Stone Ground wood Pulp - Mechanical Pulp Production

Mechanical Wood Pulping

All mechanical wood pulping makes no attempt to remove an impurity known as lignin from the fibers. A product made from mechanical wood pulp will not be durable and will degrade rapidly especially in the sunlight.

Stone ground wood Pulp

This is the most commonly used method mainly due to the simplicity of the machine. one large grindstone which breaks up the descending log into individual fibers and fiber bundles.

Mechanical pulp is produced from fiber defibrillations from a stone grindstone. the temperature around of the pulp is around 80 to 125 deg, locally at the stone where the fibers are being broken down the temperatures can reach up to 170 deg.

this high temperature is an advantage because the lignin in the logs binding the fibers together starts to become more malleable allowing for less damage to the fibers, more intact fibers are produced. 

Thermo-ground wood pulp

The thermo-ground wood mechanical pulp is an advance on the stone groundwood process where the logs are treated with hot water. the logs sit above the stone grinder in a hot water bath. the hot water is used to make the lignin softer. when the lignin becomes soft the break down of the logs into fibers becomes easier leading to less energy being needed and higher quality pulp.
Temperature around 80 to 95 can be generated locally at the grindstone. The water temperature is usually around 80 deg C. 

Pressurized groundwood

This is another advance on the original stone groundwood pulp. this is mechanical pulp production where the grinding takes place under compresses air pressure usually about 1 bar above atmospheric pressure. the water pressure is there for greater than 95 deg C (due to high-pressure atmosphere raises the boiling point of water). this allows for higher grinding temperatures without steam flashing. the high temperature promotes the softening of lignin. this improves fiber separation and reduces the specific energy consumption of the pulp.
Grindstone temp is about 124 – 130 deg.

Basic principles – Stone groundwood process

The fibers are torn from the logs and washed from the stone by means of spray showers. The supply of fibers and fiber fragments go through a screening process to remove the large particles these are known as shives. The stock is then thickened.

At grinding the logs are heated up from the frictions of the grind stone caused by pressing the logs against the stone the wooden structure is softened, the lignin becomes more malleable when heated. The bonding between the fibers will be less. The shearing forces between the stone surface and the wood mean the fibers are torn out.

This process uses very little or no chemicals but is very energy intensive (1200 – 2100kw/ dry ton)

Under the pulp stone, a basin collects the groundwood pulp

The pulp is screened after this process to remove shives (over sized particles) and then thickened

Varieties of Stone groundwood pulp

Fine groundwood pulp

·         Low moisture content in the logs

·         Gentle pressing of the log in the grinding zone (low speed of the chain drives) leads to a lower load consumption

·         Grit size – small dia grit size

·         Less sharpening of the grindstone with small dia grit on the abrasive layer

·         Less sharpening of the grindstone

·         Higher consistency in the basin

·         High specific energy consumption
As a result, the shopper riegler value is high, low dewatering ability

Coarse groundwood pulp

·         Fresh wood high moisture content

·         Intense pressing of the log in the grinding zone – increased the load on grindstone

·         Grit size :usage of grind stone with large diameter grit

·         Sharp stone surface

·         Low consistency in the basin

·         Low specific energy consumption

·         Result in good dewatering ability – high shopper riegler