There’s been some good shovel talk on here lately, hopefully it’ll keep up.
Since I’ve spent a grip of time in meetings lately discussing the merits of hydraulic and cable machines to assist in some acquisition decisions, I’ll weigh in here as well.
Most of the merits of hydraulic machines and cable machines have already been well covered in previous posts, but one thing that hasn’t been brought up is the vitally important component of any mining operation: cost-per-ton.
For several reasons, cable shovels will achieve a (sometimes drastically) lower cost-per-ton than equivalent hydraulic shovels will in equal digging conditions and operating circumstances.
Using 789 haul trucks, equivalent operating circumstances, and comparing an O&K RH200 and a P&H 2800XPC as loading tools under equal digging conditions, the O&K RH200 will move material for approximately $1.54/CY. The P&H 2800XPC will move material for approximately $1.32/CY.
I don’t consider the comparison between the RH200 and the 2800XPC the most accurate because they don’t equally pass-match to a 789, but it’s a good enough baseline to use as an example.
One of the biggest reasons for the lower cost-per-ton offered by a cable machine is attributed to their ability to work out of a greater face height than a hydraulic shovel. This allows greater bench height which decreases mine production costs for several reasons.
By increasing bench height (if rock mechanics allow the increased highwall height to be achieved safely), the total number of graded drill pads can be decreased, decreasing the total number of blasts; which reduces costs. By increasing bench heights, the powder factors can be increased which increases rock fracturing in the shot material. This provides several benefits: easier loading for the shovel, as bucket fill factors are increased, GET life is prolonged, which decreases downtime. It also increases load consolidation in the haul trucks, which is especially important in lighter materials.
Depending on material, and blasting methods, a deeper or overshot bench can also create a better floor as the face advances across the new bench, which prolongs the life of expensive haul truck tires and the tires of the RTD that maintains the floor.
The increased reach of cable shovels allows them to more safely work higher benches than a hydraulic machine. If you keep up with MSHA’s documentation of highwall failures, it’s not hard to find hydraulic shovels that worked dangerously tall faces that subsided onto the loading shovel; which has lead to fatalities in some cases. There are numerous videos on Youtube of hydraulic machines working dangerously high faces. Cable machines safely allow greater bench and face height, which helps to decrease cost-per-ton.
The increased reach of the cable machines also allows safer and more flexible dual-side loading as trucks can be spotted at a greater (read that as “safer”) distance off the counterweight of the shovel than with a hydraulic machine. Utilizing cable overpasses allows the trail cable to be wrapped in along the crest of a lower bench or toe of highwall, and then cross the path where the trucks need to access the off side of the shovel, allowing dual-side loading, thus increasing production.
In lighter excavation conditions (coal, lignite, phosphate, sedimentary overburdens, etc.) where shovel footwork patterns allow parallel tracking; an electric shovel with a cable sled can stand alone and keep truck spot-time constant; only having to stop trucks while handling his sled. Most of the time studies I’ve reviewed show that even with that hiccup in production, the increased yield off the cable shovels equates to a 53-55 min/production hour [average] for the haulage units. In this scenario, an equivalent hydraulic machine can maintain 58-60 min/production hour [average] for the haulage units due to no handling of a trail cable. The material moved per hour in this scenario will usually be equal between the two machines.
In hard rock excavation (metals, minerals, etc.), one of the biggest constraints on production off a cable shovel is time to handle and move the trail cable once the direct face has been mined to maximum reach. In a larger mining operation, with good controllers who are on the ball and not relying on CAES and FleetCommander to do everything for them, they can lull the trucks to a particular shovel to allow the machine to reset his sled or sling, realign, track back into the face and stage. When the electric shovel is the only loading tool for the haulage units; an advancing bench cut design and loading on-the-fly is more suited in most situations, but still causes a decrease in the haulage units production due to stand-by time while waiting on the shovel to handle the trail cable and realign.
Cable shovels have much greater floor level digging reach than hydraulic shovels, which increases the amount of time they can mine from a static position without needing to reposition in the face. In smaller ore bodies or scenarios that require lots of movement in the face, a hydraulic shovel will greatly outperform a cable shovel.
The digging forces of a cable shovel are greater than a hydraulic machine’s. The loading radius that an electric shovel operates on also assists in bucket fill factors, as when the bucket clears the face on a cable shovel, it’s higher, and usually close to the necessary elevation to dump into the truck bed without additional movement. That said, in free-digging, or poorly blasted material, the hydraulic shovel’s ability to curl the bucket allows much more finesse and precise handling of large rocks. As mentioned in a previous post, cable shovels require their material to be blasted.
Cable shovels require more assistance from RTD’s to maintain floor quality to save integrity of expensive haul truck tires; while hydraulic units can clean and maintain their floors better.
These diagrams are pretty strong P&H propaganda, but they do a good job illustrating some of the above points.