Cause and Effect diagrams, also known as Ishikawa diagrams, which are mostly created in the familiar shape of a fishbone, can be as complex or as simple as the problem they attempt to solve. In the case of this example, where the problem was intractable, but problem solved to root causes using our team-based Lean Six Sigma improvement initiate, the Cause and Effect diagram created was very detailed. To help prioritise problem solving, operational performance ratings were added to each activity as numbers [1 = highest importance] and letters, indicating the customer importance weighting [A = highest importance]. Looking at the attached example, which is the most important operational 'cause' performance activity that will increase capacity, without creating an 'effect' of increasing the volume of waste or increasing the reactivity of the waste? 

1. EFFECT
Recovery Gap [E] [>0%],
Reactivity [I] [<1L/kg/hr]
Output [E] [>90mt/day]
GUIDELINES
MATERIAL INPUTS
MEASUREMENT SYSTEMSOPERATOR ADJUSTED PROCESSES
Scrap
[2,3&6]
· Classification by composition [Processing differences] [2]
· Classification by configuration [Standard practice variability] [2]
· Classification by standard recovery [E]. Accuracy of standard] [2]
· Density [Melt rate] [2], [Charge time] [3] ,[Charge frequency][3]
· Compactness [Break up rate] [2], [Float and slide on top of molten bath]
· Surface area [Flame impingement],[Hot gases contact area]
· Combustible hydrocarbon contaminants from plastics, rubber greases [2]
[Variable fuel input & stoichiometric ratio] [VOC spikes] [A]
· Combustible carbonaceous contaminants [Charcoal carbon reactivity from
pallets & incomplete hydrocarbon burning under molten flux blanket] [2]
· Incombustible contaminants [inert melt and oxide burden],[Furnace capacity
loss],[efficiency loss] [2]
· Metallic contaminants [Off chemistry from high % unwanted elements, e.g.
Si, Fe, Pb, Zn, Sn, S, P],[Increased thermiting risk with high Mg] [2]
· Thickness [incipient surface melting <1mm thick oxidation increases
exponentially] [2]
· Size [Processing differences between bales and dross or return] [2]
· Shape [Coils coalesce into dented drum-shaped unmelted lumps] [2]
· Mass [Addition inaccuracies due to average weights] [3]
· Amount [Ratio],[Charging times and frequency] [3]
· Charge additions time and frequency added [3]
Salt Fluxes [5&7]
· Amount [Total flux to scrap [2,5,6&7] oxide burden ration c. 8-14% sufficient to erode and break
up molten metal from oxide skin/shell and entrap oxides into slag] [2]
· Mass [Addition inaccuracies due to dispensing method
· Particle size [Sufficient mass to resist flying up the flue in the hot gas exhaust stream]
· Fused eutectic commercial flux blend ratio [7]: NaCl 65% [Common salt], KCl 32% [Potash],
Cryolite [Na3AlF6],remelts at 712◦
C
· Charge additions time [Unfused common salt needs to melt initially at 800◦
C and potash at 770◦
C
and cryolite at 1000◦
C before eutectic remelting at 712◦
C and wetting and coating scrap with a
molten flux that prevents oxidation during metal melting and cryolite catalysing bauxite to melt at
980◦
C down from 2000◦
C] [E&I]
· Contaminants [Change ratio and effectiveness]
Current
Systems
Available
· Pulpit available trends from Citet
· Remote trends from Process Analysis via YMS
· DVR [Camera recording at low resolution]
· INCA [Recording scrap [2], metal output [E], recovery [E] but not flux
· Scales at weighbridge
· Load cells in some mobile equipment
· Direct observation [Colour, opacity and intensity of flames and flue
gases] [A]
· Direct observation [Scrap and furnace condition and molten metal
remaining when door open][6],[A]
· Direct observation [Metallic content and size and shape of oxide
aggregate in press salt cake blocks] [E&H]
· Molten temperature probe can be borrowed [D]
· Time [E]
Metrics
Available
· VOC emission AVE & instant [A]
· Opacity emission 6 min AVE &
instant [A]
· Door open & close [Also ‘flutter’], [B]
· Flue Temperature [B]
· Tilt
· Barrel rotation speed [RPM] [1]
· Rotation direction
· Burner heat request [4]
· Gas consumption [4]
· Oxygen to gas set point
Additional Useful Metrics
· Force to rotate barrel [Low kN indicates suitable time to add a
second charge as the whole unitised scrap [2,3&6] such as bales
have slumped and are floating and sliding on a low friction molten
metal pool] [E]
· Stack gas analyzer of CO, CO2 and O2. [A gas analyzer can help
understand the combustion chemical reactions Occurring. Since
VOC evolution and melting phenomena are heat transfer dependent
and combustion is the source of heat in this process, it is important
to understand the combustion reaction dynamics inside the barrel.
· Temperature distribution inside the furnace [A,C,D&E]
· Door gap [10]
Rotation RPM [1]
· Adjustment and timing [Aim to melt scrap simultaneously] [E]
· Too slow = long cycle time, slow melt rate and low recovery [E]
· Too fast = excessive scrap breakup rate, thermiting and
environmental emissions and refractory damage [B]
· Too early = scrap hitting limit switch and door flutter, burner
sputter and refractory damage [B]
Charging [3]
· Sequencing, frequency, duration, recharging and quantity [Aim to melt fluxes first [5&7] and scrap
simultaneously] [2]
· Charge into a hot barrel = less chip outs [F]
· Add fluxes first [5&7]
· Charge light to heavy scrap [2,3&6]
· Too little = longer cycle time
· Too much = scrap hitting limit switch and door flutter, burner sputter [4] and refractory damage [B]
· Recharge when scrap [2,3&6] is in a plastic state [slumped and not representative of original shape] to avoid
excessive oxidation
· Duration door open = longer cycle time and extra chip outs [F]
· Delays = longer cycle time, oxidation and thermiting [A&C]
Burner Heat
Request [4]
· Balanced burner intensity [Heat request 100%,75%,50%,25%,10%] [4]
· Balance in conjunction with charging and rotation
· Adjust occasionally and vigilantly [A]
· Too low = longer cycle time, unmelted scrap in slag = low recovery [E]
· Too high = thermiting and environmental emissions = low recovery [A&E]
· Timing, frequency and duration
· Tap out at a temperature to remain molten in transit
· Tap out to prevent thermiting [A]
· Tap out to remove molten bath where scrap floats and slides rather than rotates and tumbles
· Re-spin and tap multiple times to remove more melt from furnace [8]
· Use dam rake on last tap out [11]
O2 Boost [9] · Timing if used
· May assist with balancing hydrocarbon contamination combustion [Benefits unknown] [A]
Tapping [D&E]
RF3 Output, Efficiency and Waste Salt Cake Reactivity Minimisation Cause and Effect Diagram
Initial charging
Load fluxes and as much scrap in the compactness sequence [3&6] as quickly as possible using
INCA and the scrap and flux calculator [5&7] without:
· Overloading the barrel [Door flutter] [2&3], [B]
· Loading wrong scrap classification amounts at the wrong time in the wrong position [3]
· Impinging the flame on the scrap [2,3&6]
Melting
Heat the barrel as quickly as possible on the highest flame and the highest rotation [1] without
causing thermiting, uncontrolled scrap [6] break up high flue temperature [B], high VOC emissions
or opacity. [A]
Should any of these occur see countermeasures below
Reloading barrel timing
The timing of reloading as often as necessary when the scrap [2,3&6] has slumped and is
indistinguishable of its original shape. Reload as quickly as possible as often as required without:
· Excessive opening and closing
· Overloading
· Causing thermiting, high flue temperature [B], high VOC emissions or opacity.[A]
Should any of these occur see countermeasures below.
Slumping is determined by:
· Previous experience based on similar cycle conditions
· Flue temperature steady or decreasing
· Flame colour, intensity and transparency
· Gas consumption
· Visual appearance
Door Gap [10]
· Fixed [Benefits not fully understood]
Countermeasures
The following occurrences require action:
· High flue temperature (F) [B]
· High VOC emissions (VOC) [A]
· High opacity (O) [A]
· Thermiting (T) [A]
The countermeasures to apply are:
· Stop or lower rotation[1] for (T),(O),(F) and (VOC) [A]
· Lower gas consumption (burner) [4], but not to zero for (F)
· Press oxygen boost for (F) and (VOC) [A]
· Reloading the barrel for (T) [A]
· Mixing in scrap for (T) [A]
· Adding extra flux for (T) [A]
Standard Practice
[A to J]
Countermeasures [A]
LEGEND
Letters = Customer Importance Weighting [A = Highest]
Numbers = Operational Performance Rating [1 = Highest]