How does a rock crushing plant work? Step-by-step process

A crushing plant turns rock from a quarry into finished aggregates ready for construction, in 4 stages: primary crushing, secondary or tertiary crushing, screening by size and, when needed, washing of fines. Quality control for aggregates used in concrete in Mexico is governed by the NMX-C-111-ONNCCE-2018 standard, published in the DOF on November 12, 2018.

Key facts

• Primary crushing: reduces rock blocks as they come out of the quarry.
• Secondary and tertiary crushing: further reduce the material to the required size.
• Screening: separates the material into defined sizes using vibrating meshes.
• Grading under NMX-C-077-ONNCCE-2019; sampling under NMX-C-170-ONNCCE-2019.
• General specifications for concrete aggregates: NMX-C-111-ONNCCE-2018.

How does a rock crushing plant work, step by step?

The process starts at the quarry, where the rock that will feed the plant is extracted. From there it moves to primary crushing, which reduces the largest blocks to a manageable size for the following stages. That is followed by one or more secondary crushing stages — and, depending on the product sought, a tertiary stage — that keep reducing the material.

Once crushed, the material passes through vibrating screens that separate it into different aggregate sizes. When the product requires it, a washing stage removes fines — very small particles — before the material settles into hoppers or stockpiles for dispatch.

It is a continuous process: rock enters at one end of the plant and leaves at the other turned into different products, each destined for a specific use on site. Conveyor belts move the material between stages, so the flow from crushing to screening to storage runs without manual handling of the rock itself.

What is the difference between primary, secondary and tertiary crushing?

Primary crushing receives rock directly from the quarry, in large, irregular fragments. The typical equipment for this stage is the jaw crusher, which breaks rock by compression between two plates, one fixed and one movable.

Secondary crushing takes the material already reduced by the primary stage and processes it again, usually in a cone crusher, which follows the same compression principle but in a different machine, suited to smaller particles. When a finer product or a particular particle shape is needed — such as manufactured sand or base material — a tertiary stage is added, often an impact crusher.

Each stage reduces the size and prepares the material for the next, until it reaches the range that is later separated by screening. Not every plant needs a tertiary stage: it depends on the products the plant is set up to deliver.

How are aggregates screened by size?

Screening uses vibrating meshes with openings of different sizes, arranged in several levels: the largest openings sit at the top and the smallest at the bottom, so the material separates as it passes through each level. The result is fractions of relatively uniform size, which become the plant's commercial products — from the coarsest gravel to manufactured sand, including base and ballast materials.

Common commercial sizes for crushed gravel include 3/8", 3/4" and 1½", conforming to NMX-C-111-ONNCCE-2018, per the aggregate spec sheets from GCC and CEMEX México. The exact sizes a given plant offers depend on how its screens are configured.

This screening is verified through sieve analysis under NMX-C-077-ONNCCE-2019, which defines how to determine the size distribution of an aggregate using a series of standardized meshes. The sampling that precedes that analysis — how to take a representative sample of the material — is set by NMX-C-170-ONNCCE-2019.

What quality controls apply to aggregates?

In Mexico, NMX-C-111-ONNCCE-2018 sets the specifications and test methods for the aggregates — fine and coarse — used in hydraulic concrete: what limits on harmful substances, particle shape and other properties the material must meet. It is the reference standard for determining whether an aggregate is fit for concrete.

Alongside it are NMX-C-077-ONNCCE-2019, specific to sieve analysis, and NMX-C-170-ONNCCE-2019, which governs how to reduce a large sample to the amount needed for testing without losing representativeness. The three standards work together: without correct sampling under NMX-C-170-ONNCCE-2019 there is no reliable grading under NMX-C-077-ONNCCE-2019, and without reliable grading there is no way to verify whether the aggregate meets NMX-C-111-ONNCCE-2018.

What products does a crushing plant produce?

A stone aggregates crushing plant typically delivers gravel in different sizes, manufactured sand, base material and ballast. Each product has a different use on site: coarser gravel sizes usually go into concrete or drainage, manufactured sand into mortar or concrete, and base and ballast into the structure of pavements and roads.

The article on gravel types and sizes and the one on ballast, base and asphalt surface course detail the use of each product from a plant like this one.

How does Grupo Server help?

Triturados Gramol, a Grupo Server company founded in 2003, produces stone aggregates — gravel, sand, ballast, base and asphalt surface course — with two operating plants, one in Coahuila and one in San Luis Potosí. You can review its product line on the Triturados Gramol page.

For a project that needs aggregates with documented quality control, the rest of the group's construction operation is under the construction section, and supply questions are handled through Grupo Server's contact form in Saltillo, Coahuila.