspension is presented to a feed box at the wide end of a narrowing channel. Particles stratify along their flow path towards the narrow discharge end, where adjustable splitters separate heavies from lights. The linear, unidirectional motion minimizes the adverse impact of IBA specific shape effects. A slot separator, for this application, was developed with special attention to geometry and adjustable splitter arrangements.
The Falcon Concentrator Recovers Fine Precious Metals But Faces Limitations in Concentrate Mass for its
Batch Concentrate Discharge Mode
Fine ash in water suspension enters the spinning bowl, where particles stratify by specific gravity under a centrifugal field of 50-200 G. Particles travel upwards over a water fluidized, riffled section of the bowl, where the densest particles get retained whilst replacing lighter particles. The machine has gained worldwide acceptance for the processing of gold ores in the mining industry and is particlualry effective for particle sizes – 1mm and the recovery of free precious metals. The concentrate mass yield is limited to 1% due to its semi-batch operating mode.
The Shaking Table is Able to Generate Clean HNF Metal Products
Fine ash in water suspension is fed onto the table deck through the feed box. The linear stroke of the riffled deck spreads the material according to specific gravity into three fractions: Heavies, middlings and lights. The amount of wash water, deck angle and stroke are adjustable. The concentrate mass pull is manually controlled by splitter position.
Gravity Response of HNF and Precious Metals
Laboratory scale testwork focused on the recovery of payable heavy metals: Copper, silver and gold. Other heavy metals, including lead and zinc, yield no sales revenue. The fine size fractions contain insignificant amounts of stainless steel (i.e. hollow needles) reporting to the HNF metals. The majority of LNF (predominantly aluminum) is expected to be oxidized or posses no significant commercial value.
Figure 11 illustrates the recovery of metals as a function of mass drawn to the heavies product. A steep curve indicates that much of the metal can be concentrated in a small concentrate mass, translating to
high upgrade ratios. It can be concluded that the Falcon concentrator and shaking table are most effective for the recovery of gold: Approximately 80% Au can be recovered in less than 10% concentrate mass. The
shaking table is the preferred device for the recovery of copper and silver:
Approximately 85% Cu and 55% Ag can be recovered in less than 10% concentrate mass.
Shaking table metal recovery trends clearly level off beyond 20% mass yield: An indication of the physical limitations in gravity response at approximately 89% Cu, 69% Ag, 87% Au recovery. The graphs also demonstrate that the slot separator is effective in recovering precious and heavy metals, when operated
at a concentrate mass yield of >10-20%.
Metal Content in Fine Ash
The metal content of eight unique samples is displayed below. The grades are backcalculated from the gravity test products that were submitted for chemical analysis by aqua regia digestion and ICP finish. Backcalculation of the grade is more accurate than direct assays, because the sample mass is larger and the “nugget effect” is minimized when the gravity concentrate is assayed to extinction.
Calculated average headgrades of the 0.1 – 2 mm samples are presented in table 2.
Metal content in fine bottom ash samples
Standardized Laboratory Protocol
A standardized, cost-effective laboratory protocol was developed for testing of 20 kg samples and producing comparable results. The protocol incorporates size classification at 2 mm with rejection of the oversize, followed by magnetic separation (Ferrous removal), followed by gravity recovery of HNF by shaking table and Falcon concentrator in series, with the Falcon processing the table lights. Output is a variable amount of screen oversize, 1-5% ferrous, 1-5% shaking table heavies, 1-5% shaking table
middlings, 1-3% Falcon heavies and the residual lights from the Falcon. All products are dried, weighed and subjected to chemical analysis.
ENHANCING THE CONFIDENCE LEVEL IN PILOT SCALE
As discussed previously it can be concluded that laboratory scale testwork can only be accurate for HNF metals but not for precious metals, particularly gold, because the respective minimum sample size exceeds the laboratory capacities. Pilot plants are temporary installations to test technological design aspects and gain confidence on the grade of “nuggety” metals such as gold. A pilot operation can be executed as individual unit operations (as with lab scale) or operated in “flow through” mode, which is more representative of a commercial operation, but requires more attention to materials handling (gravity flow and pumping). As for temporary installations, process control features are not normally included and for simplicity a wet process pilot plant may be operated exclusively with fresh water. A basic flowsheet is shown below. Output streams are screen oversize, shaking table heavies (HNF1) and Falcon lights. Table middlings and the Falcon heavies (HNF2) are recirculated. The feed rate
is approximately 100 kg per hour.
PROCESS DESIGN ASPECTS IN COMMERCIAL SCALE
Bottom ash features an array of metals which differ by their specific gravity, particle size andparticle shape. It is common practice in mineral processing to apply a combination of different separationdevices to achieve efficient mass rejection and metal recovery. Devices can be distinguished according tothroughput rates and separation selectitivity, for example.
Equipment Operations in Closed Circuits
The combination of different gravity devices allows for closed circuit flowsheets to be implemented with considerably improved metallurgical performance compared to classic open circuits.
In closed circuit, devices are classified as rougher, cleaner or scavenger depending on their assignment in the process. Products are concentrates (heavies), middlings or tailings (lights). Roughers process large tonnage streams, their heavies report to the cleaner device and their lights report to the scavenager. The cleaner produces a final heavy metal concentrate and returns its lights to the rougher. The scavanger is assigned to returning previously lost values to the rougher or cleaner, its lights exit the closed
circuit flowsheet. The following practical considerations apply:
Operational experience has shown that a considerable amount of copper wires are present when processing the fines of a jig. The pulsating action of a jig facilitates wires to pass to the fines. A horizontal screen motion or upfront linear motion can retain long wires (+ 10 mm), which are otherwise difficult to recover.
Magnetic separation of ferrous material should be incorporated in the flowsheet with the objective of removing ferrous metals from the HNF metals concentrate. The options are low intensity wet drums (LIMS) or crossbelt magnets suspended in close proximity (50-100mm) above the shaking table deck.
The slot separator achieves high throughput rates (5-20 t/h per unit) with the ability to recover coarse metals (+ 1 mm) effectively but is unable to produce a clean concentrate in a single stage. The concentrate requires a cleaning step. Slot separators are low cost and simple with no moving parts. Their operating principle is best suited for rougher or scavenger applications.
The Falcon concentrator achieves high throughput rates (5-10 t/h per unit) and high recovery rates on fine precious metals (preferably – 1 mm) but does not offer the ability to produce clean metal concentrates. Operating in semi-batch mode the machine must be rinsed once the concentrate capture zone nears saturation levels, which is approximately when it contains
20-30% heavy metals. The concentrate produced by a Falcon requires a subsequent cleaning step. Semi-batch operation means that for the discharge of concentrate, the machine is stopped in certain intervals (5 min for ash applications), during which the feed is diverted (30 seconds for ash applications). Production scale equipment is therefore limited to approximately 1% mass pull to concentrate (laboratory scale may be higher depending on sample size). In fine bottom ash, where HNF metal concentrations are relatively high (measured in % as opposed to ppm), the Falcon concentrator is best applied as a scavenger.
Shaking tables are the industry standard device for selective cleaning of fine mineral and metal composites where saleable metal concentrate grades are to be achieved. Shaking tables are relatively robust towards particle shape factors and suitable for the particle sizes spectrum of 0.1 mm to 2 mm. Low throughput rates (1.0 t/h per unit) make pre-concentration a requirement for most applications.
Commercial Scale Operations
Experience from commercial scale operations has shown that closed circuit flowsheets are not only beneficial but in fact necessary for efficient heavy metal recovery. Particles varying in size, specific gravity and shape compete for recovery. This makes generous splitter settings, towards higher mass yields, necessary to avoid metal loss. As a result, a subsequent cleaner stage is required to produce saleable grade HNF concentrate. Saleable grade is considered more than 50-80% HNF content by mass. The commercial
scale flowsheet incorporates the removal of copper wires and magnetic separation prior to entering the closed gravity circuit. The slot separator acts as a rougher and retrieves a small amount of coarse HNF for
tabling, where the table does not have the capacity to process the entire stream. The lights from the slot separator bypass the shaking table and get pumped to the Falcon concentrator together with lights from the
table for recovery of previously lost values, mainly fine metals, that the slot separator missed. In this arrangement the Falcon concentrator acts as scavenger. Heavies from the slot separator (HNF3, approx 10-20% of feed mass) and Falcon concentrator (HNF2, approximately 1% of feed mass) are fed to the shaking table, acting as the cleaner, producing a final HNF product (HNF1). The shaking table is normally the bottleneck of the system.
CASE STUDY: BRANTNER OESTEREICH GMBH
Since 2013 Brantner Oesterreich GmbH has operated a wet process plant with a capacity of 50 t/h raw IBA (0 – 300 mm). The plant utilizes magnets, a jig and an eddy current separator to recover coarse (+2 mm) ferrous and non-ferrous metals. Studies on fines processing (0.1 – 2 mm) were conducted in 2016 by Sepro Laboratories and led to the following conclusions:
Table 3 – Experimentally determined metal recovery rates (laboratory)
Element Metal recovery based on fines (0.1-2 mm) [ppm]
Gravity recovery achieves the isolation of environmentally significant heavy metals (copper, lead, zinc, chrome and antimony) and the recovery of precious metals from fine ash. The extraction rates from fines are 2000 ppm for copper, 3.22 ppm for silver, 0.40 ppm for gold, 632.1 ppm for lead, 544.95 ppm for zinc, 159.95 ppm for chrome and 13.3 ppm for antimony. Based on representative sampling theory, it was understood that the laboratory results are only indicative of the precious metals contents.
In 2017, a FSTP module with a processing capacity of 8 t/h of fine IBA from the jig plant was constructed. The installation and comissioning period was completed within 3 weeks, attributable to the modular plant design.
A linear motion screen separates out wires that can be retained on a 2 mm screen deck due to their shape. The majority are copper wires with a length of + 10 mm. Wires are included in the fine HNF concentrate, where they make up approximately half of its mass. The screen undersize reports to low intensity (1000 Gauss) magnetic separation where ferrous metals and minerals are extracted. Fine ferrous accounts for approximately 200 kg/h, with a grade of 40-50% Fe. No commercial value applies at the
moment. Ferrous separation is followed by gravity concentration utilizing a slot separator, Falcon concentrators and a shaking table to produce a marketable fine HNF metal concentrate. The system produces approx 70 kg/h of combined fine HNF, with an average content of 80% metal. The material is bagged in supersacks for sale to copper smelters.
A comparison of metal recovery rates between laboratory and commercial scale has found that the laboratory protocol overestimated the actual plant recovery rate by 14% with respect to the average metal value of copper, silver and gold. This deviation is within the expected performance range and can be explained with the variablity in sampling and practical fluctuations in metal content of bottom ash over the course of a year.
Ash processing technology for incinerator bottom ash (IBA) is divided between wet and dry routes. Conventional dry technology that has evolved over the past decades progresses towards processing ever finer particle size fractions. Physical limitations in the separation mechanism of eddie current
separators and practical aspects related to moisture content in “dry” ash make heavy non-ferrous (HNF) metal recovery from fines (- 2 mm) ineffective. More efficient metal recovery from fines is possible in a
wet gravimetric process, where moisture related hydraulic adhesion forces are overcome in a slurry suspension. The differences in specific gravity between the mineral/glassy component of the ash (SG = 2.0 – 2.8 g/cm3 ) and native heavy metals (SG > 6.7 g/cm3) are distinct.
Eight unique fine IBA samples were processed at Sepro Laboratories for evaluation of the gravity response of copper, silver and gold using various, industry standard, gravity devices. The testwork provides a good understanding of the influence of particle size and shape factors that prevail in fine bottom ash, namely 1-dimensional wires and 3-dimensional spheres. The metal content of the eight samples averaged 0.33 % copper, 10.6 g/t silver and 0.51 g/t gold, whereas the theoretical recovery limits of the metals‘ response to gravity separation from one sample was identified as 89% for copper, 69% for silver and 87% for gold.
It was concluded that a combination of multiple separation devices operated in a closed circuit is necessary and that no device in isolation achieves satisfactory performance: A closed circuit flowsheet with
three stages, achieves high metal recovery and upgrade rates while certain middlings streams are recirculated to the previous stage for re-processing. The Falcon concentrator, slot separator and shaking table were found to be compatible in a closed circuit flowsheet. Brantner Oesterreich GmbH successfully commissioned a commercial fines treatment module
with a processing capacity of 8 t/h fine ash in 2017.
Production data gathered over one year of consistent operation alongside the existing wet process plant showed that metal recoveries in commercial scale justified the investment and met laboratory test results relatively closely with only a 14% deviation in combined HNF value. While current operational focus is on the recovery of heavy metals with a sales
revenue, there is also significant environmental upside potential: The ability to produce construction aggregates from the processed ash minimizes the need for landfilling. It was shown that copper, lead, zinc,
chromium and antimony levels in the processed ash were reduced.
Thresholds for heavy metals in freely applicable construction aggregates are defined by their respective content or leachability. The leachability
bears environmental importance for fine size fractions due to their high surface area. The industry outlook suggests that wet process solutions will gain relevance in comparison to conventional dry processes due to higher resource recovery, lower footprint, minimized dust emissions and
the ability to produce quality construction aggregates. The fine slag treatment plant (FSTP) can be adapted to both dry and wet IBA treatment plants.
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