Gyratory equipment

In the early 1930s, most vibratory separators had a rectangular or square design employing simple reciprocating movement. After the introduction of machines utilizing gyratory motion with orbital movements, there was a huge change in machinery industry due to the much greater screen area usage and capacity per unit mesh area.[3]

Schematic diagram of a gyratory screener

Gyratory motion

The gyratory equipment contains decks of cards on top of each other with the coarsest screen on top and the finest below. The feed is inserted from the top and gyratory motion triggers the penetration of particles into the next deck through screen openings.[4]

Casings are inclined at relatively low angles (< 15°) to the horizontal plane, with gyrations occurring in the vertical plane.[5] The eccentric masses can be varied in such as the increase of top eccentric mass leads to an increase in horizontal throw, promoting the discharge of oversize materials. Increment in bottom eccentric mass boosts the material turn over on the screen surface, maximizing the quantity of undersize-material penetration.[6] Oversize materials are discharged via tangential outlet.

The option to select number of decks enables gyratory equipment to accurately separate materials consisting particles that are very close in size. This advantage is unrivalled and proves to be significant in the powder processing industry where fine materials are involved. High separating efficiency and ease of maintenance puts gyratory screening ahead compared to other processes in terms of product quality.[5]

Existing gyratory equipment designs are already on the market, more to come with further development. Recent studies have shown that potential improvements are available for cost-saving and effective separation process.[7]

Gyratory crashers

Common applications include separation used in the process industry, food industry, chemical industry and pharmaceuticals. This includes screening, classification, sifting, fiber recovery, filtration, and scalping. Gyratory screening is capable of separating finer materials as compared to other methods, and is therefore more suitable to treat fragile materials. Several applications in respective industries are shown in the table[8] below.

General and industrial heavy duty models are available for gyratory equipment, with wooden frames for general models aiming to save cost. Industrial heavy duty models are constructed with carbon steel or stainless steel. Screen capacities vary with model sizes over a huge range to satisfy individual application requirements such as material size, bulk density, moisture contamination, etc. Models consist up to seven decks with screenings up to 325 meshes, allowing it to perform accurate separations for the finest materials. This feature comes in handy in the powder processing industry where fine powders with relatively close sizes are involved. Screens openings for different decks are to be calculated accurately to ensure accurate separation.

General models, installed with wooden frames indicating lesser reinforcements, are used for applications involving materials with distinct difference in sizes. An example for this is the removal of impurities from wood chips for biomass fuel production. In this case, the desired product will be discharged at the coarsest screen, leaving smaller impurities to sink to the bottom frames. These models are selected for more economical purposes and are less common.

Gyratory equipment is divided to a top and a bottom unit. The unit on top consists of screening frames supported with rugged springs attached to the circular base, which allows free vibration of the top unit. Secondary support springs are attached to for heavy duty operation, preventing the vibration of the top unit from reaching the floor. The base of the machine (bottom unit) consists of top and bottom eccentric weights attached to a heavy duty motor. Minimum energy is consumed with the installation of double extended shafts on the motors, which are attached to both the top and bottom eccentric weights. Screen decks can be mounted on top of another within the machine assembly with spacing frames connected together via stainless steel quick release clamps.[12]

There are large amounts of gyratory equipment designs available with some possible design characteristics include:[13][14]

• Feed rates of 1–50,000 kg/h
• Screen layers up to 7
• Operating frequency of rotation at 700–1450 rpm
• Screening area of 1800–24,800 cm²
• Screen diameter of 600–1500 mm
• Power consumption of 5.5–7.5 kW
• Mesh openings of 20 μm – 20 mm
• Construction material

Gyratory equipment is capable of handling feeds of 500 tons/(h·m²) with separation efficiency up to 98% for dry processes, with feed materials to be separated not below a diameter of 4 μm.

Wet processes in the other hand can only manage a relatively high efficiency (85%) if the moisture content is above 70%.

Eccentric weights can be varied accordingly to obtain desired ratio of coarse vs fine products.

Separation efficiency factor is given by the equation:[15]

${\displaystyle Efficiency\;Factor={\frac {m_{o}\;M_{o}}{C\;(1-m_{o})}}}$

(1)

where ${\displaystyle m_{\text{o}}}$ is the fraction of undersize in oversize and ${\displaystyle M_{\text{o}}}$ is the mass of oversize in feed.

However, correction coefficient factor is to be included in the event of multiple decks are involved, as stated in the table[15] below.

This is due to the error carried forward for every deck. Efficiency factor is multiplied by the correction factor to obtain a more accurate estimate.

The degree of removal of wet processes is lower than their dry counterparts, which is explained by the change in physico-mechanical properties of the body.

Relationship between moisture content and separation efficiency[16]

The trend of the curve displays that feed materials with a moisture content above 70% is more suited for gyratory screening.

Both top and bottom eccentric weights play a big role in sorting a ratio of coarse versus fine products. Kinetic moment produced by the additional eccentric weights changes the oscillation swing, hence producing outputs of different rates and compositions. Increasing the upper eccentric weight promotes discharge of the coarse material. An increase in the lower eccentric weight maximizes the quantity discharged below. The relationships are demonstrated in the table below for a fixed design:

The kinetic moment is linked to eccentric weights with the equations:[17]

${\displaystyle F_{u}(\theta )={\begin{bmatrix}{cos[(\theta (t)+\phi ){\frac {d}{D_{u}}}]}\;m_{u}\;r_{u}\;({\frac {dN\pi }{30D_{u}}})^{2}\;\eta _{x}\\{sin[(\theta (t)+\phi ){\frac {d}{D_{u}}}]}\;m_{u}\;r_{u}\;({\frac {dN\pi }{30D_{u}}})^{2}\;\eta _{y}\\-m_{u}\;g\;\eta _{z}\end{bmatrix}}}$

(2)

${\displaystyle F_{n}(\theta )={\begin{bmatrix}{cos[-(\theta (t)-\phi ){\frac {d}{D_{n}}}]}\;m_{n}\;r_{n}\;({\frac {dN\pi }{30D_{n}}})^{2}\;\eta _{x}\\{sin[-(\theta (t)-\phi ){\frac {d}{D_{n}}}]}\;m_{n}\;r_{n}\;({\frac {dN\pi }{30D_{n}}})^{2}\;\eta _{y}\\-m_{n}\;g\;\eta _{z}\end{bmatrix}}}$

(3)

${\displaystyle F_{s}(\theta )=\vert F_{u}(\theta )+F_{1}(\theta )+\cdots +F_{n}(\theta )\vert }$

(4)

where ${\displaystyle \theta }$ is the lower or upper wheel position (rad), ${\displaystyle \phi }$ is the phase angle (rad), ${\displaystyle m}$ is the mass of wheel, ${\displaystyle N}$ is the motor shaft input speed (rpm) and ${\displaystyle \eta }$ is the force transfer coefficient.

Gyratory equipment is only invalid if two or more materials to be separated are finer than 4 µm, which varies with different machine dimensions. The proposed value of 4 µm was calculated using the dimensions of the largest available model with the largest possible gyration radius. The critical velocity, which cannot be exceeded by the materials or else the operation fails, is given by the equation:[5][18][19]

${\displaystyle v_{c}=(D_{a}-{\frac {d_{p}}{2}}){\sqrt {\frac {g}{2D_{a}}}}}$

(5)

where ${\displaystyle D_{\text{a}}}$ is the length of side of aperture and ${\displaystyle d_{\text{p}}}$ is the particle diameter.

Gyration inertia formulae allow the calculations for different models with different dimensions.

Typical gyratory equipment operation circulates around eccentric weight and screen frames. Materials are distributed along the screen surface and undersize materials are allowed to penetrate the screen. A rule of thumb is to be followed for high separation efficiency and smooth operation:

• Slope angle of screen: 10–12° is opted for of highest separation efficiency with desired screening capacity, as displayed in the table[6] below.

• Area of screen: Larger screen area indicates a higher screening capacity. The largest area available is given as approximately 25,000 cm², a larger area will cause severe uneven distributions of material along the screen surface.

Relationship between area and output[6]

• Number of screen layers: More layers of screening are required for mixtures with materials with close sizes. Screen comes in various types, dimensions and material.[20] Fewer decks is preferred to maintain quality output (< 4).
• Material of frames: Wooden frames are opted for economical operations with simple handling. Carbon steel or stainless steel is selected for heavy duty operations for finer products.[21]
• Operating frequency: Higher revolution per minute indicates more rapid rate of separation at the expense of more energy required, which is provided by a motor.
• Feed points: Single feed port is preferred over multiple feed ports as a far more complicated control is involved despite higher separation coefficient.[7]
• Pre-treatments: It is recommended to prescreen oversize materials that are larger than a 5 mm aperture size to prevent damaging the equipment.[22] Simple sieving equipment is sufficient to screen materials with such big sizes.