Storage Dome & Reclamation System with Unique Gear Drive
Let Lafarge Corp. Use Fly Ash for Cement Mixture
ALPENA, Mich. - The installation of a new, state-of-the-art storage and handling
facility has allowed the Lafarge Corporation to use fly ash in its raw cement materials
mixture. The use of fly ash will let the Alpena, Michigan, plant, the largest cement
production facility in North America, produce a cement with wider applications, having
lower alkali properties and offer its customers both Type I and II cement. The plant
currently produces 2.3 million tons of cement per year. To improve its cement quality,
reduce the amount of raw materials used in the cement manufacturing process, and
eliminate shale-related environmental concerns, the Lafarge Corporation's cement plant
decided to change its raw materials cement mixture. Lafarge substituted silica sand and
fly ash for shale. The mixture change meant several things for Lafarge. First, the
substitution would allow them to reduce the large amount of virgin natural resources,
such as shale, it was using in producing cement. Used in the cement mixture instead of
shale would be fly ash, a recyclable waste product of coal-fired power generation plants,
and silica sand found in taconite left over from the iron ore extraction process. Secondly,
Lafarge could eliminate the environmental concerns they had with disposing a large
percentage of the high-sulfur cement kiln dust (CKD) that resulted from shale
processing.
"It was mainly an environmental concern," noted Lafarge Engineering Manager
Steve Cayton. "During processing, a high concentration of sulfur separates out in the
shale burning process. So what we would do is throw away - actually waste - some of the
valuable kiln dust because of the high level of sulfur it contained. With fly ash and
silica sand, we no longer have the high level of sulfur in the kiln dust. Also, the shale
hydrocarbon emissions that were generated during production were considered volatile. Those
substances will no longer be present." Previously, these hydrocarbons had to be
extracted through a costly and time consuming process.
Solving the Problem of Fly Ash Storage
Lafarge's plan to change mixtures seemed like a good switch except for one problem - the
physical properties of fly ash made it costly and impractical because it is difficult to
handle and store. Fly ash is a light powdery material that can literally blow away in an
open-air environment. Lafarge wanted to keep the fly ash totally enclosed in order to
prevent any of the material from becoming airborne. They believed that an enclosed system
would provide the best solution from an environmental protection standpoint. A state-of-the
art handling system had to be designed to implement the use of fly ash.
After an examination of several storage methods, Lafarge decided on a 40,000-ton
capacity concrete storage unit with an automated reclamation system from the Salt Lake
City, Utah-based company, Cambelt International.
Cambelt International works closely with Dome Technology of Idaho Falls, Idaho, in the
design and construction of automated reclamation systems. Dome Technology builds the
storage domes and Cambelt supplies overall project management and designs the unique
reclamation system for the storage domes.
"Lafarge's situation was unique," said Cambelt's Vice President of Marketing
Rexnord Wood. "They needed to handle and reclaim a very difficult to handle material that
was previously impractical to use."
Designing the Reclaim Drive System
While Cambelt's automated dome reclamation technology was proven in several
applications, Lafarge's requirements offered several "custom" challenges for the automated
dome reclamation system designers.
For example, a special drive system was needed to meet the extremely high overhung
load produced by the system's chain drive. Cambelt had been employing a European
manufactured gear drive in previous dome applications. However, Lafarge requested that Falk
gear drives be used on their dome system.
"We're glad they did," said Cambelt's Wood. "They were ready to
come to Salt Lake City to discuss our needs and their creative solution proved
invaluable." Initially, Cambelt was working with the local sales office. Because Cambelt required a "custom" solution, the local office called in engineers from
the headquarters in Milwaukee.
Rexnord account executive Ted Shushereba noted that they were able to provide Cambelt a unique,
application-engineered system from standard products.
"Cambelt needed an enlarged, high-strength, low-speed shaft and bearings because of
the very high overhung load on the system's chain drive. Cambelt gave us some design
criteria that tested the limits of our standard product offering," said Shushereba.
"We had to analyze each stressed component of the unit to determine its suitability
for the application."
To better understand Cambelt's needs, engineers went to Salt Lake City to meet and
discuss alternative drive system solutions. Rather than design an expensive, custom gear
drive from scratch, engineers were able to provide a "custom" solution
utilizing two standard gear drives, a shaft-mount and a foot-mount, working in tandem. This
enabled the company to deliver the drives in the time-frame Lafarge required and at a lower
cost.
"Additionally, we were able to adapt a low-speed shaft from our standard mixer
gear drive line to accommodate the high overhung load," said Shushereba. The tandem
drive has a primary drive mounted to the vertical parallel shaft gear drive's high-speed
shaft to achieve the low, 0.5 rpm on the low-speed shaft that Cambelt required for the
rotating center column on the reclaim system.
The drive assembly to rotate the center column consists of a chain and sprocket driven
by the Falk modified tandem gear drive. It's powered by a 7.5 HP (5.6 kW) AC electric
motor with an output of 1750 rpm. The tandem drive gives the system a 3,386:1 gear
reduction ratio with a total of six reductions. Total output torque of the drive system
is 860,000 lb-in. (97,172 Nm). The entire drive system is located atop the dome in a drive
house.
Also key to the drive system's success was the Falk solid steel plate housing. "Some
other gear housings are cast iron - which are very stiff - but are weak at the non-uniform
curvatures of the cast," said Shushereba. "Cast housings have been known to
break on applications with very high overhung load," he added.
Because of its patented Magnum No-Leak shaft seals, Shushereba notes that the Falk
drives are suited to working in the extremely dusty environment found in the fly ash
storage dome drive house. "A purgeable grease chamber and contact seal prevent the
infiltration of dirt and grime into the drive bearings. A unique drain-back passage works
in conjunction with a radial and axial bushing seal to effectively retain oil and eliminate
leaks," said Shushereba.
"This is important," said Lafarge's Cayton. "Any oil leaks from the drive
could possibly contaminate the fly ash."
The Falk drives also have an efficient splash lubrication system to ensure that
Lafarge's column rotation drive system stays lubricated. A motor-driven pump with a filter
and flow switch provide constant, positive lubrication to the drives' helical gearing.
"What we have found with European drives is that in many cases the repair parts are
more difficult to get and the delivery time is longer," said Cayton. In cases of some
gear drives, of which Lafarge has 500-plus at the Alpena facility, Cayton said that many of
the European drives had to be sent back to Germany for repairs. "This leads to
downtime that is very costly for our operation. We lose an estimated $24,000 every hour
any part of our production system is down. If we have a choice with gear drives, we'd
rather use Falk...," noted Cayton.
The Dome Reclamation System
Cambelt's mechanical reclaim system allows the inside of the storage dome to be fully
utilized, sacrificing only a small part of the dome's volume for the reclamation system.
The system pulls the material, in this case fly ash, to the center of the floor through a
controlled discharge opening into an air-slide conveyor in a tunnel under the dome
floor.
A bridge truss (additional trusses are optional) is attached to the base of a center
column equipped with screw conveyor and powered by an electric motor. Tag screws and
trailing shrouds can also be installed depending on the nature and flow ability of the
stored materials.
The system's center column is mounted on a thrust bearing and a radial bearing, and
connected to a fabricated support base. At the top of the center column, a hoist is mounted
to raise and lower the truss to the level of the fly ash. Retrieval of the stored fly ash
is by means of a 30-inch (76.2 cm) diameter, 90-foot (27.4 m) long radial screw that turns
while the center of the column drive rotates the screw truss through 360 degrees of the
dome.
The dome is filled from the top with a pneumatic conveyor feeding in the fly ash. A
pneumatic conveyor is required to handle the light, powdery fly ash.
The system gathers the fly ash to the opening in the center of the dome floor to the
air-slide conveyor that outloads it to the raw grind area where it is combined with
silica sand and eventually sent to kilns for the removal of any water or carbon dioxide.
This process produces a mixture commonly called "clinker." The clinker is either
stored, or combined with gypsum and processed in roller presses and ball mills for the
final finish grinding. The discharge from the ball mill is transported to high-efficiency
separators where it is separated into the finished cement product.
Changing the System Provides Benefits for Lafarge
Cambelt's reclamation system allows for 100 percent of stored materials to be reclaimed
from Lafarge's 40,000-ton storage dome. For Lafarge, the reclamation system has had a
greater impact than just being able to handle and reclaim a difficult material. By being
able to use fly ash and silica sand instead of shale, Lafarge is changing its Type I
portland cement to a newer low alkali product that will allow for a greater range of uses
in its markets. Cayton adds that because the consistency of the quality of the raw
materials has improved with fly ash, customers will be able to get a greater consistency in
the cement they purchase. Customers will be able to store a Lafarge cement that meets both
Type I and Type II specifications. This translates into a less expensive, yet higher
quality product for the customer.
|
