Key factors for recyclers when choosing a high-speed wood waste grinder

Recyclers have multiple options for recovering wood waste material. Slow-speed, high-torque shredders work well in lower volume applications where the infeed may be more variable, and for clean, round wood, nothing beats a disk or drum chipper. For most large-scale wood waste recycling and composting operations, the high-speed wood waste grinder, either in a horizontal or tub configuration, remains our industry's workhorse. 

Overall, the wood waste grinding industry has become more sophisticated over the last decade - for the most part, organics recyclers no longer make decisions purely based on engine horsepower or mill opening size, and they are faced with mulitiple end-market opportunities and very tight profit margins in most cases. Today's more sophisticated operations prioritize the tracking of fuel and wear part consumption against throughput, and many invest in asset protection plans. 

Today's high-speed grinders have adapted as recyclers and recycling markets have evolved, and can handle a very wide range of materials, including all kinds of wood - from wet and stringy to hardwood and stumps - as well as asphalt shingles, corn, hay, C&D wood, plastic and food waste,  just to get the list going. There are available fast-changing screens, teeth, knives and drums to easily make different products with one machine, and  integrated metal sorting systems. These machines can provide the capability to simultaneously grind and colour landscape mulch, and are available with a huge range of options designed to enable recyclers to customize their grinder to their operational needs.

Beyond the initial research into available suppliers and machines, recyclers should consider factors such as the type and volume of materials expected for processing, machine safety, cost of operation, ease of maintenance, reliability, planned end-product size and production rate requirements, as well as end market demand and future growth potential.

Following is a breakdown of: 

Major factors to consider when adding a grinder to an organics recycling operation.  

HOG vs. tub

The main difference between horizontal and tub grinders is that horizontals generally use a chain-based feed roller and "moving floor" to feed the hammermill, which is more complicated to maintain, but provides more consistent feed, especially for longer material such as whole trees. A tub uses gravity and a rotating, conical top-feed tub to feed the hammermill. Both types of grinders can handle most material they encounter, but in general horizontals will more easily handle longer material compared to a tub, while a tub grinder can provide more productivity for large material such as stumps and pallets. Some horizontals are also available with an angled cuttermill designed to employ gravity for better infeed. 

With horizontal grinders, generally, you don't have to size your product as much as you do with tubs. However, for many, the tub grinder is a preferred way of grinding wood waste for pure volume reduction, and to produce mulch because it "smashes" wood and produces less splinters, compared to a horizontal which basically "rips" material apart. Tub grinders are also  highly productive, but slightly less complicated machines, and are easier to work on and maintain.

With respect to safety, tubs do have a reputation as a less safe option, mainly due to the fact that the grinding mechanism on a tub is exposed and there is a higher possibility of hazardous flying material being thrown from the plant. Today's tubs (and some horizontals as well), however, are available with tub covers and full enclosures to prevent this.

Productivity and safety always comes down to operators at the end of the day. If an operator keeps a tub full, there is much less chance for dangerous flying material, as essentially the material on top stops material from the bottom of the tub, where it meets the hammermill, from flying out and causing a hazard. 

Electric power

While the latest Tier 4 technology has greatly improved overall fuel consumption for diesel-powered grinding equipment, an electric grinder produces no GHGs and will almost always offer a lower cost per ton (or cubic yard) of finished product, compared to a diesel grinder. Electric grinders are ideal for stationary applications where there is access to three-phase power, but are also available with mobility packages (including trailer mounts and electrical connections, for large site and multiple location applications). 

Match the machine to the task at hand

How much material is expected to be processed per hour is a key question. The amount of material throughput per hour will impact the processing speed of an entire line. Recyclers should consider their current volumes for grinding as well as future potential volumes. A basic rule of thumb, for some, is that higher speeds are needed for higher volumes. 

Where a grinder will be used should also be considered. If noise is an issue, and volume requirements are not too great, then a lower-speed, high-torque, single-shaft shredder that runs anywhere from 90 to 300 rpms, may be a better way to go. Higher speed equates to a noisier machine.

It is best to choose a grinder that works well with existing equipment and set-up. Over-sizing a grinder to match "fringe" materials that may be encountered drives up the costs of every ton or cubic yard produced. An empty grinder with the engine or motor running means an operation is grinding air. 

Metal separation needs should also be considered. Nails and other scrap metal need to be removed from wood residue in order for the end product to be sellable, especially in C&D and asphalt recycling applications. Knowing what percentage of separation you need will determine what kind of extraction system you need in your grinder. 

The amount of separation needed will most likely be determined by the specifications of the fibre buyer or based on the end use for the processed material. Cross-belt magnetic separators are commonly used and two-stage metal separation is available, which adds a magnetic pulley head and is ideal for applications where very high purity is paramount, such as composting and fibre-to-animal-bedding. 

Calculate the cost per tonne

There are basically three major cost components to wood waste grinding: power, labour and ongoing operating costs. Diesel machines tend to be more expensive to run than electric, but they are usually self-contained and portable, making them a good choice for mobile applications. For energy costs, overall, electric-powered machines will cost less to run and require less downtime for routine maintenance. It should also be noted that if high volume is not a main consideration, operating cost for low-speed technology such as a shredder, is typically lower than a high-speed grinder.

When it comes to labour costs, they of course vary greatly from one region to another. Many grinders offer the flexibility to be batch-fed or integrated in-line with conveyors and other material handling equipment. A grinder designed only to be fed in batches can result in more labour costs than one that can be dump-fed or fed automatically by a conveyor. 
Outside of energy, operating costs for high-speed grinders comes down to reliability of the machine and the routine and preventive maintenance plans that are in place. Productivity can be as high as possible, but excess downtime will quickly eat away at profitability gains. This is where the right maintenance regimen and solid dealer support with quick parts availability become key once again.

Plan for options, plan for flexibility

No matter what 2020 and the global pandemic have thrown at us, opportunities for recyclers remain healthy. Markets for wood fibre, recycled asphalt shingles (RAS) and other products that can be produced with the right grinder continue to grow and change, and the movement toward circular economies will continue to push this trend. There are solid and developing markets for coloured mulch, natural mulch, biofuel, refuse-derived-fuel, fuel pellets, compost, animal bedding and other engineered fibres, as well as hot mix asphalt supplement - all of which can be made with the right grinder.

Today's grinders are available with a wide range of options to accommodate flexibility, including track-mounted and mobile models, integrated mulch colouring capability, metal separation, as well as quick-change drums, grinder teeth and screens designed to equip one machine for multiple applications and varying end products. For example, easy change-out chip drums for horizontal grinders are now available from some manufacturers, which enable operators to quickly and easily switch from turning wood waste to mulch, to making a uniform-sized wood chip preferred over standard grindings by wood-fuelled waste-to-energy plants and pellet mills. 

Other options that can be integrated into today's grinders include: a secondary hammermill for very fine particle sizing applications; customized infeed and outfeed set-ups, including conveyor belts, vibrating conveyors and metal separation; and high-abrasion packages for processing tough materials like asphalt shingles, C&D and yard waste. In addition, the modern grinder includes sophisticated, "intelligent" machine control and monitoring systems, remote capabilities, as well as feed controls and infeed protection systems that automatically stop the machine before metal and other contaminants contact the hammermill, potentially preventing catastrophic damage. 

Know your supplier

Today's grinders can produce several hundreds of tons per hour of saleable material if all the parameters are just right. But when downtime occurs, for hard-facing, cleaning or maintenance or broken parts, production and profitability gains can be lost very quickly. All manufacturers will tell you that regular maintenance combined with consistent production is key to maintaining the highest long-term output, including a lower cost-per-tonne or cubic yard.

 Most manufacturers in the industry will also tell you that support is the number one factor you want to consider prior to purchase. Parts and service can make or break a recycling business, so this has to be an initial consideration. For grinder owners and operators, parts should be available locally, or if not, should be shippable direct to the customer in one day. 

Research your supplier and once decided, take advantage of the relationship on an ongoing basis. Today's major manufacturers of horizontal and tub grinders for wood waste recycling are experts that can offer help determining what size and type of grinder will fit for an operation or application, they can help in finding end markets for products produced, they can help optimize a machine's operation over its entire life, and they can help in obtaining financing options for an equipment purchase. 

For recyclers considering a grinder purchase, especially if they don't have a solid, positive experience with a given supplier to work from, good market research is paramount. Ask a lot of questions, ask to talk to current customers. It is the right supplier, over the long run, that will make the greatest amount of difference to your bottom line, with respect to lower downtime and lower overall cost of operation per tonne produced. RPN

This article was originally published in the April 2021 edition of Recycling Product News, Volume 29, Number 1.