Temperature drop can be reduced by switching to larger ingots. For example, melting a single 1t ingot takes longer than melting forty 25kg ingots, because the surface area is smaller. It takes longer for the heat transfer and therefore less of a temperature drop. The stretched-out melt time means that the furnace now has more time to get heat into the bath.
This solution requires a redesign of the loading area in the furnace but will significantly improve the production rate and reduce temperature fluctuations. By adding a slopped loading shelf inside the furnace chamber, the newly loaded cold zinc will be exposed to radiation from the walls and convection from the hot gases. This effectively pre-heats and melts the zinc before it slowly flows into the bath. Not only is zinc being added to the bath very gradually, but it is also already hot, which minimizes the bath temperature drop.
If your goal is to minimize the sudden drop in bath temperature, then adding less zinc more frequently will help the situation. This loading method uses the same amount of energy, but it reduces temperature fluctuations in the liquid. This, however, has a potential drawback. Constantly opening the furnace door to load zinc, allows cold air to enter the furnace and this can reduce the heating efficiency. The situation is much worse if the burners turn down to low fire for safety while the loading door is open. Ideally, use an automated loading system, which can add small ingots through a small opening at regular intervals.
The furnace’s performance depends on how fast the heat can be transferred to the liquid zinc. This in turn depends on burner technology, furnace geometry, choice of refractory materials, and the liquid zinc surface area, to name just a few factors.
There is a limit to how much energy can be transferred from the furnace chamber to the zinc bath. Simply adding more, or larger burners does not necessarily mean that the zinc will heat up any faster. In fact, it will probably just result in overheating of the bath surface and generation of zinc oxide, resulting in even worse heat transfer rate and metal waste. The way to get more heat into the bath faster is to increase the zinc surface area or to inject the heat directly into the zinc by using immersion burners. Incidentally, Burns was involved in development of the original immersion burner technology over 40 years ago in the UK, and the latest version of this technology is available from us today.
It is important during the furnace design process to choose the correct refractory lining materials. The hot-face refractory choice is important because the highest rate of heat transfer in a hot furnace is by energy radiating from the walls and roof onto the zinc. Choice of suitable insulation is crucial to minimizing heat losses, so that the energy transferred into the zinc bath is not being lost unnecessarily.
Although your melting furnace may have originally been designed for your current production rate, lack of regular maintenance, or regular wear-and-tear, may have left it struggling to keep up.
Thermal cameras are a great way to find damaged refractory, missing insulation, or leaking doors, flues, badly fitted access panels, seized dampers, leaking sight ports, etc. In a properly designed furnace these will result in leaks into the furnace. Any energy wasted on heating cold air leaking into the furnace, is no longer available to the process. If these leak points cause the hot air to flow out of the furnace, then there is a problem with the flue or exhaust damper.
Another inefficiency could be caused by the combustion system set up. After all, it is the combustion system that converts the energy stored in the fuel into available heat. The burners, blowers, filters, valves, regulators, etc. all require regular maintenance to ensure that all the energy is extracted from the fuel.
Unless immersion burners are used for heating, the energy from a conventional burner or electric radiant heating system must enter the liquid zinc through its surface. It is therefore important to keep that surface relatively clean to minimize the amount of ash or oxide floating on the surface. The surface should not be so clean that it is shiny. A shiny surface reflects the heat away from the metal making it harder for heat to be transferred. The ash floating on the surface is not very conductive, so if it is allowed to become too thick, then it will act like a blanket slowing down heating of the bath.
If the operators take a long time to load the furnace or clean the surface, then the furnace will lose a lot of heat and take a longer time to recover. Operators should always work safely, but efficiently. In fact, our experience shows that a quality operator has a significant impact on the efficiency of the process.
Too much ash on the surface of the zinc bath might be an indication that the furnace is operating too hot. Although it may be tempting to turn the furnace up a few degrees in order to increase the production rate, it risks overheating of the bath surface. The zinc bath can only absorb heat from the furnace chamber at a certain rate. The addition of more heat will drive up the surface temperature and this increases the oxidation rate of the zinc leading to waste of metal. The formed zinc oxide acts like a blanket on the surface of the bath, impeding the heat transfer and lowering the production rate. To increase the production rate in a melting furnace without overheating the zinc, the bath surface needs to be regularly cleaned, or the surface area of the metal needs to be increased.
Check the furnace thermocouple. It is possible that the thermocouple is in a cold spot, is damaged, is the wrong type, is grounded at the head, not fitted with the correct cable, or is not sensing the true furnace temperature. A bad cold junction, for example, may result in a low measured temperature and this will result in overheating.
Correct burner placement is crucial to the efficient operation of a furnace. So although the furnace might be operating at the appropriate 'average' temperature, it is possible that the burner flame is too close to the bath surface, causing local overheating. Flame temperature from a natural gas burner could be 1750C, which is more than enough to cause local overheating and zinc oxide generation. Verify that the flame has enough space to fully develop before the hot gases reach the zinc surface. If that is not possible, then the burner firing rate might have to be limited to minimize overheating the zinc.
Using a properly calibrated flue gas analyzer, check the flue gas to make sure the burners are set up properly. 2%-3% oxygen is ideal. Setting the burners any lower than 2% O2 risks generating CO, so be careful. If faster heating/melting is a priority then other options should be examined, such as the addition of immersion burners, installation of a loading shelf, adding a mixing pump, or upgrading the furnace to increase the surface area.
If the surface ash is not zinc oxide, then the material is being introduced with the feed metal. This is not a problem with SHG zinc but secondary zinc contains impurities such as lead, iron, aluminum, etc., which may react with each other to produce a precipitate, some of which will sink to the bottom of the melter but others will float on the surface. Check your source zinc to verify its purity.
Careful zinc handling and storage prior to use will help to minimize the risk of introducing contaminants into your furnace.
A common problem with thermocouple signals is magnetic interference caused by nearby wiring. Ensure that the thermocouple wires are installed in separate ducts, trays, etc, away from high voltage wiring. Damaged or failing temperature sensor will also report incorrect temperature causing the control system to adjust the firing rate incorrectly.
The temperature sensor might me installed in the wrong location. For example, too close to a burner, door, or the flue. The varying temperatures in those areas would result in changes to heat demand and therefore fluctuations in the firing rate of the burners heating system.
If the furnace uses temperature sensors in both the bath and the hood for control, what is known as Thermal Head Control, then it is possible that the settings may need adjustment to reduce the switching between the sensors, and therefore reduce oscillations.
Temperature fluctuations in the furnace could be the result of a poorly set up control device. The heat demand response is usually controlled by PID settings programmed either into the controlling PLC or the temperature controller. The PID settings will be unique to your process and coming up with the correct settings takes time and expertise, which is why it should be carried out by a qualified person who can monitor the furnace while they make the changes.
In general, reducing the P parameter will slowdown how quickly the heating system reacts to temperature changes in the furnace, and should therefore reduce the over and under shoots. The I parameter relates to how long the system analyzes the rate of temperature change. So increasing this value will typically make the system slower to react. Sometimes the temperature fluctuations can be caused by the D parameter which anticipates the effect of heat demand changes on process temperature. When the ideal value has been determined, the furnace will approach the desired temperature smoothly. In slow and steady systems, it is sometimes better to ignore the D parameter as it can introduce unintended fluctuations. Setting D=0 might solve the fluctuation problems.
If the temperature sensor is installed in a liquid bath where the level fluctuates above and below its location, it will sense the changes in temperature and unnecessarily oscillate the heat demand. It is important for safe and efficient operation of the furnace to maintain a relatively constant, and preferably a high, liquid level.
An obvious cause of temperature fluctuation in the liquid bath could be the addition of cold material for melting. In this case oscillations in temperature cannot be avoided completely but they can be controlled. See question #1 above for suggestions on how to deal with this common issue.
Another cause of temperature fluctuations, or lack of temperature uniformity in the furnace, is cold air entering the chamber. This could be from opening the furnace door too long, from leaks through poor or damaged seals and dampers, or air ingress through the flue during low production times.
Consider ways to reduce operational swings. This will enable you to operate your equipment closer to its operational maximums and this will increase the production output. Increasing output will reduce your unit labor, maintenance and overhead costs based on the tonnage produced.
If the production line is already operating at its maximum capacity, look for bottlenecks that may be limiting the ability of the production line to produce more material. A melting furnace may be the bottleneck, therefore take a close look at performance over time, could this be improved by:
- Reducing the time it takes to load new material?
- Changing the shape of the new material to better fit your operator practice?
- Minimizing the time the door is held open?
- Improving bath metal temperature control?
- Improving the heating system control?
Remember that most of these waste materials have some value, usually at a discount rate from the LME SHG zinc price. The following suggestions identify ways that the yield of Grade A product may be maximized.
- Can the ratio of grit to grade A material in the grit catchers be improved?
- Can the amount of grit being generated be reduced?
- Can candle production be reduced. Note that the frequency of candle removal has a direct impact on the mass of candles being generated. The growth rate, by observation, probably follows a parabolic law, which means that growth after cleaning is very slow. If candle removal is done before the fast growth phase, the mass of candles being generated will be drastically reduced.
- Check the ash being generated at the melting furnace. How much zinc is being lost with the ash? Can this be economically recovered? Can the ash formation rate be controlled by improvements in the way the melting furnace is managed by the operators?
- Is the operator using high hood temperatures in the melter to recover bath temperature quickly? If so, is this generating high volumes of ash? The zinc oxidation rate also follows a parabolic law with temperature. This means that a few degrees of temperature change will have a very large influence on the rate that ash is produced. Check out earlier questions that deal with melting furnace temperature swings and ash formation.
- Schedule product changes to minimize transitions and hold transition material so that it may be fed back into the product stream at a controlled rate, usually via a table feeder. Maintain careful records of the set-up parameters required to produce the required product physical properties.
Determine the true energy cost for your process and take this as your baseline. Then compare this with the production rate. If the data show a direct response to changing production rates, then it is probable that the equipment is operating as efficiently as it was intended. If the response to changes is small when compared to the total energy demand, consider implementing measures to reduce energy waste:
- Start with the furnace island costs and begin with the vaporizing furnace, as this is the largest contributor to the utility bill.
- Check control system set-up. Temperature control automation will most often result in about 15% fuel saving. This is an observation made based on our experience.
- Check the temperature control response and make sure that the PID loops are properly tuned, so the combustion system operates smoothly, and therefore efficiently.
- Check that the burner system is operating with the correct oxygen volumes in the flue gas, as recommended by the furnace supplier. Too little combustion air means that not all the fuel will be burned and CO will be generated, but too much air means that the fuel is unnecessarily heating the extra air and therefore wasting energy.
- Check that the waste gas damper, if fitted, is working properly and is in good condition. If there is no damper, consider fitting one, as large heat losses may be caused when the furnace is at low fire.
- Is the crucible filled to its ideal level and is the operator able to maintain it? Note: a low level in the crucible will reduce the amount of heat that can be transferred to the zinc and so will reduce the production rate.
Consider using heat recovery to capture some of the heat that would otherwise be lost to the environment.
- Some heat may be captured by the cold blocks or bundles of zinc being loaded into the melting furnace. Approximately 57% of the total heat needed to melt the zinc goes into getting the cold zinc to the melting temperature. A reasonable target would be to supply 50% of this heat using a preheater. This practice has the added benefit of reducing the risk to the operator when loading material into the furnace, as any moisture will be removed before the block or bundle enters the metal bath. Furthermore, preheating the zinc reduces temperature fluctuations and recovery time of the melter.
- Addition of recuperative or regenerative burners are probably best considered for new installations. Retrofit of an old furnace is usually complicated and, in some cases, not feasible, as would be the case for the use of a pair of regenerative burners on a small single pot furnace. Check out our equipment descriptions provided elsewhere on this web site. Muffle furnace technology would not be an ideal candidate for a regenerative burner retrofit for structural reasons.
- While there is a lot of heat being generated by the combustion of zinc at the crucible or retort pipe, recovery of this heat is not recommended because:
1) There will be a system requirement for a minimum quantity of heat that is required to maintain ideal conditions in the bag-filter and the connecting ductwork. This heat is provided by the zinc flame.
2) The obvious place to extract heat from the zinc flame is in the immediate location of the zinc flame, but this would influence the product quality. Check out the question about product quality for more details.
3) Finally, extraction of heat from the zinc flame may increase the tendency of the zinc oxide crystals to grow as needles, which is the case for American process zinc oxide. This process soaks up a lot of the surface electrons and makes the zinc oxide less suitable for catalytic applications
Generally, a retrofit or modification will be expensive as it will require planning and engineering time. This would not normally be the case for new equipment as these costs are built into the price of the equipment. Only if the utility cost is very high, would it be possible to make a business case for a retrofit or modification of existing furnaces.
The scale of the equipment upgrades would have to be large enough so that the detailed engineering overhead would be reduced to a very small part of the overall project.
A further consideration would be the operating permit for the equipment at your site. Older equipment is usually grandfathered (allowed to operate under a code that has been superseded by a later code). The new code is usually enforced by requiring that major modifications to old equipment trigger an inspection under the new regulations. You must find out from your local agency what is considered a major modification, and would the proposed upgrades require you to re-apply for your operating permits. Failure to comply with these rules may void your insurance policy.
Electrical savings are harder to justify as the costs are usually significantly less than the heating cost. Certain measures may be considered, but it is probably better to evaluate these options against a corporate strategy for the reduction of carbon footprint.
- For most larger motors, VFD control should be considered. When a VFD is used to control air flow, the speed of the blower is regulated to provide the pressure required to drive the air flow though the pipe. If instead, a modulating valve is used to regulate flow, the pressure upstream of the valve is roughly constant and this requires much more energy to maintain. Also, in most modern plants, motors larger than 30kW require soft starters to limit the electrical demand on start up. As the price differential between a soft starter and a speed controller are usually small in the range of 10kW to about 200kW, you should be specifying speed controllers for new equipment.
- Air compressors are a huge electrical energy burden, and a well-designed compressed air and storage system can produce significant savings. Also, find and fix any leaks!
- Pneumatic conveyors are a very convenient way to move finished product around, but they are very expensive to operate. If they must be used, consider using in pulse mode at high transfer capacity to limit conveyor on time. Also note that not all zinc oxide is suitable for pneumatic conveying, as some grades may clump and plug the system.
Avoidance of the SHG zinc price premium is a very good reason to consider a change to secondary zinc as a feed source. There are several articles on this website that explain the process and provide details of the equipment needed and the opportunities for savings.
For most of the projects we have examined so far, the savings generated are at least the zinc price premium, and in most cases, savings fall in the range $250 - $450(US) per ZnO tonne.
It is possible, by careful management of the system, to produce car tire grade zinc oxide from secondary zinc sources. In fact, this material is already in extensive use in the tire industry.
Note though, that switching to secondary zinc will involve practice changes at your plant, to accommodate the extra production steps required to produce grade A zinc oxide.
If you are currently producing pharmaceutical grade zinc oxide, carful planning is needed to ensure there is no cross contamination with products from the secondary line.
Look for ways to improve Grade A product yields. For an 8,000 t/y plant operating with a yield rate of 1.20, increasing the yield to a realistic 1.225 will reduce the SHG zinc costs by about $340,000/y. At the same time, the waste off-grade material is halved, making a further $300,000/y worth of material saleable at Grade A prices. In some cases, it may be possible to do even better than the 1.225 conversion rate. We will be happy to work with you to identify opportunities for savings and we are able to help you estimate the bottom-line contribution of some of these measures. These opportunities are listed below:
Consider ash being generated and can any zinc be recovered from it? See the question dealing with ash control.
Is ash present on the feed material? This material cannot be recovered by processing.
Spillages can usually be recovered, however, care should be taken as it is best to assume that collected spillages will contain contaminants. In most cases the spillage material may be returned to the melting furnace, however, is some cases it may be better to sell it at a discount.
Increasing the frequency of candle removal will go a long way to the improvement of the product yield. This must be done carefully so that dust from the crucible covers and the collapsing candle is not drawn into the hood system with the product.
We have designed several manual and automated systems which may help with the control of candle formation during production both for retort and crucible type furnaces.
The hood design will determine how air is introduced into the system. The important thing is to avoid flows which can carry product in the direction of the crucible lid on a vertical crucible vaporizer, or to the base of the combustion chamber in the case of the horizontal retort vaporizer.
Air entering the system will be cooler than air in the vicinity of the zinc vapor flame, and will therefore be at a higher density. There will be a tendency for this air to fall with gravity. If the air happens to be in an area where zinc oxide particles are forming, these particles will get caught in a down draft and will be deposited on the first inclined or horizontal surface it encounters.
By careful design of the area under the hoods, it is possible to minimize this material loss. This is an area where the furnace and the collection system are engineered as a complete system, which we do by CFD modelling of these flows before finalizing a design for a particular plant.
When candles are removed, some zinc oxide may fall into the crucible or the vapor tube. This material floats on the surface of the zinc and if it becomes too thick it can cause quality problems, and in extreme cases can be dangerous to the operator. The floating zinc oxide in the crucible or the retort must therefore be regularly removed. The periods between each cleaning cycle may be extended by careful operation of the equipment. This waste material may be collected and sold at a discount to manufacturers of fertilizer and soil amendments.
Operator practice can go a long way towards the minimization of these losses.
The grit catcher is really a system for your quality assurance manger. There are costs associated with its proper operation. In operation it is designed to catch 98% of the particles that are larger than the specification size, and it must minimize the quantity of good material that gets captured.
Generally, the material from the grit catcher will be sieved to -325 mesh and any product passing though the screen is added to the good product, usually via a table feeder or other such device. Prevention of grit will help minimize tramp losses with the grit.
It is inevitable in an operation where it is necessary to change the physical properties of the product that there will be a period where the production is off grade for both grades. This material may be stored and fed back into the product at a rate and time whereby the addition will not cause the produced grade to go out of specification.
In this case there is no point looking for savings, as the only losses that might occur are when this material is transferred to storage and returned to the material handling system. Since freshly produced zinc oxide ages, it is recommended that it be returned to the system as soon as it is practically possible.
Spillage should be avoided as most spilled material will absorb some contaminants and will not be suitable for reintroduction into the product stream. Spillage material can be collected and sold at a discount based on the assay of metallic zinc present.
Most of the collected material, such as dust from the material handling system transfers and the fume from the melter hood, will be zinc oxide dust. However, all this material must be assumed to be contaminated with environmental dust and it will not normally be suitable for reintroduction to the product stream. This material may also be collected and sold at a discount, based on the assessed metallic zinc present.
Careful design of the locations where dust and fume are generated, along with good operator practice, for example by minimizing the time the melter door is left open during charging, will go a long way to minimizing all of these losses.
First and foremost, it is important to practice good record keeping. To reproduce the conditions when Grade A material is being produced, it is important to keep set-up and configuration records such as the actual furnace temperature and production rate, bag-filter temperature, and flow rate, etc. By maintaining accurate benchmark data, it will be possible for the operators to quickly identify an upset and allow them to implement changes that will return the system to its production normal.
Recognizing that upsets from time to time will be unavoidable, the plant design should be configured so that off specification material can be quarantined and reintroduced into the grade A product in a such a way that the bulk material is still within its specification limits and waste from the process is minimized.
Look at the various datalogging and automation systems described on the website for some ideas of what may be achieved. We will be happy to help you begin the process of implementation of these ideas at your plant using your existing systems wherever possible.
- Lead: Although SHG only has a very small quantity of lead in its specified composition (Pb <0.003%) over the lifetime of a 3000 kg crucible operating at 300 kg/h, the quantity of lead will concentrate to about 26kg. In normal circumstances this will not be a problem as the expected lead in the product will only be about 16 ppm. However, if crucibles are accidentally run low it is possible that lead levels may rise in the product, causing it to go out of specification.
If you are processing secondary zinc, a careful watch must be maintained to ensure that lead levels remain within the grade specification limits. Refer to the sections describing the secondary process for further details.
The only way to return the production to the required lead level limit will be to remove lead from the system and a use liquation to remove and collect the lead that has concentrated in the vaporizer.
- Iron will normally be in solution in the zinc, provided the composition is managed. In an operation where SHG is being processed, iron is can be introduced with the feed at the melter when banding material is dissolved or when chains and hooks are accidentally lost in the zinc bath.
As with lead, any dissolved iron will tend to concentrate inside the crucible. Once it reaches a compositional limit it forms a zinc / iron crystal which is solid at the zinc boiling point. Although iron has a very high evaporation temperature it may still be detected in the product if it is not managed.
- Minor Elements: Of the minor elements, cadmium is the most problematic as its boiling point is well below the boiling point of zinc. This means that any cadmium in the zinc alloy in the crucible will be present in the product at roughly the same proportion as it appears in the liquid metal. The only way to handle the situation will be to dilute the contaminated feed with low cadmium zinc and then process this as normal, making sure that the dilution is sufficient to ensure that the cadmium specification for the product is not exceeded.
For heavily contaminated feeds, standard zinc oxide furnace equipment is not suitable, and you should not attempt to run it as it may put you operators in danger from the highly toxic fumes.
The other minor elements, except for magnesium, are usually present in trace or minor amounts in the liquid. To estimated how much of the element may be present in the product you must multiply the crucible concentration by the relative vapor pressure of the element at the zinc boiling point, which for most of these elements will be in the ppm-ppb range. It is therefore highly unlikely that any Fe, Ni or Cu contamination of the zinc oxide will be generated in the vapor phase.
- Magnesium will also be dilute in the vapor at about 2500:1 and as the probability of Mg being in the liquid zinc is very small, it is not considered to be a major issue. It is still important to monitor the crucible from time to time to ensure that the concentrations are not building up.
If Fe, Cu, Ni etc. are present in the zinc oxide then the most likely source will be droplet ejecta from the crucible. In operation zinc vapor is generated at imperfections at the base and on the walls of the crucible as the vapor bubbles require an imperfection in order nucleate. As they have a long way to travel and as the pressure at the base of the crucible is very high, compared with the pressure at the surface of the zinc, the vapor bubbles grow and accelerate towards the surface.
As vapor bubbles break at the surface the energy of the bubble surface holding the liquid zinc back, is suddenly released, and the bubble rapidly collapses. The energy released by the collapsing bubble causes a small jet of liquid to be formed at the former base of the bubble.
The effect of this process is to generate a mist of fine droplets with very small droplets at the top of the jet and larger droplets formed at the base of the jet. There is considerable momentum acting on these droplets, which is, in some cases, enough to shoot them out of the crucible.
To control this, it is important to operate the crucible with a space between the liquid metal and the crucible lid, such that most droplets generated by boiling process are contained and do not have sufficient momentum to leave the crucible.
Most of the grit generated will be from the lids of the crucible, and most of this will be generated by the act of candle removal and clean-up by the operator, during production.
There is a possibility of metallic zinc contamination during cleaning, as zinc may condense inside the candle if they grow large enough, and these frozen droplets may be released when the candles are removed.
The installation of a grit catcher between the bag filter and the combustion hoods will go a long way to minimize metallic grit in the product. Good operator practice will also help to limit production losses by minimizing the quantity of grit being generated.
There is a possibility that grit, and dust may be drawn into the system with the zinc flame. Check the positioning of the hood in relation to workshop drafts, open access doors and areas where dust and grit can be blown into the hood area. Operate with a closed workshop wherever possible. If that is not practical install baffles to prevent dust from entering the production area.
Take care during workshop maintenance and cleaning. When cleaning make sure that any operating hoods are protected from the possible generation of dust by the cleaning process.
Try to keep the loading door of the melting furnace away from the hoods and design the area so that any drafts tend to flow towards the melting furnace, if possible.
During the process of ash removal from the melting furnace, there is a risk that fume may be drawn into the hoods. It is recommended that, for environmental purposes, the melting furnace loading door should be under a hygiene hood with a forced draft of at least 5m/s across the open area. We can help with the efficient design of these various hygiene systems.
Any dilution vent that may be opened during production, for whatever reason, may draw contaminants into the product. Any such area should be routinely checked for debris. Consider adding protection to ensure that the vents are free of potential contaminants.
If this is a persistent problem, consider commissioning a workshop draft analysis. This is an area where we may be able to offer some assistance. Our consultation services will assist you in the formulation of your forward planning.
During collection and handling
The collection and handling systems must be covered at all times. It is important to check the condition of this equipment regularly as failures of shafts and bearings may cause contamination of the product with metal shards and filings etc. Simple sensors monitoring critical items in real time, for example monitoring the current of a motor that drives a conveyor is a very good idea. As most speed controllers provide torque feedback, adding software routines to your existing plc program will enable you to highlight system changes and issue maintenance warnings.
Checking for the proper rotation of a conveyor or rotary valve is now simple matter of using proximity sensors. Using these digital devices, integrated volume flow volumes can be calculated. For calculating flow rates, consider using shaft encoders.
When ducts are modified or replaced, poor quality duct fabrication can be very troublesome as problems will show up randomly. Use grade A rust free steel for any duct work. Use GMAW and avoid using stick welding. Consider using a steel band under welds to prevent weld blow though. Thoroughly clean any surface affected by oxyacetylene cutting. Prevent, as far as is possible, burning into the duct. Ensure proper clean up practice to make sure the interior of the duct is clean and free from wind blown debris, swarf, filings, oils, rust, welding spatter, and flux before and after fabrication.
It is important to use IP54, or better, covers for all conveyors, bins, hoppers, diverters, feed tables, chutes, and transitions. An IP54 cover will only be effective if it is closed and secured. In some cases, seals and covers may be damaged during maintenance. Sealing the cover after the completion of the maintenance task is important and should be inspected by a supervisor.
Air in the packaging plant should be free from drafts as far as possible. Locations where the product may be exposed to outside air should be protected by baffles and hygiene hoods. It is important to make sure that spills and areas where spilled material may build up are kept clean.
Care must be taken when material systems are blended, to make sure that contaminants are excluded from these spaces.
The exact conditions that will provide you with the required particle size will vary from plant to plant, therefore it is important to test the limits of your equipment in the commissioning phase of your project. Taking careful notes of how this varies in your plant is important for bench-marking your operation.
In general Particle size/Surface area will be controlled by the speed of the vapor leaving the vapor tube or the crucible orifice. The faster the vapor flows though the orifice, the smaller the particle size and the higher the surface area of the product will be.
As vapor speed is a function of vapor volume and the orifice size, particle size is strongly influenced by the production rate, and as the production rate is usually set up to be as constant as possible, the choice of orifice size will define the median particle size for a given grade. It is not recommended that changes to the production rate be used as a means of particle size control.
The speed of the dilution air entering the combustion hood influences the shape of the zinc vapor flame, with high air speed causing a greater dispersion of zinc oxide crystals. Dispersion of the crystals makes it more difficult for them to grow. Generally, the faster the air speed under the hood, the greater the surface area. This effect is localized so the changes to the average particle size will be relatively small. Therefore, use the position of the collection hoods for fine control.
There is usually a by-pass damper installed downstream of the combustion hoods and this is usually used to control temperature in the duct. The effect of opening this damper will be to decrease the volume air flow at the hood. In an automated system changes to the bypass damper will cause the elevation of the hood to be changed to maintain the crossflow velocity set by the operator. In a manual system the operator must adjust the height of the hoods.
Make sure there is a sampling point at each hood to check the setup. Changes will react very quickly on the surface area quality of the product. Make sure that the system is given some time after the adjustments before sampling.
To make a consistent product it is important to provide a uniform air flow around the emerging zinc vapor. If this flow is perturbed in any way, it may cause product losses and stretch out the PSD (Particle Size Distribution) curve.
This is a QA measure that produces a consistent batch of product for packaging. The system uses a shallow fluid bed that is pulsed for short periods using high pressure compressed air. Short duration pulses from a large air receiver are designed to keep operating costs low. The product flow generated by the pulses mixes the batch to produce a homogenous product.
Homogenizing widely dissimilar surface area grades will not work in this type of equipment and should not be attempted. Homogenization is necessary if the end user requires that the properties of the shipment be the same from the first bag to the last bag.
Density will naturally increase in the collection systems as the fine particles are electrostatically attracted to each other to make clusters. It is possible to increase product density by putting energy into the flowing powder. The more energy you put in, the denser the powder will become. To do this consider:
- Increasing the power draw on some of the feed conveyors by the addition of extra flights or adding tags to the flights.
- Adding custom mixing conveyors with long residence times and special flights designed to convey and homogenize material.
- Using compression flights in some of the material handling systems.
- Adding compression or granulation rollers to transfer points in the system.
- Adding de-aeration rollers at key transition points.
- Compression of the product in the sacks and supersacks, using hydraulic rams.
- Vacuum packaging.
The achievable delivered density using these methods will be about 950kg/m3 to achieve grater densities it will be necessary to pelletize.
For some users, zinc oxide pellets are attractive. The physical properties of the pellets must be controlled in a very tight range from large to small and from soft to hard. The problem is that most end users require small hard pellets, and this goes against the physics of pellet formation.
The more energy you put into the pellet the harder it will become, unfortunately for most pellets the longer they are in the pelletizer the larger they grow.
For both disc and drum style pelletizers, screens will be required to separate the ideal sized pellets from the over and undersized materials.
Equipment must therefore be oversized, as only a very small proportion of the material exiting the pelletizer will be in the correct property range.
To get the system going, it is necessary to seed the process with pellet nuclei.
Any oversized pellets must be crushed and returned, along with unpelletized fines. The return material provides the nucleation sites for the pellets.
If the produced pellets are too dusty, it is probable that oversized particles are breaking down in the packaging system.
Care must be taken not to make the pellet too hard; hard pellets will not break down and disperse in the client’s mixing equipment.
Hardness is usually a function of the speed of the pelletizer and the residence time. For a disc pelletizer this is controlled by setting the speed of the disc and setting the angle of tilt. For a drum pelletizer, adjustment of the angle is possible in a narrow range of a few degrees. Residence time is therefore usually set by setting the dam height. For a drum pelletizer only small speed changes are usually possible due to mechanical limitations.
Drum pelletizers are usually better at producing small hard pellets. However, as the residence time can be quite long, care must be taken to ensure that the pellets are not too hard. There is less flexibility with the set-up of a drum pelletizer.
Seeding the pelletizer with the correct sized pre-pellet is crucial in maximizing the yield of the correct sized pellets. It is not usually a good idea to attempt to pelletize fresh material. It is better to allow the material to rest prior to pelletization.
If the zinc oxide is treated, or produced using the American Process, material may not be susceptible to pelletization as the surface electron density will not be sufficient to hold them together.