In a cement plant, the clamshell grab and the electrohydraulic orange peel grab rarely handle just one material. The same storage hall may hold clinker fresh from the cooler, quarried gypsum rock, powdered lime and petroleum coke. Each one punishes the equipment in a different way: through abrasion, dust, reactivity or lump size.
That is why at Stemm we don't design equipment "for cement" in the abstract. We adapt shells, lips, sealing and hydraulic circuit to the specific material being handled. Below we review what each material demands and which features meet that demand. If you are looking for the design details of our range for this sector, you can find them in special clamshell grabs for the cement industry.
Clinker: abrasion, irregular particle size and residual heat
Clinker is the intermediate product of the cement manufacturing process: nodules that leave the kiln at around 1,450 ºC and are later ground with gypsum. It is the material that wears a grab the most in a cement plant, and it does so in three ways at once.
The first is abrasion. Its mineral phases are hard and act like a grinding wheel on lips and shells with every penetration cycle. That is why the closing lips must be made of wear-resistant steel such as HARDOX 500, and the shells must be interchangeable. Wear is then solved by replacing parts without touching the grab body.
The second is particle size. A single load contains fines, 3 to 4 cm nodules and, at times, larger balls broken off from the kiln. A shell profile calculated for that angle of repose and a high penetration force achieve full loads without leaving the coarse material outside the grab.
The third is temperature. Clinker may still be hot when it reaches the storage hall, and the heat is transferred to the hydraulic unit. A sealed, pressurised tank with an anti-condensation system, together with a safety thermostat like the one we describe in our equipment for extreme environments, protects the oil and the seals.
There is a less visible detail that determines service life: the joints. Because of the abrasive dust, grease turns into a paste that sands down pins and bushings. Our maintenance-free, grease-free joints eliminate this problem at the root. For clinker we work with the 2CH-1.8 and 4CH-1.8 models, designed specifically for cement, clinker, limestone and gypsum.
Lime: fine dust, moisture and tight sealing
With lime, the problem is not so much wear as reactivity. Quicklime reacts with water, releasing heat, and absorbs moisture from the air. It also generates a very fine dust that gets through any gap. In a lime kiln, the grab usually handles both the limestone feed and the already calcined lump lime, two materials that behave very differently.
For this material, the priority is closure. The shells must fit without play along the entire length of the lip, because any gap lets fines escape during transfer. That dust ends up on the floor, in the air of the hall and on the grab's own components.
The second priority is to isolate the hydraulic and electrical parts from the environment. A pressurised tank prevents dust-laden air from entering, and the anti-condensation system stops moisture from building up in the oil. The protected electrical cabinet built into the structure completes this isolation. Since lime has a low bulk density, it is advisable to size the shell capacity by volume and not only by the crane's payload.
Gypsum rock and limestone: lumps, compaction and penetration
Gypsum is the setting regulator in cement, and plants almost always receive it as natural gypsum from a quarry. It is a soft mineral that causes little wear, but it arrives in irregular lumps and, when damp, cakes in the stockpile. Crushed limestone is more abrasive and can also include coarse fractions.
With compacted gypsum rock, the grab above all needs to penetrate. Penetration teeth can be welded to the lips for this purpose, and the variable closing speed motor pump adjusts the force to the resistance the shell meets. This way the grab neither stays half closed nor overloads the circuit.
When the lump exceeds what a shell can enclose, the right equipment becomes the electrohydraulic orange peel grab. Its tines, with a profile adapted to the product, hold irregularly shaped pieces and work at any inclination on an uneven stockpile. In a gypsum quarry or in the plant's receiving yard, combining both types of equipment makes it possible to move both the fine material and the lumps.
Alternative fuels: when to use a clamshell grab and when to use an orange peel grab
Cement plants feed their kilns with petroleum coke and, increasingly, with alternative fuels such as shredded tyres or solid recovered fuel (SRF). These materials are stored indoors and dosed into the feed hopper with an overhead crane, often in continuous cycles.
Coke is fine and dusty, so it suits a crane grab with tight sealing, just like lime. SRF and shredded tyres are a different story: light, fibrous and tangled. A shell tends to slide over them or close with little load, whereas an orange peel grab sinks its tines in, untangles the stockpile and takes full bites.
In this application, availability matters most. An orange peel grab feeding the kiln cannot stop for hours because of a hydraulic fault. That is why our electrohydraulic orange peel grabs are fitted with standard, interchangeable manifold blocks that can be replaced in less than 15 minutes, and grease-free joints that eliminate maintenance stoppages.
Port unloading of clinker with clamshell grabs and electrohydraulic orange peel grabs
Clinker is stable and is traded in bulk by ship, so many plants receive it at dry bulk terminals. There the grab faces the same material, but with two additional demands: speed and dust control on the quay.
In the hold, every second of the cycle counts. A grab with fast opening and closing and a full load in every bite reduces the vessel's time in port. At the end of unloading, when material remains in the corners and at the bottom of the hold, it is essential that the grab can work at an angle without losing oil or pump performance.
Tight sealing is once again decisive. Every leak of fines between the hold and the hopper is lost material and dust over the port. We discuss in more detail how the different grabs fit into port operations in the article on stevedores' work.
Stemm, manufacturer of clamshell grabs and electrohydraulic orange peel grabs
Clinker, lime, gypsum and fuels are not handled the same way, and neither should the equipment be. At Stemm we have spent more than 40 years designing grabs and orange peel grabs tailored to the material, with shells, tines and hydraulic circuits designed for each application in the cement industry.
If you are considering renewing or expanding your equipment, our online quote tool lets you calculate an approximate investment in a few minutes. And if you would rather review your case with our technical team, we will help you choose the right configuration.
